What are macronutrients? |

Structure and Functions

Macronutrients are generally considered to include carbohydrates, lipids, and proteins; these are dietary constituents consumed in the largest quantities. While water and oxygen are also needed in large amounts, they are not usually considered to be food. Although fiber ingested in substantial quantity is desirable for optimum health, it is, by definition, a non-nutritive dietary constituent. In addition, calcium, sodium and chloride (salt), magnesium, potassium, phosphorus, and sulfur are sometimes added to the list of macronutrients because they are needed in large amounts compared to vitamins and other minerals; however, they are best referred to as macrominerals.

Carbohydrates, lipids, and proteins provide the energy (calories) needed for maintenance, growth, thermoregulation, and physical activity, as well as pregnancy and lactation. All three also provide for other needs: Carbohydrates are components of structural polysaccharides inside cells and on their surfaces; lipids are important in cellular membrane structure and function and as precursors of some hormones; and proteins are a source of amino acids needed for the synthesis of body proteins, nucleic acids, and other hormones. Lipids also facilitate the uptake of the fat-soluble vitamins A, D, E, and K. As sources of energy, all three macronutrients are virtually interchangeable, although the nitrogen of the amino acid constituents of proteins must be concomitantly disposed of, mostly in the form of urinary urea. The carbon in these substances is combusted with oxygen, producing carbon dioxide, and during the process the metabolic energy and heat are generated that support living processes. Total dietary intake of
macronutrients (and hence energy) should be kept in balance with energy expenditure. Excess intake over expenditure will lead to overweight and eventually obesity, which is detrimental to health, as it is associated with some cancers, coronary heart disease, diabetes, and other chronic diseases. Acceptable macronutrient distribution ranges in human diets have been set for carbohydrates, lipids, and proteins to prevent frank deficiencies and minimize incidences of chronic diseases from overconsumption.

Dietary carbohydrates, primarily starches and sugars, are digested to monosaccharides (primarily glucose and fructose), and transported into the blood. Carbohydrate uptake in excess of that needed for immediate use is stored as glycogen, principally in liver and skeletal muscle, or is converted to fatty acids and stored as triglycerides in adipose depots. Subsequently, glycogen can be broken down to glucose for use as a ready fuel source for the body. Humans do not have a dietary requirement for carbohydrates. However, a diet devoid of carbohydrates would have to compensate with much larger quantities of protein. Some amino acids can be converted to the glucose needed in the body by a process termed gluconeogenesis, whereas little of the fat content can be so converted. However, a high-protein diet would be ketotic, disrupting mineral balance. Because the brain and nervous tissue have a requirement for glucose, an acceptable macronutrient distribution range for carbohydrates has been set at 45 to 65
percent of the energy content of the diet. Furthermore, no more than 25 percent of the energy needs should be met with added sugar, which is metabolized rapidly and can thereby disrupt whole body metabolism. Added sugar is also associated with a lower intake of essential vitamins and minerals, as well as with a high risk of dental caries (cavities).

Dietary lipids, primarily triglycerides, are essentially digested to their constituent fatty acids for absorption and then are reconstituted to triglycerides in intestinal tissue. Because triglycerides are insoluble in water, they are transported in the body via a complex system of lipoproteins—chylomicra, low-density lipoproteins (LDLs), and high-density lipoproteins (HDLs). Their constituent fatty acids are used as fuel by various tissues, and any excess triglycerides are stored in adipose tissue. Two fatty acids, linoleic and α-linolenic, termed essential fatty acids because they cannot be synthesized by the human body, are required in small quantities. Human diets should contain a minimum of 10 percent diversified lipids (linoleic is found in a variety of plant oils and α-linolenic in fish oil) to ensure that the requirement for these essential fatty acids is met. The acceptable macronutrient distribution range for lipids has been set at 20 to 35 percent of the energy content of
the diet for adults. Lipid intake should be slightly higher for infants and children because of the demands of growth.

Dietary proteins generally serve as energy sources once the need for their constituent amino acids as precursors of body protein has been met. Dietary proteins are digested to their constituent amino acids and absorbed across the intestinal tract into the blood, where they are taken up by various tissues for synthesis of body proteins, which are continually being broken down and resynthesized. Amino acids absorbed in excess of that need are used as fuel, either directly or after conversion to glucose by gluconeogenesis. Adult humans have a requirement for 0.8 grams (g) of well-balanced protein per kilogram (kg) of body weight each day; this corresponds to 56 g of protein per day for the average man and 46 g per day for the average woman. The acceptable macronutrient distribution range for proteins has been set at 10 to 35 percent of the energy content of the diet. Protein intake should be slightly lower for infants and children to compensate for the higher proportion of lipid intake and to minimize the need to dispose of excess nitrogen.

