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Wednesday, January 12, 2011

Exercise may lower risk of death for men with prostate cancer

Boston, MA – A new study of men with prostate cancer finds that physical activity is associated with a lower risk of overall mortality and of death due to prostate cancer. The Harvard School of Public Health and University of California, San Francisco researchers also found that men who did more vigorous activity had the lowest risk of dying from the disease. It is the first study in men with prostate cancer to evaluate physical activity after diagnosis in relation to prostate cancer-specific mortality and overall mortality.

The study appears in an advance online edition of the Journal of Clinical Oncology.

"Our results suggest that men can reduce their risk of prostate cancer progression after a diagnosis of prostate cancer by adding physical activity to their daily routine," said Stacey Kenfield, lead author of the study and a Harvard School of Public Health researcher. "This is good news for men living with prostate cancer who wonder what lifestyle practices to follow to improve cancer survival."

Prostate cancer is the most frequently diagnosed form of cancer among men in the United States and affects one in six U.S. men during their lifetime. More than 2 million men in the U.S. and 16 million men worldwide are prostate cancer survivors.

The study was conducted in 2,705 men diagnosed with prostate cancer in the Health Professionals Follow-Up Study over an 18-year period. The participants reported the average time per week they spent doing physical activity, including walking, running, bicycling, swimming and other sports and outdoor work.

The results showed that both non-vigorous and vigorous activity were beneficial for overall survival. Compared with men who walked less than 90 minutes per week at an easy pace, those who walked 90 or more minutes per week at a normal to very brisk pace had a 46% lower risk of dying from any cause.

Only vigorous activity—defined as more than three hours per week—was associated with reduced prostate cancer mortality. Men who did vigorous activity had a 61% lower risk of prostate cancer-specific death compared with men who did less than one hour per week of vigorous activity.

"We observed benefits at very attainable levels of activity and our results suggest that men with prostate cancer should do some physical activity for their overall health, even if it is a small amount, such as 15 minutes of activity per day of walking, jogging, biking or gardening," said Kenfield. "However, doing vigorous activity for three or more hours per week may be especially beneficial for prostate cancer, as well as overall health," she said.
Boston, MA – A new study of men with prostate cancer finds that physical activity is associated with a lower risk of overall mortality and of death due to prostate cancer. The Harvard School of Public Health and University of California, San Francisco researchers also found that men who did more vigorous activity had the lowest risk of dying from the disease. It is the first study in men with prostate cancer to evaluate physical activity after diagnosis in relation to prostate cancer-specific mortality and overall mortality.

The study appears in an advance online edition of the Journal of Clinical Oncology.

"Our results suggest that men can reduce their risk of prostate cancer progression after a diagnosis of prostate cancer by adding physical activity to their daily routine," said Stacey Kenfield, lead author of the study and a Harvard School of Public Health researcher. "This is good news for men living with prostate cancer who wonder what lifestyle practices to follow to improve cancer survival."

Prostate cancer is the most frequently diagnosed form of cancer among men in the United States and affects one in six U.S. men during their lifetime. More than 2 million men in the U.S. and 16 million men worldwide are prostate cancer survivors.

The study was conducted in 2,705 men diagnosed with prostate cancer in the Health Professionals Follow-Up Study over an 18-year period. The participants reported the average time per week they spent doing physical activity, including walking, running, bicycling, swimming and other sports and outdoor work.

The results showed that both non-vigorous and vigorous activity were beneficial for overall survival. Compared with men who walked less than 90 minutes per week at an easy pace, those who walked 90 or more minutes per week at a normal to very brisk pace had a 46% lower risk of dying from any cause.

Only vigorous activity—defined as more than three hours per week—was associated with reduced prostate cancer mortality. Men who did vigorous activity had a 61% lower risk of prostate cancer-specific death compared with men who did less than one hour per week of vigorous activity.

"We observed benefits at very attainable levels of activity and our results suggest that men with prostate cancer should do some physical activity for their overall health, even if it is a small amount, such as 15 minutes of activity per day of walking, jogging, biking or gardening," said Kenfield. "However, doing vigorous activity for three or more hours per week may be especially beneficial for prostate cancer, as well as overall health," she said.
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Can diet protect against asthma?

Researchers found zinc, vitamins A, D, and E, as well as fruits and vegetables, seemed to have a protective role, but they stress the data are too preliminary to draw any firm conclusions.

