2 Macronutrients

The major nutrients

Macronutrients are nutrients the body needs in relatively large amounts. Carbohydrates, proteins, and fats provide energy, while water is essential but supplies no calories. Fiber is a type of carbohydrate that the body does not fully digest.

A varied pattern of nutrient-dense foods can include grains, fruits, vegetables, beans, milk, and other sources of these nutrients. Dietary reference values are useful guides, but they do not describe one-size-fits-all needs.

Carbohydrates and energy

Carbohydrates are molecules built from sugar units. Simple carbohydrates include sugars, while complex carbohydrates include starches, which are chains of sugar units. The body breaks down many carbohydrates into glucose, an important fuel for cells. Carbohydrates provide about $4$ kilocalories per gram.

Sources include grains, fruits, vegetables, beans, milk, and foods with added sugars. Nutrient-rich choices include whole grains, legumes, fruits, and vegetables. Dietary guidance emphasizes whole, nutrient-dense foods and reducing highly processed foods with refined carbohydrates and added sugars; frequent reliance on sugary drinks and sweets is best limited.

Proteins and amino acids

Proteins are chains of amino acids. The body uses amino acids to build and repair tissues and to make enzymes, hormones, antibodies, and other proteins. Some amino acids are essential amino acids: the body cannot make enough of them, so they must come from food. Protein provides about $4$ kilocalories per gram.

Sources include meat, fish, eggs, dairy products, beans, lentils, peas, soy foods, nuts, seeds, and some grains. A varied eating pattern can include both animal and plant protein sources.

For healthy adults, the Recommended Dietary Allowance for protein is $0.8$ grams per kilogram of body weight per day. Individual needs vary with age, growth, pregnancy, activity, and health.

Fats and their roles

Most food fat is a triglyceride, a molecule of glycerol attached to three fatty acids. Fat provides about $9$ kilocalories per gram. It helps form cell membranes, provides energy storage and insulation, supports absorption of vitamins A, D, E, and K, and supplies essential fatty acids that the body cannot make.

Fatty acids are commonly grouped as saturated, monounsaturated, or polyunsaturated according to their chemical structure. Oils, nuts, seeds, avocados, and fatty fish provide unsaturated fats; butter, fatty meats, and some tropical oils provide more saturated fat.

A practical choice is to replace foods high in saturated fat with sources of unsaturated fat, such as using plant oil instead of butter. Avoid industrial trans fats where possible.

Water and hydration

Water is a major component of the body. It is needed for chemical reactions, transport of nutrients and waste, temperature regulation, and maintaining blood volume. Water comes from drinking water, other beverages, and food.

Water needs vary with age, activity, climate, health, and other factors, so one fluid target does not suit everyone. The National Academies’ Adequate Intakes for total water, including water from foods and all beverages, are $3.7$ liters daily for adult men and $2.7$ liters for adult women. These reference values are not individual prescriptions.

Fiber and digestion

Dietary fiber is found in plant foods. Soluble fiber holds water and can form a gel during digestion; it occurs in foods such as oats, beans, lentils, and some fruits. Insoluble fiber adds bulk to stool and occurs in foods such as whole grains and vegetables. Fiber supports bowel function, and some types are fermented by gut microbes.

Increase fiber gradually and drink enough fluid to support digestive comfort. Useful sources include whole grains, vegetables, fruits, beans, peas, lentils, nuts, and seeds.

For adults, the National Academies’ Adequate Intake for fiber is $14$ grams per $1{,}000$ kilocalories. For example, this corresponds to $28$ grams at $2{,}000$ kilocalories. This reference is for generally healthy people; needs differ by age and life stage.

Using dietary reference ranges

For adults, the Acceptable Macronutrient Distribution Ranges (AMDRs) are:

• Carbohydrate: $45\%$–$65\%$ of calories.

• Protein: $10\%$–$35\%$ of calories.

• Fat: $20\%$–$35\%$ of calories.

These broad ranges help with planning a balanced diet. They are not a required ratio for every meal and do not replace individualized advice. Use reference values as guides while building dietary choices around varied, nutrient-dense foods.