I once heard someone say, “I’m not worried about what I eat because I lift.” This statement feels emblematic of the widespread narrative that unhealthy eating can be effectively cancelled out with physical exercise and that exercise is the healthiest endeavor. Of course, exercise makes sense from the traditional calorie management perspective: the rationale being that if you have ingested excess calories, you need to burn them, or they will transform into fat. If you think of the unhealthy substances rampant in processed foods, however, claiming that exercise negates the effects of these potentially toxic compounds seems a bit ridiculous. After all, could you consume arsenic then go to the gym to cancel it out? To put it simply, the calorie approach is illogical and, as I learned during my research for this article, outdated. And it is no coincidence that the companies that promote the exercise narrative are the ones that produce highly processed foods and beverages.  

So where does this narrative originate? A seemingly obvious explanation for rising obesity rates and metabolic diseases is that, as societies develop and modernize, daily life becomes increasingly sedentary. People are less active so they burn less calories, which is why it is all the more important to exercise today, to provide the body with what it is now lacking. But this rationale has been contradicted by a recent study published in the Proceedings of the National Academy of Sciences journal: the amount of calories burned per day is actually very similar across populations and time. In other words, a hunter-gatherer and an American office worker burn a very similar amount of calories.

What this means is that the problem does not stem from a lack of exercise, but from our diet. The quality of the foods and beverages we ingest, and their quantity, are the culprits. These two dimensions (quality and quantity) are actually connected. As far as quantity is concerned, the more we eat a specific food, the more we crave it. And regarding quality, not all calories are created equal: the same amount of calories in vegetables and ultra-processed candy do not have the same effects on the body. After all, in hunter-gatherer times, nobody had access to bottles of Coca Cola with 65 grams of added sugar and caramel color alongside a slough of other harmful additives.

Now it’s time to share the discovery that surprised me the most. It turns out that we are controlled by a “second brain”: our gut bacteria. These bacteria make up our microbiome, a world teeming with activity that we host in our digestive tract. A balanced, harmonious microbiome, in which millions of varieties of “good” bacteria thrive, ensures physical health as well as mental health. These bacteria impact, among other things, our immune system, mood, energy levels, and of course, metabolism and weight. There are also “bad” bacteria living in our gut. Our job is to feed the “good” bacteria and to starve the “bad” bacteria. But this is not as easy a task as it seems. Our gut bacteria trigger cravings, especially the “bad” ones, such as those that feed on sugar and saturated fats. The more we eat what these bad bacteria crave, the more they grow, and eliminate the good ones. The good bacteria feed off of whole plants, and produce butyrate, a vital fatty acid that maintains the gut barrier, reduces inflammation, and supports immune health. So maintaining a healthy microbiome means eating a wide variety of fresh vegetables, fresh and dried fruit, legumes, and nuts. While exercise can enhance microbial diversity, it cannot supply the raw materials those bacteria need. You cannot lift weights to produce butyrate: you have to eat for it. It is not a coincidence that the most effective and popular weight loss medications today, such Ozempic, Mounjaro, and Wegovy, are GLP-1 receptor antagonists which rebalance the microbiome.

Despite our ever increasing understanding of the importance of metabolism, many people continue to believe that exercise matters more than  diet. It turns out that Big Soda has played a major role in funding research and messaging designed to shift the obesity debate away from dietary causes and toward a focus on physical activity. A study by the University of California, San Francisco reviewed 60 studies exploring the relationship between sugary drinks and obesity and diabetes. The researchers found that 34 did report a link. Twenty six, all of which were funded by the beverage industry, did not. Between 2011 and 2015, PepsiCo and Coca-Cola funded 96 national health organizations, including the American Diabetes Association and the American Red Cross, according to a Boston University School of Medicine study. Coca-Cola also extensively supported and funded a nonprofit known as the Global Energy Balance Network, which argued that Americans focus too heavily on food and beverage consumption while failing to prioritize physical activity.

Through aggressive influence campaigns, these corporations push their messaging all the way up to the federal government. In 2011, for example, Congress declared pizza a vegetable, as a result of intensive lobbying. In 2009, Coca-Cola and PepsiCo poured tens of millions of dollars lobbying when Congress was considering a soda tax.

I am not trying to downplay the importance of exercise. As an avid runner, I recognize how incredibly valuable it is. However, the idea that exercise can cancel out what you eat can also be comforting, and thus it is particularly convenient for corporations to promote for their own gain. The microbiome makes clear that diet and exercise are not interchangeable inputs. Physical activity can enhance metabolic health, but it cannot substitute for the nutrients that shape the gut environment in the first place.