Macronutrients

The three functions of macronutrients

The main function of macronutrients is to provide the body with energy, measured as calories. While all of the macronutrients yield energy, the amount of calories provided by each one varies. Carbohydrates and proteins provide four calories per gram, while fat provides nine calories per gram. Macronutrients also have specific roles in maintaining body functions and contribute to the taste, texture, and appearance of foods, which helps to make the diet more varied and enjoyable.

Proteins, from the Greek proteios meaning “first,” are important biological molecules (biomolecules) that consist of strings of smaller units called amino acids, the “building blocks” of proteins. These amino acids are linked together in sequence as polypeptide chains that fold into compact shapes. Proteins vary in shape and size, some consisting only of 20–30 amino acids and others of several thousands. They are present in every living cell. In the skin, hair, calluses, cartilage, muscles, tendons and ligaments, proteins hold together, protect, and provide structure to the body. As enzymes, hormones, antibodies, and globulins, they catalyze, regulate, and protect the body chemistry. Important biomolecules like hemoglobin, myoglobin, and various lipoproteins that carry oxygen and other substances within the body are also proteins.

Besides providing energy to the body, dietary protein is also required for growth—especially by children, teenagers, and pregnant women. It is essential for tissue repair, immune system function, hormone and enzyme production, and for lean muscle mass and tone maintenance. When eaten, the proteins contained in foods are broken down into amino acids, an important dietary source of nitrogen. To make the proteins that it needs (protein biosynthesis), the body requires amino acids. There are 20 amino acids. The body can make some of them from components within the body, but it cannot synthesize nine of them, accordingly called the “essential amino acids^”since they must be provided in the diet. They include: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Proteins that contain all nine essential amino acids come from primarily animal sources and are called “complete proteins.” Protein from plants, legumes, grains, nuts, seeds, and vegetables are called “incomplete proteins” because they are lacking one or more essential amino acid(s). Nonmeat sources of complete proteins include soy and a grain called quinoa.

Proteins are complex molecules and the body needs time to break them down. This is why they are longer-lasting source of energy than carbohydrates. If more protein is consumed than is needed, the body stores its components as fat, which can be broken down and used for energy as the need arises. Proteins are broken down during digestion, which exposes them to acid in the stomach and to degradation by the action of enzymes called proteases. Some ingested amino acids are converted to carbohydrates (gluconeogenesis), a process that is also used under starvation conditions to generate glucose from the body's own proteins, particularly those found in muscle.

The major role of carbohydrates is to be an energy source for the brain, nervous system, and the body. Dietary fiber, usually called roughage, helps keep the bowel functioning properly and may help decrease the risk of developing heart disease, diabetes, GI disorders, colon cancer, and obesity. The simple carbohydrates are broken down quickly and are the fastest source of energy for the body. Fruits, dairy products, honey, and maple syrup contain large amounts of simple carbohydrates, which provide the sweet taste in most candies and cakes. Another type of carbohydrate are the polyols, the so-called sugar alcohols. They do occur naturally, but most are made commercially by the transformation of sugars. Complex carbohydrates also include starch, the main energy reserve in root vegetables and cereals. Nonstarch carbohydrates are the main components of dietary fiber. These are the indigestible portion of plant foods, such as cellulose, which is the major component of plant cell walls that consist of several thousand glucose units. Simple sugars are absorbed directly by the small intestine into the bloodstream, where they are then transported to where they are required. Complex carbohydrates are broken down by enzymes into their constituent sugars, which are then absorbed into the bloodstream while dietary fiber moves food through the digestive system.

Besides being a source of energy, fat stores protect the internal organs of the body. Some essential fats are also required for the formation of hormones. Fats are the most energy-efficient form of food, but take the most time for the body to convert into energy. Each gram of fat supplies the body with about nine calories, more than twice that supplied by the two other macronutrients. Because fats are such an efficient form of energy, they are stored by the body either in the abdomen (omental fat) or under the skin (subcutaneous fat) for use when the body needs more energy. Fats that are in foods are combinations of four main types:

The three types of macronutrients do not have the same chemical composition. When compared with carbohydrates and fats, proteins are very different. Fats largely consist of hydrocarbon chains, containing 75%–85% carbon. Carbohydrates are roughly 50% oxygen, and like fats, they usually have less than 5% nitrogen or none at all. Proteins, on the other hand, consist of 15%–25% nitrogen and about an equal amount of oxygen. The three macronutrients are often found together in most foods, but in varying amounts, or alone in other foods. The Nutrition Facts labels provide a breakdown of the macronutrient composition of various foods.

