Nutrition is a vital physiologic process necessary for the development and maintenance of the human body, its organs, and its physiological processes. In order to maintain good health, individuals require daily intake of protein, fat, and carbohydrates—nutrients that provide energy, vitamins, minerals, and water. Nutrition provides necessary nutrients that are not produced by the body. The daily requirements of these nutrients vary depending on individuals’ age, sex, and physiologic state (that is, pregnancy).
In the United States dietary guidelines are updated every five years and are the basis for the Food Guide Pyramid. These guidelines are issued by the U.S. Department of Agriculture (USDA) and the U.S. Department of Health and Human Services (HHS). The Food Guide Pyramid groups foods into major categories (fruit, vegetable, dairy, grain, meat, and bean) and recommends serving sizes and the number of servings needed to achieve balanced nutrition and health. In 2005, to address the dramatic rise in obesity, the USDA modified the Food Guide Pyramid to emphasize lower energy intake and more physical activity and provide an individualized approach. The USDA promotes nutrition based on the fact that certain dietary patterns may place individuals at risk of chronic diseases, such as cardiovascular disease, diabetes, and cancer. Diets rich in saturated fat and low in fruits and vegetables are associated with heart disease, cancer, and diabetes, while diets rich in fruits and vegetables may protect individuals from these conditions.
The balance between energy intake and energy expenditure determines weight maintenance or weight gain and loss. When regular energy intake is greater than the expenditure, the body stores the surplus of energy as extra body fat or adipose tissue. Energy expenditure is a result of the basal metabolic rate, the thermic effect of food (energy spent in processing food), and the energy spent in physical activity. The contribution of physical activity to energy balance is small compared to food intake. Energy intake and dietary patterns are influenced by hormonal mechanisms and psychosocial factors. People eat in response to hunger signals that the body creates, such as changes in blood glucose levels. Hunger can also be experienced in response to sensory cues, such as the smell or sight of favorite foods.
Satiety is regulated by a complex hormonal signaling system with input from proteins secreted by the brain, the gastrointestinal tract, and the liver. In the brain the hypo-thalamus secretes small proteins called neuropeptides. The neuropeptides inhibit food intake after receiving input from hormones and proteins synthesized in the stomach and intestines. Ghrelin, which is synthesized predominantly by the stomach, stimulates feeding. In the fasting state ghrelin levels are high, and levels fall after eating. The intestine releases cholecystokinin (CCK) after a meal, which stimulates gut motility, the contractions of gastrointestinal muscles that enable food to move along the digestive tract and ensure absorption of nutrients and inhibits food intake by signals to the brain. Other organs and hormones are also involved in the regulation of food intake and satiety. After a meal, the pancreas secretes insulin, which in turn signals the brain to inhibit feeding. Leptin is a hormone synthesized in adipose (fat) cells, and its secretion is regulated by the obese (ob) gene. Leptin levels are low during starvation and high when nutrients are plentiful and in obesity. High levels of leptin signal the brain to reduce appetite. In obesity people are resistant to the effects of leptin. The hormone adiponectin is secreted by adipose cells, and its secretion is stimulated by food restriction. Adiponectin is also a factor that increases energy expenditure, and low levels may contribute to the development of obesity.
The body seems to tolerate positive energy balance, that is, when energy intake is higher than expenditure. The mechanisms described above strongly defend from weight loss but not from weight gain because of an evolutionary process that favored intake of nutrients when food was easily accessible and in preparation of times when food was scarce. This evolutionary perspective can explain in part the obesity epidemic that emerged during the last decades of the twentieth century when, in developed countries, there was abundance of food without periods of scarcity.
Nutrition is not only influenced by biological factors. Environment, culture, and social factors also play important roles in nutrition. The environment determines the type and quality of foods accessible to individuals and communities. A diet rich in refined sugars, carbohydrates, and fat is related to increased availability of fast foods, sugared beverages, and low-nutritional value snacks and decreased access to produce (fruits and vegetables). A positive energy balance in turn is caused by increasing calorie intake, especially when reinforced with a sedentary lifestyle. Social factors, such as low income and low socioeconomic status, are associated with less healthy dietary patterns with higher saturated fat intake and lower intake of fruits and vegetables possibly due to the facts that low-income individuals are more likely to live in communities with less access to healthy foods and that healthier food items are proportionally more expensive that unhealthy ones. Industrialized societies eat a diet rich in meat and saturated fat and lower in fiber compared to agrarian societies, who consume more fruits, vegetables, and grains. Research indicates that immigration changes dietary patterns of individuals, who over time adopt the diet of their host country.
In people with excess weight, a diet restricting calories but maintaining balanced nutrient composition can achieve healthy weight loss and reduce the risk of diabetes and cardiovascular disease, which are conditions associated with obesity. However, when the calorie restriction is severe and the diet does not allow for sufficient amounts of essential nutrients, dieting can lead to metabolic and heart problems and even death. The psychological factors that are associated with nutrition are best exemplified by obesity and eating disorders. Obese individuals frequently have low self-esteem and depression. Whether these are causes or consequences of obesity is still under debate. Some individuals may increase their food intake in response to feelings of stress, loneliness, and anxiety, which place them at risk of overweight and obesity. Eating disorders include anorexia, characterized by being severely underweight from excessive food intake restriction, and bulimia, characterized by purging. Either disorder may also have periods of binge eating. Eating disorders, especially anorexia, are associated with body image dissatisfaction, depression, and low self-esteem.