"The short answer is that it is too early to tell," said Dr. Graham Devereux of the University of Aberdeen in the UK, who worked on the study.

As a result, pregnant women and parents should not change their diets solely for the purpose of protecting their kids from allergies, Devereux added in an e-mail to Reuters Health.

"Currently there is no evidence that changing diet makes any difference," he said.

More than seven percent of adult Americans, and even more kids, have asthma, and the lung disease has been on the rise in recent decades for unknown reasons.

According to the U.S. Centers for Disease Control and Prevention, it now causes more than 13 million annual visits to emergency rooms and doctors' offices.

Devereux and his colleagues, whose findings appear in the Journal of Allergy and Clinical Immunology, reviewed 62 recent studies that looked at diet and risk of allergy and asthma.

All studies were based on records of women's diets and other observations, a much weaker design than clinical trials in which people are given supplements or foods to eat.

Of the 22 studies that looked at fruit and vegetable intake, 17 linked healthier diets to lower risks of asthma and allergies. And two reports suggested that children with higher levels of vitamin A in their bodies had a 75 percent lower risk of developing asthma.

Pregnant women who ate a lot of vitamins D -- found in fatty fish -- and E -- found in nuts and seeds -- were between 30 and 40 percent less likely to have a child who wheezed, often a sign of asthma.

Also, sticking to a Mediterranean diet -- rich in vegetables, fish and monounsaturated fats from olive oil and nuts, but low in saturated fat from meat and dairy -- during pregnancy was also tied to a drop of nearly 80 percent in babies' risk of wheezing.

But the studies didn't find any apparent benefits from vitamin C or selenium.
Researchers found zinc, vitamins A, D, and E, as well as fruits and vegetables, seemed to have a protective role, but they stress the data are too preliminary to draw any firm conclusions.

"The short answer is that it is too early to tell," said Dr. Graham Devereux of the University of Aberdeen in the UK, who worked on the study.

As a result, pregnant women and parents should not change their diets solely for the purpose of protecting their kids from allergies, Devereux added in an e-mail to Reuters Health.

"Currently there is no evidence that changing diet makes any difference," he said.

More than seven percent of adult Americans, and even more kids, have asthma, and the lung disease has been on the rise in recent decades for unknown reasons.

According to the U.S. Centers for Disease Control and Prevention, it now causes more than 13 million annual visits to emergency rooms and doctors' offices.

Devereux and his colleagues, whose findings appear in the Journal of Allergy and Clinical Immunology, reviewed 62 recent studies that looked at diet and risk of allergy and asthma.

All studies were based on records of women's diets and other observations, a much weaker design than clinical trials in which people are given supplements or foods to eat.

Of the 22 studies that looked at fruit and vegetable intake, 17 linked healthier diets to lower risks of asthma and allergies. And two reports suggested that children with higher levels of vitamin A in their bodies had a 75 percent lower risk of developing asthma.

Pregnant women who ate a lot of vitamins D -- found in fatty fish -- and E -- found in nuts and seeds -- were between 30 and 40 percent less likely to have a child who wheezed, often a sign of asthma.

Also, sticking to a Mediterranean diet -- rich in vegetables, fish and monounsaturated fats from olive oil and nuts, but low in saturated fat from meat and dairy -- during pregnancy was also tied to a drop of nearly 80 percent in babies' risk of wheezing.

But the studies didn't find any apparent benefits from vitamin C or selenium.
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Tuesday, January 11, 2011

Feline Nasopharyngeal Polyps

Klose TC, MacPhail CM, Schultheiss PC et al: Prevalence of select infectious agents in inflammatory aural and nasopharyngeal polyps from client-owned cats, J Feline Med Surg 12:769, 2010.
Feline inflammatory polyps are the most common non-neoplastic masses affecting the middle ear of cats. While these polyps can affect cats across all age ranges, they most commonly occur in younger cats. Polyps present in the middle ear result in chronic otitis externa, otitis media, or otitis interna with clinical signs such as nystagmus, head tilt, Horner’s syndrome and ataxia. When polyps extend into the nasopharynx, they cause chronic nasal discharge, dyspnea, dysphagia, stertor, and stridor. This study looked at the prevalence of feline herpesvirus-1 (FHV-1), feline calicivirus (FCV), Mycoplasma spp, Chlamydophila felis, and Bartonella spp. in samples collected from the ears of normal cats and inflammatory polyps submitted from client-owned cats. Samples were taken from 30 clinically affected and 12 clinically normal cats. All normal bulla samples yielded positive bacterial cultures, suggesting that the epithelium of the inner ear is routinely exposed to bacteria. The study failed to consistently amplify RNA or DNA of the select agents from the polyp tissue, suggesting that these agents were not directly associated with the pathogenesis of the inflammatory polyps studied in the cats tested. At this time, the cause of feline inflammatory polyps remains elusive and may be multi-factorial. [VT]

