The Weight Loss Scientist: You've Been LIED To About Calories, Dieting & Losing Weight: Giles Yeo

When adults lose even a small amount of weight, the brain perceives a threat to survival and deploys automatic mechanisms to restore the prior weight. Hunger increases and metabolism quietly drops so that even eating the same foods leads to more storage. This is why weight usually comes back after stopping a diet—keto or otherwise—and why regaining (often with a bit extra) is the norm, not a personal failure.

A calorie only tells you how much energy is in food, not its quality or how your body will process it. Protein calories are effectively ~30% 'overstated' because so much energy is spent digesting and metabolizing them, and they keep you fuller longer. Yeo’s practical template for any diet: about 16% of daily energy from protein, at least 30 g of fiber, and no more than 5% of energy from added sugars. Apply these ratios to any eating style (keto, Mediterranean, flexitarian) rather than obsessing over raw calorie totals.

Caloric availability—the energy you can extract from a food—depends heavily on processing and cooking. The same starting ingredient can yield very different usable calories: raw celery (~6 kcal per stick) can become ~30 kcal when stewed; whole sweetcorn passes largely intact while cornmeal tortillas from the same corn deliver much more energy. Two foods both labeled “100 calories” can result in 50 vs. 80 usable calories, undermining precise calorie-counting and highlighting why whole, less processed foods are generally more forgiving.

Body weight is influenced by hundreds to thousands of genes, including key pathways like leptin and MC4R that regulate hunger and fat sensing in the brain. Some people are biologically set to defend a higher 'weight range' than others and find it far harder to say no to food. About 0.3% of people carry MC4R mutations that make them, on average, ~18 kg heavier by age 18. Families often share body types because they share both genes and environments; willpower alone cannot fully override these biological set points.

For most people, exercise is a poor primary strategy for losing weight. We typically don’t do enough to outweigh compensatory hunger and the psychological sense of having 'earned' more food. However, once weight has been lost through dietary change, regular activity—especially resistance training—is extremely valuable for keeping it off and protecting health, particularly by preserving muscle mass as we age.