Diet vs. exercise: Which should you focus on for weight loss? By Lainey Younkin, RD

Are you diligently exercising but seeing no results around your midsection? Good news: It’s not just you. Two new studies explain why many people who begin exercise programs often lose little to no weight.

In the first study published in PLoS, researchers from the U.S. and U.K. compared total daily energy expenditure (TEE) – how many calories are burned each day – between Westerners and the Hadza, a population of hunter-gatherers living in Northern Tanzania.

The Hadza hunt and forage for food without modern tools (e.g. vehicles or guns). According to the study, their diet consists mainly of “tubers, berries, small- and large-game, baobab fruit, and honey.”

Many believe that modern Western lifestyles lead to decreased energy expenditure, as people, in general, burn fewer calories than in the past because their lives have become more sedentary.

Researchers were interested in determining the difference, if any, in daily energy expenditure between the Hadza and Westerners. They measured the TEE of 30 Hadza adults over an 11-day period using the doubly labeled water (DLW) method and also calculated their physical activity levels (PALs). Then they compared the TEE of the Hadza to the TEE of individuals in Western countries (U.S. and Europe) by gathering TEE data from previous DLW studies and taking new measurements from 68 U.S. adults. They also gathered comparative data from non-Western market economies such as Siberia and from a farming population in Bolivia. There were 221 subjects from comparative datasets.

Not surprisingly, the Hadza were active, lean, and had lower body fat percentages than the average Westerner. They were also found to have greater PALs (calculated as TEE/estimated basal metabolic rate). Men walked about seven miles each day, while women walked an average of 3.5 miles. The researchers attributed the greater PAL to the differences in body size, as the Hadza are smaller in general than individuals in the West.

What was surprising was that the researchers found little difference in TEE  (calories burned) between the Hadza and their Western counterparts. Though the Hadza seem to be more active, both groups expended about the same amount of energy each day.

This means that minutes of physical activity per day may have little to no effect on the number of calories burned. So what does?

An important part of the equation is missing: energy intake. Since energy expenditure did not significantly differ between the Hadza and Westerners, the researchers concluded that “differences in obesity prevalence between populations result primarily from differences in energy intake rather than expenditure.” In other words, “active ‘traditional’ lifestyles may not protect against obesity if diets change to promote increased caloric consumption.”

Diets have indeed changed, especially in the West. It’s long been known though that in order to lose weight, you have to burn more calories than you consume. But why do so many people have trouble burning calories and keeping the weight away? The second study, published in Obesity Reviews, was a review on the effect of exercise interventions on body composition that revealed why people lose little weight when enrolled in exercise programs.

Contrary to popular belief, the researchers found that when people exercise and keep their energy intake (i.e. number of calories) constant, their resting metabolic rate (i.e. metabolism) is actually reduced. This was especially true of individuals who had a lower amount of lean body mass, which is the total mass of the body minus fat.

However, the researchers did find that aerobic exercise (e.g. running, cycling, swimming) caused people to lose more fat mass than lean body mass when keeping calorie intake constant. In addition, when people increased the number of calories they ate during aerobic training, they gained more lean body mass and less fat mass than previously predicted. But if you increase your calories by too much, it negates the effects of your hard work.

Therefore, the reason people lose less weight than expected when exercising is two-fold, according to the scientists. First, exercise programs may not lead to as much calorie-burn as you would think. Second, many people concomitantly increase their calories when exercising and may sometimes increase them too much. This could cause you to either gain weight while exercising or not lose as much weight as expected.

BOTTOM LINE:

If you enroll in an exercise program or start a form of aerobic training to lose weight, you won’t succeed with the mentality “I can eat anything because I’ll burn it off later.” You will have better weight loss results if you choose a healthy diet of whole grains, fruits and vegetables, lean proteins, and healthy fats while exercising. And, though these two studies show that diet is more important than exercise for weight loss, don’t discount the other benefits of exercise, including decreased stress and anxiety, improved mood, and reduced risk of cardiovascular disease, diabetes, and some cancers.

Lainey Younkin, RD is starting her second year as a Nutrition Communication student at Friedman. She looks forward to learning more about diet and exercise and hopes to help people find a proper balance between the two.

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