The New York Times’ Gretchen Reynolds reports on two new studies about how exercise affects food intake. There has already been a somewhat similar study in rats; these ones are in humans. Both studies come to the conclusion that exercise will generally result in a lower overall food intake. (The rat study measured actual food eaten as well as body weight; the human studies measured how appealing food was on an MRI, and one also measured change in body weight.) However, the study that measured change in body weight, which was conducted on people with a BMI >30, found that some people, the “non-responders”, had the opposite response. Based on their MRIs, high-energy foods became more appealing to them after they exercised, and they lost little or no weight.
Now, in this study, the subjects were doing exercise meant to expend 500 calories, 5 days a week. For reference, I plugged “1 hour of walking 3 miles/hour by a 200 lb person” into this calculator, and the number it came up with was 317 calories; running 5 mph [12 minute miles] for 40 minutes was 483; so they were doing a lot of exercise, and they were doing it for 12 weeks, which seems to me like a significant time period. I seem to remember that for some types of metabolic indicators, there will be measurable differences after just one session. (The people in the first study, if they weigh ~150 lbs, doing 1 hour of vigorous cycling, would have burned 682 calories according to that same site–assuming they were doing vigorous cycling for the entire hour.)
So how does the following conclusion make any kind of sense?
Those 14, dubbed nonresponders, also had displayed the highest brain responses to food cues following exercise when the study began. After three months, they retained that undesirable lead. Their food-reward networks lit up riotously after exercise at the sight of food, and in fact showed more enthusiasm now than at the start of the study.
The responders’ brains, in contrast, responded with a relative ‘meh’ to food pictures after exercise.
What all of this suggests, Dr. Hagobian of Cal-Poly says, is that “exercise has a definite impact on food reward regions. But that impact may depend” on who you are and what kind of exercise you do.
His group of fit young people, he points out, completed prolonged, strenuous endurance sessions. “It’s likely that, in order to achieve weight loss and weight maintenance, you need to do a fair amount of exercise and do it often,” he says.
For exercise noticeably to dampen your desire for food, in other words, you may need to sweat for an hour. It may also help if you’re already lean and in shape.
The quote is from the author of first paper, not the one that found non-responders, so it’s possible that he’s just not familiar with, or barely familiar with, the second paper, in which case his strange interpretation may have more to do with Gretchen Reynolds than Dr. Hagobian. (Maybe that quote was not really responding to the second study, or maybe she gave him a summary that did not include just how much exercise the subjects were doing?) Or maybe he just doesn’t think that 40 minutes straight of vigorous exercise 5x a week is “do[ing] a fair amount of exercise and do[ing] it often”. Reynold’s following paragraph (“you may need to sweat for an hour”) is at least more specific about the time period (40 minutes of running is “a fair amount of exercise” IMO, but it’s not an hour). But their responses are written as though the results were just, “thin people’s appetite goes down; fat people’s appetite goes up”, when only a minority of the people in the >30 BMI study were non-responders–which makes the “maybe you need to be lean” part make less sense. They seem to be assuming that more exercise will work because, well, exercise HAS to work. ELMM!!!!11
Further “exercise must work”:
But Dr. Hagobian is optimistic that research might help almost everyone to better deploy exercise against appetite control. “There may be doses or types of exercise that are more effective for some people than for others.” Eventually, brain research may help to point people to the exercise program best suited to them.
In the meantime, he says, don’t take to the couch, even if exercise makes you ravenous. “Being fit can have psychological effects,” he says, perhaps increasing your desire to consume a better diet and, in the long term, shed pounds.
Well, at least he said “may”. The second paragraph–I thought they were going to say, “exercise is good for you even if your weight doesn’t change”. But they disappointed me by not even mentioning that, instead solely connecting it back to weight. AND doing it in a way that was not borne out by the study the article is talking about. Exercise will make you want to lose weight, even though 12 weeks of exercise resulted in little or no change in the non-responders’ weight! That ‘perhaps’ is the mark of an evidence-free assertion. (I know of at least one study–of people training for a marathon, hardly casual exercisers–showing no weight change for most subjects. And that study wasn’t limited to “non-responders”. [Warning; link talks about weight loss & eating rules.])
I notice that Dr. Hagobian is the only study author quoted. I wonder if it was because the authors of the 2nd study just didn’t want to talk to the press, or if their quotes didn’t conform to the simplistic ELMM narrative, so their quotes weren’t as quotable?