Related articles:
Kudnig ST: Nasopharyngeal polyps in cats, Clin Tech Small Anim Pract 17:174, 2002. 

More on cat health: Winn Feline Foundation Library
Klose TC, MacPhail CM, Schultheiss PC et al: Prevalence of select infectious agents in inflammatory aural and nasopharyngeal polyps from client-owned cats, J Feline Med Surg 12:769, 2010.
Feline inflammatory polyps are the most common non-neoplastic masses affecting the middle ear of cats. While these polyps can affect cats across all age ranges, they most commonly occur in younger cats. Polyps present in the middle ear result in chronic otitis externa, otitis media, or otitis interna with clinical signs such as nystagmus, head tilt, Horner’s syndrome and ataxia. When polyps extend into the nasopharynx, they cause chronic nasal discharge, dyspnea, dysphagia, stertor, and stridor. This study looked at the prevalence of feline herpesvirus-1 (FHV-1), feline calicivirus (FCV), Mycoplasma spp, Chlamydophila felis, and Bartonella spp. in samples collected from the ears of normal cats and inflammatory polyps submitted from client-owned cats. Samples were taken from 30 clinically affected and 12 clinically normal cats. All normal bulla samples yielded positive bacterial cultures, suggesting that the epithelium of the inner ear is routinely exposed to bacteria. The study failed to consistently amplify RNA or DNA of the select agents from the polyp tissue, suggesting that these agents were not directly associated with the pathogenesis of the inflammatory polyps studied in the cats tested. At this time, the cause of feline inflammatory polyps remains elusive and may be multi-factorial. [VT]

Related articles:
Kudnig ST: Nasopharyngeal polyps in cats, Clin Tech Small Anim Pract 17:174, 2002. 

More on cat health: Winn Feline Foundation Library
Read More


Can sitting too much kill you?

We all know that physical activity is important for good health—regardless of your age, gender or body weight, living an active lifestyle can improve your quality of life and dramatically reduce your risk of death and disease. But even if you are meeting current physical activity guidelines by exercising for one hour per day (something few Americans manage on a consistent basis), that leaves 15 to 16 hours per day when you are not being active. Does it matter how you spend those hours, which account for more than 90% of your day? For example, does it matter whether you spend those 16 hours sitting on your butt, versus standing or walking at a leisurely pace? Fortunately or unfortunately, new evidence suggests that it does matter, and in a big way.

What is sedentary behavior?

Before we go any further, it’s important that we define the term "sedentary behavior". Sedentary behavior is typically defined as any behavior with an exceedingly low energy expenditure (defined as <1.5 metabolic equivalents). In general, this means that almost any time you are sitting (e.g. working on a computer, watching TV, driving) or lying down, you are engaging in sedentary behavior. There are a few notable exceptions when you can be sitting or lying down but still expend high energy expenditure (e.g. riding a stationary bike), but in general if you are sitting down, you are being sedentary.

The above definition may seem rather intuitive, but this is not the way that the term sedentary has been used by exercise science researchers for the past 50 years. Up until very recently, referring to someone as sedentary meant simply that they were not meeting current guidelines for physical activity. In simple terms, if you were exercising for 60+ minutes/day, you were considered physically active. If you were exercising 10 minutes/day, you were sedentary. Case closed. But as we will discuss below, sedentary time is closely associated with health risk regardless of how much physical activity you perform on a daily basis. Further, it is entirely possible to meet current physical activity guidelines while still being incredibly sedentary. Thus, to quote researcher Marc Hamilton, sitting too much is not the same as exercising too little. (if you take only one thing from this post, let it be that quote from Dr Hamilton). Which is why it is so important that when we use the term "sedentary", we are all on the same page about what that means.

