Macronutrients

DEFINITION: The dietary ingredients required in large quantities for health and activity.

ANATOMY OR SYSTEM AFFECTED: All

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 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 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 the 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 converted. However, a high-protein diet would be ketotic, disrupting mineral balance. Only a small percentage 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 (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. 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 the 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. 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.

In the early twenty-first century, diets that focused on counting macronutrients rather than calories become more popular. A particularly common subtype of macronutrient-focused diets are low-carbohydrate, high-fat diets, such as the ketogenic diet and the Atkins diet. Diets that reduce macronutrient intakes by 20 to 40 percent, primarily from carbohydrates and lipids, while maintaining adequate intakes of protein and other nutrients, have long been associated with increased longevity and decreased incidence of some cancers and other diseases of aging, as well as with weight loss. However, few people are willing to restrict their intake voluntarily to such an extent and to live with a continuous feeling of hunger. In addition, studies have shown that the benefits of reducing carbohydrates are not entirely agreed upon. Although still popular in diet culture, extreme diets will never replace balanced and healthy eating.

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 has also existed in other parts of the world. Obesity is associated with an 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 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 dietitians and public health professionals to tailor recommendations and 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. Just as a person can undergo testing to personalize their drug choices, such as in the case of antidepressant medications, personalized genomic-based nutrition advice is available that can not only determine ideal macronutrient intake but also food allergies or intolerances. While available to the public on an outpatient basis, this process remained prohibitively expensive for most.

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