Diet myths put to the test, from muscle-building surpluses to intermittent fasting

Diet advice is crowded with myths. The global weight management market is estimated at $428 billion, and a Google Scholar search for "diets health" returns over 4 million results. This narrative review, part of a series from sports nutrition researchers, sets out to answer 16 frequently asked questions, from calorie surpluses and fasting to carnivore diets, superfoods, protein and kidney health, and diets for longevity. The text available here covers the first four questions in detail.

On building muscle, the evidence is mixed and depends on training status. In elite athletes (Garthe), a roughly 544-kcal increase brought lean mass gains of 1.7 kg versus 1.2 kg for controls, but fat gain was 1.1 kg versus 0.2 kg. In an 8-week trial (Helms) of 0%, 5% and 15% surpluses, biceps thickness rose only 0.2-0.3 cm in all groups, though participants ate well beyond their assigned intakes and training volume had dropped. In elite bodybuilders (Ribeiro), a 6087-kcal diet produced 1.0 kg of lean gain versus 0.4 kg on 4501 kcal, with a far larger rise in fat (7.4% versus 0.8%). Resistance-trained people who added about 400-800 kcal of protein (3.3-4.4 g/kg/day) did not gain fat. The authors suggest a 10-20% surplus (about 250-500 kcal), protein-heavy, for men at or below the mid-teens in body fat percentage and women at or below the mid-20s. Novices can often recompose without a surplus.

Weight loss without explicit calorie restriction is also possible. In a 12-week trial in obese women (Pescari), low-carbohydrate, ketogenic and high-protein diets produced losses of 3.01, 2.38 and 2.88 kg, more than the calorie-restricted comparison groups. A meta-analysis of 24 RCTs (1,063 participants; Wycherley) found that higher-protein diets cut weight by 0.79 kg and fat mass by 0.87 kg and preserved 0.43 kg of fat-free mass. Skov reported 8.9 kg lost on an ad libitum high-protein, low-carbohydrate diet versus 5.1 kg on a high-carbohydrate diet. Low-fat, high-fiber diets also reduced weight by 2.6 kg and fat by 2.4 kg (Saris). The proposed mechanisms are higher GLP-1 and PYY, lower ghrelin, greater thermogenesis and lower energy density.

For intermittent fasting (IF) versus chronic caloric restriction (CCR), IF is defined by consensus as repeated fasts of 48 hours or less, including alternate-day fasting and time-restricted eating. A meta-analysis found about 0.94 kg more weight loss with IF at 6 months, a modest difference that narrows beyond six months. An umbrella review linked IF to larger reductions in waist circumference and fat mass, and short-term trials showed greater improvements in fasting insulin and HOMA-IR. IF was less effective than CCR for systolic blood pressure. Adherence is highly individual.

On sex differences, women showed higher free fatty acid appearance after about 12 hours of fasting, with greater nonoxidative disposal. After a 48-hour fast, women showed higher insulin responses to a mixed meal with similar glucose, and one 22-day alternate-day fasting trial showed worsened glucose clearance in women. Still, trials have not clearly supported different long-term effects by sex, so the authors do not advise favoring or avoiding IF based on sex alone. Most trials in exercising populations enrolled mainly men.

Overall, the review argues energy balance still governs body weight, but macronutrient composition influences appetite and body composition. Key caveats are short trial durations, small samples and the narrative design. The text provided ends partway through the fourth question.