Now that we know what sedentary behavior is, let’s look at its relationship with health risk.

Epidemiological Evidence

In 2009 Dr Peter Katzmarzyk and colleagues at the Pennington Biomedical Research Center published an influential longitudinal paper examining the links between time spent sitting and mortality in a sample of more than 17,000 Canadians (available here). Not surprisingly, they report that time spent sitting was associated with increased risk of all-cause and cardiovascular disease mortality (there was no association between sitting and deaths due to cancer). But what is fascinating is that the relationship between sitting time and mortality was independent of physical activity levels. In fact, individuals who sat the most were roughly 50% more likely to die during the follow-up period than individuals who sat the least, even after controlling for age, smoking, and physical activity levels. Further analyses suggested that the relationship between sitting time and mortality was also independent of body weight. This suggests that all things being equal (body weight, physical activity levels, smoking, alcohol intake, age, and sex) the person who sits more is at a higher risk of death than the person who sits less.

The above findings linking excessive sitting with poor health are far from isolated. For example, a similar longitudinal study from Australia reports that each hour of daily television viewing (a proxy of sedentary time) is associated with an 11% increase in the risk of all-cause mortality regardless of age, sex, waist circumference, and physical activity level. And as my colleagues and I summarize in a recent review paper (PDF), numerous epidemiological studies have linked sedentary behavior with obesity, cardiometabolic risk, and even some cancers.

New evidence also suggests that in addition to the quantity of sedentary time, the quality of sedentary time may also have an important health impact. For example, Genevieve Healy and colleagues examined this issue in participants of the Australian Diabetes, Obesity and Lifestyle (AusDiab) Study. A total of 168 men and women aged 30-87 years wore an accelerometer (an objective measure of bodily movement) during all waking hours for 7 consecutive days, which allowed the researchers to quantify the amount of time that participants spent being sedentary, as well as how frequently they interrupted these sedentary activities (e.g. standing, walking to the washroom, etc).

What did they find?

The greater the number of breaks taken from sedentary behavior, the lower the waist circumference, body mass index, as well as blood lipids and glucose tolerance. This was true even if the total amount of sedentary time and physical activity time were equal between individuals—the one who took breaks more frequently during their time at the office or while watching television was less obese and had better metabolic health. Importantly, the breaks taken by the individuals in this study were of a brief duration (<5 min) and a low intensity (such as walking to the washroom, or simply standing).

Taken together, the epidemiological evidence strongly suggests that prolonged sitting is an important health risk factor. But what explains these relationships? Let’s now look at the multiple mechanisms linking sedentary time with increased health risk.

Mechanisms

Reduced Energy Expenditure

Quite obviously (and by definition), when you are sedentary, you are not being physically active. And so one common assumption is that people who sit more are at increased health risk simply because they are getting less physical activity. However, somewhat surprisingly, sitting time and physical activity do not appear to be related for most people. For example a paper from the European Youth Heart Study published in PLoS Medicine reports no association between physical activity and TV watching in a sample of nearly 2000 children and teenagers, and other reports suggest that there is little evidence that sedentary behavior displaces moderate or vigorous physical activity. So while it makes intuitive sense that being sedentary reduces energy expenditure, it is likely through the reduction of very light intensity physical activity (e.g. standing, walking at a slow pace), rather than by reducing the volume of what we typically think of as exercise. This may also help explain why the relationship between sedentary behavior and health risk are often independent of moderate or vigorous physical activity.

Increased Food Intake

In addition to reducing our energy expenditure, sedentary behaviors may also promote excess food intake. For example, a recently published study in the American Journal of Public Health suggests that the amount of commercial television (e.g. television with advertisements) that children watch before the age of 6 is associated with increased body weight 5 years down the road, even after adjustment for other important variables including physical activity, socio-economic status and mother’s BMI. In contrast, watching non-commercial television (DVD’s or TV programs without commercials) showed no association with body weight. Similarly, it has also been reported that each hour of daily television watching in children is associated with an increased consumption of 167 calories per day (PDF), mainly through increased consumption of high calorie, low nutrient foods (e.g. the foods most commonly advertised on television). Much of this is likely just a learned behavior—watching TV exposes us to food ads promoting unhealthy fare, which is likely to have a disproportionate influence on younger viewers. Just as importantly, people may just really enjoy munching on food while relaxing on the couch. Either way, excess sitting (and TV watching in particular) seems to put us in situations where we choose to eat more than we would otherwise.

