2 Macronutrients
Learn how carbohydrates, fats, and proteins provide energy and support body functions, how they are digested, and how to consider their food sources.
and energy
are nutrients needed in relatively large amounts. Carbohydrates, fats, and proteins each provide energy and also have specialized roles in the body.
On average, digestible carbohydrate and protein provide about , while fat provides about . These values describe food energy; they do not indicate a food’s overall nutritional quality.
Carbohydrates and
Carbohydrates are molecules made chiefly of carbon, hydrogen, and oxygen. They include sugars, starches, and dietary . Sugars contain one or two sugar units: and fructose are single sugars, while sucrose and lactose are double sugars. Starches are long chains of sugar units found in foods such as grains, potatoes, and beans.
The body breaks down digestible carbohydrates mainly into simple sugars, especially , which cells use for energy. can also be stored as glycogen in the liver and muscles. Carbohydrate-containing foods include grains, fruits, vegetables, legumes, milk, and foods with added sugars.
and digestion
is a group of carbohydrates that human digestive enzymes do not fully break down. Some passes through the digestive tract largely intact; some is fermented by microbes in the large intestine, producing compounds the body can use. contributes to stool bulk and regularity, and -rich foods can support a healthful eating pattern.
Sources of include beans and lentils, vegetables, fruits, nuts, seeds, and whole grains. Gradually increasing and drinking enough fluids can help reduce digestive discomfort.
Carbohydrate digestion begins with salivary amylase in the mouth. It continues in the small intestine with pancreatic amylase and enzymes on the intestinal surface, which break starches and sugars into absorbable simple sugars. resists this digestion and continues into the large intestine.
Fats and their digestion
Most dietary fat is in the form of triglycerides, each consisting of a glycerol backbone attached to three fatty acids. Fatty acids may be saturated, meaning they have no carbon–carbon double bonds, or unsaturated, meaning they have one or more. Monounsaturated fats have one double bond; polyunsaturated fats have more than one. Foods contain mixtures of fatty acids, although some sources are richer in particular types.
Fat supplies concentrated energy, forms part of cell membranes, helps protect organs, and supports absorption of the fat-soluble vitamins A, D, E, and K. Some fatty acids are essential because the body cannot make them in sufficient amounts.
Vegetable oils, nuts, seeds, avocados, and many fish are sources of . Saturated fats occur in varying amounts in butter, some dairy products, fatty meats, and certain tropical oils. Trans fats can occur in some foods, although the former major industrial source—partially hydrogenated oils—has been removed from the U.S. food supply.
Fat digestion and absorption
A small amount of fat digestion occurs earlier in the digestive tract, but most takes place in the small intestine. , made by the liver and released into the intestine, disperses fat into small droplets; it is not a digestive enzyme. then breaks triglycerides into smaller molecules that can be absorbed.
Many long-chain fatty acids are repackaged inside intestinal cells and transported first through the lymph and then into the bloodstream.
Proteins and their digestion
Proteins are chains of joined by peptide bonds and folded into specific shapes. Their structure enables them to form and maintain tissues, act as enzymes, transport substances, help regulate body processes, and contribute to immune defenses. The body can also use for energy, but protein’s primary dietary value is not simply its calorie content.
Food proteins are digested into and small peptides. The body uses these building blocks to make its own proteins. Some are essential, meaning they must be obtained from food.
Protein sources include meat, poultry, seafood, eggs, milk and other dairy foods, soy foods, beans, peas, lentils, nuts, and seeds. Both plant and animal foods can contribute protein; choosing a variety of foods also provides different accompanying nutrients.
Protein digestion and absorption
Protein digestion begins in the stomach, where acid unfolds proteins and the enzyme pepsin starts breaking peptide bonds. Digestion continues in the small intestine with enzymes from the pancreas and intestinal lining. and small peptides are absorbed through the intestinal wall and enter circulation.
Choosing foods and considering individual needs
Consider both the nutrient and the food that supplies it. Choose carbohydrate sources that commonly provide , such as vegetables, fruit, legumes, and whole grains; limit reliance on refined grains and foods high in added sugars. Include sources of unsaturated fat, and vary protein foods across plant and animal sources according to preference and needs.
Current U.S. dietary guidance emphasizes whole, nutrient-dense foods, including protein foods, healthy fats, and whole grains. Individual needs vary with factors such as age, activity, health, and life stage, so broad guidance is not a personalized prescription.
Foods with similar can differ substantially in , fatty-acid profile, and other nutrients. Food choices therefore matter in addition to the amount of energy or macronutrient a food provides.