Disorders and Diseases

The main disorders associated with macronutrients result from their inadequate or excess consumption, namely starvation and obesity. The former is particularly problematic for infants and children because of their higher demands for growth and brain development; some early deficits lead to permanent impairment. Two general types of starvation are recognized; marasmus is due to a general deficiency of macronutrients, also referred to as protein-calorie malnutrition, whereas kwashiorkor is primarily attributed to a deficiency of dietary protein.

Anorexia nervosa (restricted intake) and bulimia nervosa (binge eating followed by purging, vomiting, or misuse of laxatives or diuretics) are two eating disorders with complex and variable etiologies that can lead to reduced macronutrient intake, starvation, and even death.

Ironically, calorie-restricted diets that reduce macronutrient intakes by 20 to 40 percent primarily from carbohydrate and lipids, while maintaining adequate intakes of protein and other nutrients, appear to be associated with increased longevity and decreased incidence of some cancers and other diseases of aging. However, few people are willing to restrict their intake voluntarily to such an extent and to live with a continuous feeling of hunger.

Obesity is a modern epidemic largely because of the ready availability and consumption of inexpensive food coupled with a sedentary lifestyle. While particularly a problem in Western societies, it is making inroads in the rest of the world. Obesity is associated with increased risk of coronary heart disease, some cancers, and type 2 diabetes. Excessive weight also puts added stress on knee and ankle joints. Obesity is, by definition, an energy imbalance, where energy intake (from macronutrients) exceeds energy expenditure, as from physical exercise. Obesity is a multifactorial disease, however, influenced by both genetic and environmental factors. Some people appear to be more susceptible to weight gain, as genetic factors have an impact on appetite, endocrinology, metabolism, and activity. The environmental factors include access to palatable food and lack of exercise. While not a major contributor to the current epidemic, binge eating disorder can lead to obesity.

Perspective and Prospects

The ancient Greeks noted that a wide variety of foods were converted into the organs and tissues of people consuming them. They concluded that the differences between food and human protoplasm must be superficial and that they must be made from the same substance. They also assumed that the need for food after growth had ceased was caused by the wearing out of organs and tissues and the continuous need to replace them. In the late 1700s, Antoine Lavoisier demonstrated that carbon dioxide
expiration increased with exercise and that the oxidation of fats and carbohydrates accounted for most of the energy needed for animal heat production. In the nineteenth century, the need for nitrogen in the diet was demonstrated for dogs and by analogy for humans; proteins were first described as the main nitrogen-containing substances in food. Sophisticated calorimetry equipment large enough for humans to live in for several days made it possible to quantitate energy balance and, by 1900, permitted the conclusion that the metabolism of lipids and carbohydrates could be used for mechanical work with similar efficiency. In that same period, dietary proteins were shown to be broken down to amino acids in the digestive tract, absorbed, and used to rebuild body protein.

Relative to macronutrient consumption, the dilemma of undernutrition and overnutrition in the world, often within a country and even within the same household, makes it difficult for nutritionists and public health professionals to tailor recommendations and to inform political decisions. The concept of an optimum intake of macronutrients, while easy to grasp, is difficult to enact. A further complication is the individual variation in metabolism and taste, even within the same culture. Cases have been confirmed where individuals with distinct genotypes respond differently, even oppositely, to the same macronutrient intervention. In the future, one can foresee personalized genomic-based nutrition advice.

Bibliography

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Otten, Jennifer J., Jennifer Pitzi Hellwig, and Linda D. Meyers. Dietary Reference Intakes: The Essential Guide to Nutrient Requirements. Washington, D.C.: National Academies Press, 2006.

Shils, Maurice E., et al. Modern Nutrition in Health and Disease. 10th ed. Baltimore: Lippincott Williams & Wilkins, 2006.

United States Department of Agriculture National Agricultural Library. “Macronutrients.” USDA, July 12, 2013.

Webster-Gandy, Joan, Angela Madden, and Michelle Holdsworth. Oxford Handbook of Nutrition and Dietetics. New York: Oxford University Press, 2012.

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