Physiological Adaptations

I don’t think the mechanisms described above—that sitting too much may lead to reduced energy expenditure and increased food intake—will come as much of a surprise. But what I find truly fascinating is that sedentary behavior also results in rapid and dramatic changes in skeletal muscle. For example, in rat models, it has been shown that just 1 day of complete rest results in dramatic reductions in muscle triglyceride uptake, as well as reductions in HDL cholesterol (the good cholesterol). And in healthy human subjects, just 5 days of bed rest has been shown to result in increased plasma triglycerides and LDL cholesterol, as well as increased insulin resistance—all very bad things. And these weren’t small changes—triglyceride levels increased by 35%, and insulin resistance by 50%!

These negative changes are likely related to reductions in the activity of lipoprotein lipase, an enzyme which allows muscle to uptake fat, thereby reducing the amount of fat circulating in the blood (it also strongly influences cholesterol levels—the details can be found here). Animal research has shown that lipoprotein lipase activity is reduced dramatically after just six hours of sedentary behavior—not unlike a typical day at work or school for many individuals. Sedentary behavior may also reduce glucose transporter protein content in the muscle, making it more difficult for glucose to be taken into the muscle and resulting in higher blood sugar levels. What is most interesting to me personally is that these physiological changes in skeletal muscle have little or nothing to do with the accumulation of body fat, and occur under extremely rapid time-frames. This means that both lean and obese individuals, and even those with otherwise active lifestyles, are at increased health risk when they spend excessive amounts of time sitting down.

Should we be concerned about the health impact of sedentary behavior?

Yes.

Western society is built around sitting. We sit at work, we sit at school, we sit at home, and we sit in our cars as we commute back and forth. In fact, a recent survey reports that the average American accumulates more than 8 hours of sedentary behavior every day—roughly half of their waking hours. The situation in children is, unfortunately, no different. There is evidence that children in both Canada and the USA (PDF) accumulate more than 6 hours of screen-time (time spent in front of the TV, computer, or other screen-based device) on a daily basis. Keep in mind that screen-time is almost exclusively sedentary (active video games excluded), and that all these hours of sedentary behavior are in addition to the hours and hours (and hours) that kids spend sitting at school. In fact, a recent study reports that roughly 70% of class time, including physical education class, is completely sedentary (while slightly better than class time, children were also sedentary for the majority of lunch and recess).

In short, given the consistent links between sedentary behavior and both death and disease, and the ubiquity of sedentary behavior in our society, we should be very concerned about the health impact of sedentary behavior.

What is the take-home message?

There is a rapidly accumulating body of evidence which suggests that prolonged sitting is very bad for our health, even for lean and otherwise physically active individuals. The good news? Animal research suggests that simply walking at a leisurely pace may be enough to rapidly undo the metabolic damage associated with prolonged sitting, a finding which is supported by epidemiological work in humans. So, while there are a lot of questions that remain unanswered (e.g. Is there a “safe” amount of daily sedentary time?), the evidence seems clear that we should strive to limit the amount of time we spend sitting. And when we do have to sit for extended periods of time (which, let’s face it, is pretty much every single day for many of us) we should take short breaks whenever possible.
We all know that physical activity is important for good health—regardless of your age, gender or body weight, living an active lifestyle can improve your quality of life and dramatically reduce your risk of death and disease. But even if you are meeting current physical activity guidelines by exercising for one hour per day (something few Americans manage on a consistent basis), that leaves 15 to 16 hours per day when you are not being active. Does it matter how you spend those hours, which account for more than 90% of your day? For example, does it matter whether you spend those 16 hours sitting on your butt, versus standing or walking at a leisurely pace? Fortunately or unfortunately, new evidence suggests that it does matter, and in a big way.

What is sedentary behavior?

Before we go any further, it’s important that we define the term "sedentary behavior". Sedentary behavior is typically defined as any behavior with an exceedingly low energy expenditure (defined as <1.5 metabolic equivalents). In general, this means that almost any time you are sitting (e.g. working on a computer, watching TV, driving) or lying down, you are engaging in sedentary behavior. There are a few notable exceptions when you can be sitting or lying down but still expend high energy expenditure (e.g. riding a stationary bike), but in general if you are sitting down, you are being sedentary.

