Protein, carbs and fat: what each one actually does

How many calories are in a gram of each macro?

A gram of protein or carbohydrate carries about 4 calories, and a gram of fat about 9. Nutrition labels in the US may calculate calories using those general factors for protein, total carbohydrate and total fat (21 CFR 101.9). Fat is described as energy dense because a gram of it carries more than twice the energy of a gram of protein or carbohydrate.

What does protein do?

Protein supplies the amino acids the body uses to repair cells and make new ones. Digestion breaks the protein in food back down into amino acids. The National Library of Medicine explains that you need protein “to help your body repair cells and make new ones” (MedlinePlus Medical Encyclopedia). Muscle is one of the tissues made and remade that way.

How much protein helps depends on the situation. A meta-analysis in Advances in Nutrition (2020) found that eating more than the Recommended Dietary Allowance limited lean mass loss during calorie restriction by about 0.36 kg on average. Extra protein made no difference to lean mass when people were neither restricting calories nor resistance training, where the allowance was adequate. With resistance training, a meta-analysis in the British Journal of Sports Medicine (2018) found protein supplements increased gains in fat-free mass. The gain stopped growing beyond a total intake of about 1.6 g per kilogram of body weight a day.

Reviewing weight loss in Advances in Nutrition (2017), Cava and colleagues conclude that a high protein intake helps preserve lean mass. It does not improve muscle strength, though, and could have adverse metabolic effects. The reviewers recommend adequate but not excessive protein alongside exercise. Protein also generally increases satiety more than carbohydrate or fat (The American Journal of Clinical Nutrition, 2008), which can make a given intake easier to keep to.

What does carbohydrate do?

Carbohydrate mostly becomes glucose, the body’s everyday fuel. It comes in three main forms: sugars, starches and fiber. The body breaks sugars and most starches down into glucose, which circulates in the blood to be used for energy (MedlinePlus Medical Encyclopedia).

Glucose that is not needed straight away can be stored as glycogen, mainly in the liver and skeletal muscle. Liver glycogen helps keep blood glucose steady, especially when you have not eaten for a while. Muscle holds the largest total store and uses it to power contraction (StatPearls Publishing). Muscle glycogen is the reason carbohydrate comes up so often in discussions of training.

Neither end of the carbohydrate range is automatically right. How much suits a person depends on their activity, health, preferences and any medical advice they have been given.

Is fiber a carbohydrate?

Fiber is counted as carbohydrate but is not broken down for energy the way sugars and starches are. Fiber adds bulk, can help you feel full sooner, and aids digestion. MedlinePlus puts the recommendation for older children, adolescents and adults at 21 to 38 grams a day. It notes that Americans eat about 16 grams on average (MedlinePlus Medical Encyclopedia).

Two days with the same carbohydrate total can therefore be quite different: one built on beans, whole grains and vegetables, and one built mostly on refined starches and sugar.

What does fat do in the body?

Dietary fat supplies essential fatty acids, helps the body absorb some vitamins and builds cell membranes. Omega-3 and omega-6 fats are essential fatty acids that the body does not make, so they have to come from food (MedlinePlus Medical Encyclopedia). Vitamins A, D, E and K are “absorbed more easily by the body in the presence of dietary fat” (MedlinePlus Medical Encyclopedia).

Lipids are an essential part of every cell membrane, and cholesterol is the starting material for steroid hormones such as estrogen, testosterone and cortisol (StatPearls Publishing).

How should you read a macro split?

A split such as 40/30/30 describes proportions of energy. It is a planning choice. The body does not follow the percentages an app displays.

For adults, the Institute of Medicine sets acceptable ranges rather than one ideal figure (Institute of Medicine):

In The New England Journal of Medicine (2009), 811 adults with overweight were assigned to one of four reduced-calorie diets for two years. The diets had different shares of fat, protein and carbohydrate. Weight loss was similar across them, and attendance at group sessions was strongly associated with how much people lost. The authors concluded that reduced-calorie diets result in meaningful weight loss regardless of which macronutrients they emphasize.

Macros also say nothing about vitamins and minerals. A day can hit every macro target and still be low in several micronutrients.

What nutrition research says about macros