Obesity and Inactivity: The Evolutionary Perspective

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AICR’s expert report concluded that carrying excess body fat is a convincing cause of six different cancers (colorectal, postmenopausal breast, esophageal, endometrial, kidney and pancreatic) and a probable cause of gallbladder cancer as well.  That means that in the US alone, obesity is responsible for over 100,000 cancer cases every year.

And as obesity figures continue to rise, that number is likely to grow even larger.

How did we get here?

Last week, at the 3rd International Congress on Physical Activity and Public Health, many researchers presented data tracking recent trends in the amount of leisure-time physical activity we’re getting, nowadays. But one of the keynote speakers, Dr. William Leonard of Northwestern University, presented an intriguing talk that took the long view.

The very long view.

Like, hundreds of thousands of years long.

Dr. Leonard is an  anthropologist, you see.  He talked about how humans evolved, specifically how how changes in our diet and activity level changed our body type.

Basically, said Dr. Leonard, as our brains got bigger, they placed larger demands on our metabolism, and our diets became more nutrient- and calorie-dense to support them.

Leonard proposed that the recent and much-talked about uptick in the calorie-density of our foods over the past few decades (higher fat content, larger portion sizes) is simply an extension of what’s been happening to us, on an evolutionary scale, for millions of years.  But the difference is key: dietary changes that used to to take thousands and thousands of years to occur have happened within a single lifetime.

Even so, he suggested that we might be missing the real story by focusing so much on the increase in calories in our diets.  In fact, he notes, while calorie content of the diet in the developed world has increased since the fifties, that increase leveled off in the 80s.  Yet obesity rates continued, and continue, to rise.

To explain this, he suggests that it’s decreased calorie expenditure that plays a larger role in obesity than caloric intake.

Throughout our evolution, our caloric intake increased to match greater and greater needs we placed upon our bodies – hunting calorie-dense animals is more demanding than gathering low-calorie-dense crops. But this eon-old trend toward increased calorie-burning is now experiencing a dramatic reversal: Our jobs have become much more sedentary since the 80s and the advent of the computer.

Our bodies have evolved over millions of years to actively and effectively meet our caloric needs from the environment. But now we’re suddenly accelerating the trend toward more calories – and by changing the physical environment to make things easier, we’re reversing the evolutionary trend toward burning more calories.

The net effect: we’re upsetting an equilibrium our species has managed to maintain for thousands and thousands of years.

The Fitness Factor

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A lot of this issue’s Cancer Research Update focuses on the benefits of physical activity for cancer prevention, which is a relatively new area of study.

It’s clear that being physically active helps us lose and/or control weight. Weight control is the obvious reason to be active, now that research shows excess body fat causes seven different cancers.

Less is known about exactly how physical activity helps fight cancer independent of weight.  Studies show that it does, but can physical activity reduce cancer risk regardless of the number on the scale?

To learn more, read the full story in CRU.

And if you want a mental health boost as well as a physical one, try exercising outside. A new study in Environmental Science & Technology found that just five minutes of “green exercise” per day improves mood and self -esteem. Green exercise is any activity that takes place outdoors, such as a backyard garden or walk in a park.

With warmer weather, there are precious few excuses not to add some fun activity into your day. AICR has plenty of “Moving More” strategies here. If you have a fitness tip of your own, please share.

Be Active, Get Smarter, Reduce Your Cancer Risk

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The reasons to make physical activity part of a daily routine just keep building. For one thing: there’s the evidence linking physical activity to reduced cancer risk. The latest incentive to get active comes from a new study that found exercise speeds learning and improves blood flow to the brain in monkeys.

Previous studies have linked improved learning to exercise in rodents; but this study examines this link in monkeys. The study is published in the journal Neuroscience; you can read the release here.

Physical activity can reduce cancer risk; can it make us learn better?

In the study, one group of monkeys was aerobically active – running on a treadmill for an hour each day, five days per week, for five months. Another group simply sat on the treadmill for the same amount of time.

Cognitive tests found that the exercising monkeys learned one task twice as quickly as the sedentary animals.

When it comes to exercise and cancer prevention, the link between physical activity and reducing cancer risk is clear. Regular activity acts with weight control – and excess body fat causes several different cancers – and also appears to have biological effects that lower cancer risk, such as strengthening the immune system.

Want to see if you are active enough? Take our quiz.

If you already incorporate physical activity into your day, how did you get into the habit? Any tips?