The above definition may seem rather intuitive, but this is not the way that the term sedentary has been used by exercise science researchers for the past 50 years. Up until very recently, referring to someone as sedentary meant simply that they were not meeting current guidelines for physical activity. In simple terms, if you were exercising for 60+ minutes/day, you were considered physically active. If you were exercising 10 minutes/day, you were sedentary. Case closed. But as we will discuss below, sedentary time is closely associated with health risk regardless of how much physical activity you perform on a daily basis. Further, it is entirely possible to meet current physical activity guidelines while still being incredibly sedentary. Thus, to quote researcher Marc Hamilton, sitting too much is not the same as exercising too little. (if you take only one thing from this post, let it be that quote from Dr Hamilton). Which is why it is so important that when we use the term "sedentary", we are all on the same page about what that means.

Now that we know what sedentary behavior is, let’s look at its relationship with health risk.

Epidemiological Evidence

In 2009 Dr Peter Katzmarzyk and colleagues at the Pennington Biomedical Research Center published an influential longitudinal paper examining the links between time spent sitting and mortality in a sample of more than 17,000 Canadians (available here). Not surprisingly, they report that time spent sitting was associated with increased risk of all-cause and cardiovascular disease mortality (there was no association between sitting and deaths due to cancer). But what is fascinating is that the relationship between sitting time and mortality was independent of physical activity levels. In fact, individuals who sat the most were roughly 50% more likely to die during the follow-up period than individuals who sat the least, even after controlling for age, smoking, and physical activity levels. Further analyses suggested that the relationship between sitting time and mortality was also independent of body weight. This suggests that all things being equal (body weight, physical activity levels, smoking, alcohol intake, age, and sex) the person who sits more is at a higher risk of death than the person who sits less.

The above findings linking excessive sitting with poor health are far from isolated. For example, a similar longitudinal study from Australia reports that each hour of daily television viewing (a proxy of sedentary time) is associated with an 11% increase in the risk of all-cause mortality regardless of age, sex, waist circumference, and physical activity level. And as my colleagues and I summarize in a recent review paper (PDF), numerous epidemiological studies have linked sedentary behavior with obesity, cardiometabolic risk, and even some cancers.

New evidence also suggests that in addition to the quantity of sedentary time, the quality of sedentary time may also have an important health impact. For example, Genevieve Healy and colleagues examined this issue in participants of the Australian Diabetes, Obesity and Lifestyle (AusDiab) Study. A total of 168 men and women aged 30-87 years wore an accelerometer (an objective measure of bodily movement) during all waking hours for 7 consecutive days, which allowed the researchers to quantify the amount of time that participants spent being sedentary, as well as how frequently they interrupted these sedentary activities (e.g. standing, walking to the washroom, etc).

What did they find?

The greater the number of breaks taken from sedentary behavior, the lower the waist circumference, body mass index, as well as blood lipids and glucose tolerance. This was true even if the total amount of sedentary time and physical activity time were equal between individuals—the one who took breaks more frequently during their time at the office or while watching television was less obese and had better metabolic health. Importantly, the breaks taken by the individuals in this study were of a brief duration (<5 min) and a low intensity (such as walking to the washroom, or simply standing).

Taken together, the epidemiological evidence strongly suggests that prolonged sitting is an important health risk factor. But what explains these relationships? Let’s now look at the multiple mechanisms linking sedentary time with increased health risk.

Mechanisms

Reduced Energy Expenditure

Quite obviously (and by definition), when you are sedentary, you are not being physically active. And so one common assumption is that people who sit more are at increased health risk simply because they are getting less physical activity. However, somewhat surprisingly, sitting time and physical activity do not appear to be related for most people. For example a paper from the European Youth Heart Study published in PLoS Medicine reports no association between physical activity and TV watching in a sample of nearly 2000 children and teenagers, and other reports suggest that there is little evidence that sedentary behavior displaces moderate or vigorous physical activity. So while it makes intuitive sense that being sedentary reduces energy expenditure, it is likely through the reduction of very light intensity physical activity (e.g. standing, walking at a slow pace), rather than by reducing the volume of what we typically think of as exercise. This may also help explain why the relationship between sedentary behavior and health risk are often independent of moderate or vigorous physical activity.

