Having trouble losing weight with exercise? Evolution may be to blame!

Do you feel frustrated because your workouts are not leading to your desired weight loss? This seems to be a common problem among many exercisers. I often hear students complaining that though they burn over 1,000 calories in a single stair master session, they are still not losing weight. Why would working out not promote weight loss?

An interesting report in a recent issue of Scientific American shed some light on this question. This article entitled, The Exercise Paradox, was written by Dr. Herman Pontzer, an anthropologist at Hunter College (https://www.scientificamerican.com/article/the-exercise-paradox/). The paradox is this: why does burning calories during exercise not directly translate into weight loss?

The answer has to do with evolution.

Dr. Pontzer spent many years following the Hadza (or Hadzabe) people of northern Tanzania. Unlike other industrialized societies, these people spend their time as hunter-gatherers. During the day, the men travel far distances to hunt for animals while the women and children stay close to home to gather foods such as berries and tubers. At the end of their day, the men and women share their bounty.

In this non-agricultural society, physical activity levels are high. Therefore, Dr. Pontzer and colleagues hypothesized that daily caloric burn would be significantly higher in this population than in the average, sedentary American populace.

While the Hadza hunted and gathered, the scientists gave them special water to drink, which was enriched with two rare isotopes, deuterium and oxygen 18. They then collected their urine samples and used a process known as the doubly labeled water method to determine the participant’s daily energy expenditure or the amount of calories burned per day. “[The motivation] for measuring Hadza metabolism,” Dr. Pontzer notes, “was to determine…just how deficient we Westerners were in our daily energy expenditure.”

But, the data showed something very surprising. The average energy expenditure of the Hadza were equivalent to the average American (and European). Their overall conclusion was that, “humans tend to burn the same number of calories regardless of how physically active they are”.

Though at first, this finding seems confusing, it really does make a great deal of sense from an evolutionary perspective. Our hunter-gatherer ancestors were required to be physically active in order to obtain food. The problem, though, was that sometimes, they returned without a meal. Our bodies, therefore, developed the means to limit or constrain our metabolism so that we would not starve.

The big question is, how does the body do this? Scientists are still discovering the answer, but they think that what might be happening has to do with the inner workings of our cells. The “housekeeping work” that our cells and organs participate in to keep our body functioning properly requires energy. Scientists think that by increasing physical activity levels, our cells and organs have to do less work (or expend less energy) to keep us going. For example, exercise decreases levels of inflammation in the body. Therefore, the immune system of a physically active person may be less active (and burning fewer calories) than the immune system of a sedentary person.

Of course, this is not to say stop exercising because you won’t lose weight. Exercise is good for the body in a huge number of ways. And exercise can, of course, contribute to weight loss when combined with a healthy, well-rounded diet full of fruits, vegetables, whole grains, proteins, and healthy fats (like those from avocados, nuts, and fish). Building muscle also helps to increase your metabolism and therefore burn more calories. This research is interesting, however, as it reveals that evolution may be to blame for why the number on the scale refuses to go down despite your exercise efforts.

References:

Pontzer, H. (2017). The Exercise Paradox. Scientific American, 316(2), 26-31.