Overview

The Guardian’s Science Weekly sits down with Kevin Hall, a prominent researcher at the US National Institutes of Health, to unpack what science really says about diet, body weight, and public health. The discussion centers on why weight loss is not simply a matter of counting calories, how the body adapts its metabolism and appetite during dieting, and why exercise, while beneficial for health, does not always translate into large, immediate weight loss. Hall emphasizes two intertwined negative feedback loops: a reduction in energy expenditure and an increase in appetite that can undermine dieting efforts, leading to weight plateaus and regain unless lifestyle changes are persistent and environment‑aligned.

"the calories in, calories out idea ... misses the physiological adaptations in both the number of calories the body burns through metabolism, as well as how our appetite changes as we gain or lose weight." - Kevin Hall, NIH

Calories, metabolism and appetite

The conversation delves into how caloric intake and expenditure are not static. Hall explains that over days and weeks, caloric burn can fall more than expected with weight loss, and appetite can rise as people lose weight, making adherence to strict diets challenging. The discussion also covers how weight regulation is defended by the body and how a persistent lifestyle shift is often required to achieve lasting change. The role of genetics is highlighted, with many obesity‑related genes acting in the brain to influence food intake, suggesting biology plays a substantial role alongside environment.

"Calories in, calories out" is a useful heuristic but incomplete, because metabolism and appetite adapt during weight change, complicating simple energy balances.

The episode also touches on physical activity, noting that exercise’s real value lies in metabolic health and maintenance of weight loss, rather than guaranteeing substantial short‑term pounds shed. Hall cautions against stories that equate gym workouts with rapid, durable weight loss, while underscoring exercise’s broader health benefits.

"where exercise has its sort of superstar qualities when it comes to weight loss is not in the weight loss itself, but the ability to maintain the weight loss over long periods of time." - Kevin Hall, NIH

Ultra‑processed foods, policy and science integrity

The talk turns to ultra‑processed foods, energy density, and the ways highly engineered foods may spur overeating. Hall describes how reducing water content in foods concentrates calories and discusses potential effects of hyperpalatable combinations on intake. He cites ongoing, nuanced findings that suggest the relationship between ultra‑processed foods and obesity is not black‑and‑white, but is shaped by energy density and nutrient interactions. The conversation also covers potential policy levers, such as regulating energy density and product formulations that promote excessive intake.

"these sorts of what they count as maha wins are not likely to move the needle on public health when it comes to diet‑related chronic diseases." - Kevin Hall, NIH

Scientific integrity, censorship and policy impact

In a candid conversation about politics and science, Hall recounts a turning point in his career, describing censorship attempts around a study on ultra‑processed foods and brain responses. He explains that he was advised to downplay a major finding and eventually chose early retirement rather than continue under conditions that compromised scientific reporting. The discussion broadens to critique current policy actions, arguing that rhetoric aired by the administration does not translate into meaningful public‑health gains, and that constructive reforms require more than symbolic changes.

"they are very interested in downplaying science that seems to not support their narrative in much the same way a trial lawyer might." - Kevin Hall, NIH

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