Foods that are a source of protein include:

Proteins are noted for their nutritional contribution of amino acids, phosphorus, vitamins B₆, B₁₂, zinc, magnesium, iron, niacin, and thiamin. Foods from the dairy group such as milk, cheese, and yogurt also provide calcium, riboflavin, and (when fortified) vitamins D and A.

Sources of dietary carbohydrates include:

Of the carbohydrates, fruits provide vitamins A and C, potassium, and fiber, as well as flavonoids and phytochemicals. Grains contain riboflavin, thiamin, niacin, iron, magnesium, and fiber. Vegetables supply rich amounts of vitamins A and C, folate, potassium, magnesium, and soluble and insoluble fiber.

Sources of dietary fats include:

Fats do not supply vitamins or minerals to the body, but they do assist in the makeup of hormones, bile, and vitamin D. Triglycerides carry all of the fatsoluble vitamins (A, D, E, and K) in foods to be absorbed in the body.

According to the dietary reference intakes (DRIs) published by the U.S. Department of Agriculture (USDA), between 10% and 35% of a person's total caloric intake should come from protein. Adults need to eat approximately 60 grams of protein per day (0.6 to 0.8 g per kg of weight). Adults who are physically active most days or trying to build muscle need about 1.0 to 1.8 g per kg of weight. Infants need 1.5 g per kg each day, children age 1 to 3 need 1.1 g per kg each day, children age 4 to 13 need 0.95 g per kg each day, teens age 14 to 18 need 0.85 g per kg each day, and women who are pregnant or lactating need 1.1 g per kg each day, based on their pre-pregnancy weight.

Between 50% and 60% of calories should come from carbohydrates and 20–35 g of dietary fiber per day should be consumed by all those over two years of age.

Overall, fat intake should be no more than 30%– 35% of total calories, with no more than 10% of calories coming from saturated fats and 300 mg of cholesterol daily. This means that the remaining 20%–25% of calories should come from mono and polyunsaturated sources. It is also recommended to include more omega-3 polyunsaturated fats in the diet while limiting trans fats as much as possible.

In the case of an allergic reaction, the only way to treat sensitive individuals is to eliminate the food or food component from the diet. In the case of food intolerance, limiting the food to smaller servings may be sufficient to avoid symptoms. The number of calories required to correct or maintain weight depends on several factors, including age and activity level. This is why conditions such as nutritional deficiencies and obesity require professional care that should be supervised by a physician working with a dietitian.

A common complication of unbalanced intake of macronutrients is diabetes, a metabolic disorder whereby the body cannot regulate blood glucose levels properly. There is no evidence that sugar consumption is linked to the development of any type of diabetes. However, there is now good evidence that obesity and physical inactivity increase the likelihood of developing non-insulin dependent diabetes, which usually occurs in middle age. Weight reduction is usually necessary and is the primary dietary aim for people with non-insulin dependent diabetes. Consuming a wide range of carbohydrates is an acceptable part of a diabetic diet, and the inclusion of low–glycemic index foods is beneficial as they help regulate blood glucose control.

Furthermore, research suggests that a diet that contains whole grains, lean proteins, and low or nonfat dairy products (as well as an abundance of fruits and vegetables) will assist in reducing the risk for many diseases and cancers. In contrast, a diet low in these foods is associated with increased risk of developing these diseases.

Today's lifestyles are vastly different from those of the past. The fast pace of modern lifestyles and the increase in the number of households where both parents work have lead to marked changes in food preparation and consumption habits. A positive consequence has been the emergence of convenient foods and important advances in food technology that help ensure the safety and wholesomeness of the food supply. However, a negative consequence has also been a significant increase in ready-to-eat foods of low nutritional value (junk food). Parents are accordingly concerned about their kids developing bad nutritional habits. Fortunately, there is a wealth of information about food made available to help ensure that diets are nutritious, which parents can use to teach their kids to make informed decisions regarding which foods, and in what quantities, are best for good health.

See also Bodybuilding diet ; Calories ; Dietary supplements ; Fad diets ; Functional foods ; High-protein diet ; Inflammatory bowel disease ; Nutrigenomics ; Phytonutrients .