Increased Food Intake

In addition to reducing our energy expenditure, sedentary behaviors may also promote excess food intake. For example, a recently published study in the American Journal of Public Health suggests that the amount of commercial television (e.g. television with advertisements) that children watch before the age of 6 is associated with increased body weight 5 years down the road, even after adjustment for other important variables including physical activity, socio-economic status and mother’s BMI. In contrast, watching non-commercial television (DVD’s or TV programs without commercials) showed no association with body weight. Similarly, it has also been reported that each hour of daily television watching in children is associated with an increased consumption of 167 calories per day (PDF), mainly through increased consumption of high calorie, low nutrient foods (e.g. the foods most commonly advertised on television). Much of this is likely just a learned behavior—watching TV exposes us to food ads promoting unhealthy fare, which is likely to have a disproportionate influence on younger viewers. Just as importantly, people may just really enjoy munching on food while relaxing on the couch. Either way, excess sitting (and TV watching in particular) seems to put us in situations where we choose to eat more than we would otherwise.

Physiological Adaptations

I don’t think the mechanisms described above—that sitting too much may lead to reduced energy expenditure and increased food intake—will come as much of a surprise. But what I find truly fascinating is that sedentary behavior also results in rapid and dramatic changes in skeletal muscle. For example, in rat models, it has been shown that just 1 day of complete rest results in dramatic reductions in muscle triglyceride uptake, as well as reductions in HDL cholesterol (the good cholesterol). And in healthy human subjects, just 5 days of bed rest has been shown to result in increased plasma triglycerides and LDL cholesterol, as well as increased insulin resistance—all very bad things. And these weren’t small changes—triglyceride levels increased by 35%, and insulin resistance by 50%!

These negative changes are likely related to reductions in the activity of lipoprotein lipase, an enzyme which allows muscle to uptake fat, thereby reducing the amount of fat circulating in the blood (it also strongly influences cholesterol levels—the details can be found here). Animal research has shown that lipoprotein lipase activity is reduced dramatically after just six hours of sedentary behavior—not unlike a typical day at work or school for many individuals. Sedentary behavior may also reduce glucose transporter protein content in the muscle, making it more difficult for glucose to be taken into the muscle and resulting in higher blood sugar levels. What is most interesting to me personally is that these physiological changes in skeletal muscle have little or nothing to do with the accumulation of body fat, and occur under extremely rapid time-frames. This means that both lean and obese individuals, and even those with otherwise active lifestyles, are at increased health risk when they spend excessive amounts of time sitting down.

Should we be concerned about the health impact of sedentary behavior?

Yes.

Western society is built around sitting. We sit at work, we sit at school, we sit at home, and we sit in our cars as we commute back and forth. In fact, a recent survey reports that the average American accumulates more than 8 hours of sedentary behavior every day—roughly half of their waking hours. The situation in children is, unfortunately, no different. There is evidence that children in both Canada and the USA (PDF) accumulate more than 6 hours of screen-time (time spent in front of the TV, computer, or other screen-based device) on a daily basis. Keep in mind that screen-time is almost exclusively sedentary (active video games excluded), and that all these hours of sedentary behavior are in addition to the hours and hours (and hours) that kids spend sitting at school. In fact, a recent study reports that roughly 70% of class time, including physical education class, is completely sedentary (while slightly better than class time, children were also sedentary for the majority of lunch and recess).

In short, given the consistent links between sedentary behavior and both death and disease, and the ubiquity of sedentary behavior in our society, we should be very concerned about the health impact of sedentary behavior.

What is the take-home message?

There is a rapidly accumulating body of evidence which suggests that prolonged sitting is very bad for our health, even for lean and otherwise physically active individuals. The good news? Animal research suggests that simply walking at a leisurely pace may be enough to rapidly undo the metabolic damage associated with prolonged sitting, a finding which is supported by epidemiological work in humans. So, while there are a lot of questions that remain unanswered (e.g. Is there a “safe” amount of daily sedentary time?), the evidence seems clear that we should strive to limit the amount of time we spend sitting. And when we do have to sit for extended periods of time (which, let’s face it, is pretty much every single day for many of us) we should take short breaks whenever possible.
Read More