“You can’t outrun a bad diet.” This phrase became something of a mantra in recent years among some researchers and diet experts. When it comes to weight loss and weight gain, diet matters a heck of a lot more than exercise does.
This huge report from Vox is a comprehensive and user-friendly resource on the subject. The title? “Why you shouldn’t exercise to lose weight, explained with 60+ studies.”
A 2014 statement in the International Journal of Epidemiology, for example, put the case plainly: “increases in physical activity of the amount common for most individuals, such as 3 days/week of 1 h of aerobic activity, will not lead to weight loss, nor will it help prevent weight gain, for the majority of the population.”
And a much-discussed 2015 editorial in the British Medical Journal, authored in part by pioneering keto doctors Tim Noakes and Stephen Phinney, portrays widespread confusion on the issue as a public health crisis. The editorial alleges that public health messaging around diet and exercise “has been corrupted” by the food industry, leading many to “wrongly believe that obesity is entirely due to lack of exercise.” The consequences for the growth of the twin epidemics of obesity and Type 2 diabetes are obvious.
To be clear, that none of this means that exercise is without benefits. There’s absolutely no dispute among experts: even if it doesn’t really help you lose weight, exercise is marvelously beneficial. A 2017 article in Diabetes Spectrum , acknowledging this issue, suggested that diabetes experts should move away from describing exercise as a weight loss tool, and instead emphasize its “myriad of other health-related benefits.”
While we’ve known the basic facts for some time, the precise details of exactly how and why exercise seems so unrelated to weight are still somewhat of a mystery to experts. Luckily, recent studies have helped to flesh out our understanding of what’s going on in the body.
Modern Diet or Modern Lifestyle?
Last month, The Journal of Nutrition published a thought-provoking study of children living in Amazonian Ecuador. A team led by a Baylor University professor compared children that still lived a traditional Shuar rural lifestyle—in a village of 300, accessible only by canoe or on foot—with those of the same ethnicity that had moved into town. The rural kids had to forage for a high percentage of their food; the semi-urban kids had much greater access to store-bought packaged foods, not to mention running water, schools, and other amenities of modern civilization.
You won’t be surprised to learn that the urban Shuar kids were a lot heavier than their rural counterparts. The difference in average body fat percentage was immense: 20.6% compared to 12.5%. About a third of the urban children qualified as “overweight,” compared with precisely zero percent of the rural kiddos.
What is surprising is that the rural kids did not expend more energy than the urban ones. They were neither more active nor less sedentary. They were leaner, but it had nothing to do with how many calories they burned. Instead, it had everything to do with how many calories they consumed. The only material difference, researchers concluded, was the food that they ate.
The New York Times concluded that “how much children eat influences their body weight more than how much they move.”
A Weight Loss Threshold
There must, of course, be a point at which exercise does legitimately cause weight loss. Extreme examples should make this clear: it is the rare thru-hiker of the Appalachian Trail, for example, that does not lose weight after walking two thousand miles and climbing over hundreds of mountains, even while eating as much calorically dense food as they possibly can. Of course, it’s both impossible and undesirable for you and me to work out like that.
So where is the threshold at which exercise can actually begin to induce real weight loss? Another recent study tried to answer this question. The experiment, which also got a write-up in the New York Times, suggested that the body finds ways to compensate for the first 1,000 calories per week burned in exercise, either automatically (by adjusting metabolic rates) or by influencing behavior (by making you hungrier). That first 1,000 calories per week—roughly ten miles of jogging for an average adult—doesn’t seem to contribute to weight loss in the slightest.
It was above that 1,000 calorie threshold that researchers noticed a significant increase in leptin sensitivity, reducing hunger levels. At that point, participants stopped eating so much extra food to replace all of those calories that they had burned.
How much exercise did the researchers recommend to trigger real weight loss? Try 3,000 calories per week, about 30 miles of jogging or running, or 6 hours per week of committed exercise. No wonder exercise can seem so ineffective! It takes an extraordinary amount to make a noticeable difference on the bathroom scale.
For better or worse, the science is clear: if you want to lose weight, you probably should not rely on exercise as a significant part of your effort. Diet is by far the most important factor.