Pushing Exercise to the Limit

Getting more exercise, maintaining a healthier diet, and losing weight are—along with saving money—the most common New Year’s resolutions that Americans made in January. So, here we are two months into 2021. How’s it going for you?  

We know that getting regular exercise and eating healthy foods are generally good for our mental and physical health. It’s also true that eating more calories than you burn leads to weight gain. But when it comes to weight loss, we’re still figuring out how the relationship between energy intake, metabolism, and output actually works in the human body.  

Researchers have been working with the Hadza, a modern Indigenous hunter-gatherer community in Tanzania, to understand the evolutionary roots of how we consume and expend energy. Hadza people are constantly on the move. Duke University anthropologist Herman Pontzer and his team have used GPS technology, heart rate monitors, and calorie measurement techniques to study how much energy they use. The team found that Hadza activity levels are equivalent to over two hours of moderate to vigorous exercise every day. But surprisingly, the number of calories they burn is about the same as adults in more sedentary Western societies—demonstrating that increasing activity isn’t a guarantee for weight loss. 

Pontzer has also studied ultra-endurance runners and found that while their metabolism speeds up at the start of a race, it settles to a more sustainable rate of 2.5x baseline over the course of a five-month race. Elite cyclists and Arctic trekkers? Same 2.5x baseline. Even during energy-intensive pregnancy and breastfeeding, humans hit that same limit. A possible explanation is that our digestive systems can physically only process so many calories.  

With a cap on how much total energy we can burn, there’s a limit to how much weight you can lose through exercise. The more likely benefit is that when you spend energy being active, your body has fewer calories to spend on potentially harmful functions like chronic inflammation. Chronic inflammation is a long-term process in which your immune system starts to attack healthy body tissues, and is associated with diseases like diabetes, heart disease, and arthritis. Along with exercise, a diet high in fruits, vegetables, whole grains, and healthy fats can help reduce inflammation (always consult your healthcare provider about any specific health issues). 

What does this mean as we try to keep our resolutions going for the rest of the year? Science suggests: don’t look at the scale for motivation to exercise. Instead, take the long view and think about exercise as a way to keep your body working better.