How should I split protein, carbs and fat?

No split is the right one for everyone. The International Society of Sports Nutrition states three things in order in its position stand on diet and body composition: energy balance decides your weight, protein is the macronutrient that matters most for body composition, and the ratio of carbs to fat after that is largely a question of preference and adherence.

Intervention studies point the same way. In POUNDS LOST, 811 adults were randomly assigned to four diets with different targets for fat, protein and carbohydrate. After two years the weight loss was roughly the same regardless of which macronutrient the diet emphasized. In DIETFITS, 609 adults were assigned to a healthy low-fat or a healthy low-carb diet, and the difference in weight loss after a year was not statistically significant (−5.3 vs −6.0 kg).

Protein is the part that actually carries a number. The ISSN protein position stand puts 1.4–2.0 grams per kilo of body weight per day as enough for most people who train, with room upward when eating in a deficit and wanting to keep muscle. The rest of the plate, how carbs and fat divide around that, can follow your type of training, your fullness and what you can hold over time. Someone training a lot often does well on more carbohydrate; someone else prefers less. Neither split is inherently more correct.

An app that shows three exact percentages makes it feel like the body has an optimal setting just waiting to be found. The number gives a sense of control, and control is soothing when you want to change something. At the same time, each diet camp has its own answer to sell: low-carb, high-carb and keto all market their particular split as the key, and a book or a program sells better with a clear verdict than with the message that it matters less than you think. That one split is hard to tell apart from another in trials makes a poor headline but an honest conclusion.

Set protein first. Aim for roughly 1.6 grams per kilo of body weight if you train and want to keep muscle. Then let carbs and fat fill the rest of your calories in whatever mix you enjoy and can hold over time. Try a split for a few weeks and change it if it does not fit your training or your fullness. Work out your grams in the macro calculator rather than chasing a perfect percentage.