The more you run, the less your diet affects your weight


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Another month, another neat paper from Paul Williams’ prolific National Runners’ Health Study, online at Medicine & Science in Sports & Exercise. With his sample of >100,000 runners, Williams took two well-established correlations — those who eat more meat tend to weigh more, as do those who eat less fruit — and checked to see whether these links between diet and weight were affected by how much exercise the subjects did.

The first thing to emphasize: don’t get hung up on whether or why meat and fruits are “good” or “bad.” That’s not what this study is about. It’s just a cross-sectional study, so individual dietary markers (like meat and fruit consumption) may simply be markers of other behaviours. But whatever the reason for these correlations, they’re well established. The question is: if you do lots of exercise, are you less likely to get fat from whatever dietary patterns are associated with high meat and low fruit intake?

The answer is yes:

Specifically, compared to running < 2 km/d, running >8 km/d reduced the apparent BMI increase per serving of meat by 43% in men and 55% in women, and reduced the apparent BMI reduction per serving of fruit by 86% in men and 94% in women.

Williams suggests two possible explanations for this effect:

  • Aerobic exercise trains your body to burn a higher percentage of fat rather than carbohydrate for fuel. In contrast, people who burn a lower percentage of fat compared to carbohydrate are thought to be at higher risk of gaining weight.
  • Exercise improves “coupling between energy intake and expenditure.” In effect, researchers have found that people who exercise more tend to develop stronger appetite cues that tell them when they’re hungry or full. There have been some neat studies of this, where subjects were fed “disguised” drinks that had either high or low energy content. As a later meal, regular exercisers unconsciously adjusted by eating more or less (depending on which drink they’d received) compared to sedentary people.

So why is this important? It goes back to the never-ending “diet versus exercise” debate for weight loss, a false dichotomy if there ever was one. Williams notes that a recent review of epidemiological studies looking for links between reported levels of physical activity and prospective weight gain concluded that they “generally failed” to show any links. This contrasts sharply with Williams’ own findings, which clearly show that “running attenuates age-related weight gain prospectively in proportion to the exercise dose, and that increasing and decreasing exercise produces reciprocal changes in body weight.” He speculates that the difference arises because running is relatively vigorous. It’s also easy to quantify compared to vague epidemiological studies that have subjects estimate how much time they spend playing soccer or mowing the lawn, at what subjective level of effort.

Whatever the mechanism, it’s a good reminder that exercise can play a role in weight control. It certainly doesn’t give you a free pass on your diet — but if you do enough, it seems to give you a little more wiggle room.

7 Replies to “The more you run, the less your diet affects your weight”

  1. “those who eat more meat tend to weigh more”
    “are you less likely to get fat”
    “reduced the apparent BMI”

    Did the study investigate fat loss, or did you inject that in there? BMI reveals nothing about body composition, and it would seem to me that higher meat intake could promote higher levels of muscle mass, and thus higher weight (and BMI).

    Of course running would decrease BMI increases to an extent, since endurance work would antagonize the mTOR anabolic pathway, and thereby prevent muscle mass gains.

    Why is BMI still used in studies? It provides no useful information.

  2. @David Csonka
    “Did the study investigate fat loss, or did you inject that in there?”

    The study used waist, hip and chest circumferences as proxies of fat. I’d be happy to send you the paper if you’re interested in reading more.

  3. Very interesting. Also wonder why the discrepancy between men and women, says the runners were people “who ran 5.30+/-3.23 km/d if male and 4.79+/-3.00 km/d if female”.

    Also while this is great, a 5k a day might seem like not a lot to people who run already but it’s pretty significant to society in general. And still doesn’t (I feel) get past whether it’s the jogging, or whether there is some already present dietary or genetic component that serves to affect both someone’s ability to run longer as well as BMI. As in maybe the same people who have a higher BMI due to some tertiary factor also find it difficult to get above a certain mileage. Still, would be interested to see this incorporated into further studies.

  4. @FrauTech
    Thanks for the comments. The discrepancy between men and women is because this was a survey study: people just reported how much they ran, they weren’t told to run a certain distance. It happens that the men in the study group were running marginally more than the women, but all things considered those two numbers are pretty close.

    And of course, I agree that running 5K a day isn’t something that everyone can just get out of bed and start doing tomorrow, particularly for people who are already obese. This study isn’t a recipe for weight loss, but it tells us something about what mechanisms play a role in determining stable weight. As Williams puts it in the paper: “We also caution that the runners may represent a unique group of individuals who may not be representative of the general population, however, we believe that the basic biological processes relating exercise, diet, and body weight are likely to be shared by all individuals.”

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