Normal nutritional function requires a healthy diet. Healthy eating incorporates a variety of nutrients that are essential for energy expenditure, prevention of disease, and maintenance of normal physiological function. An unhealthy diet can result in malnutrition, and this contributes to illness and death throughout the world. The core principle of healthy eating is obtaining an adequate balance, and the diseases resulting from overnourishment differ greatly from those resulting from undernourishment. In the third world, diets tend to rely heavily on staple crops, and can be very seasonal. Energy sources are predominantly cereals, whereas meat and fish are limited. Malnutrition tends to occur from a lack of essential nutrients, leading to conditions such as vitamin deficiencies, kwashiorkor, and iodine deficiency syndromes. In first-world countries, people have more freedom to choose what they eat. Thus, diets tend to be high in fat and dense in energy. Obesity, diabetes, coronary heart disease, cancer, and hypertension are major contributors to morbidity and mortality. A healthy diet should contain adequate proportions of carbohydrates, fats, proteins, vitamins, and trace elements. The intake of these constituents is sporadic, with meals constituting major boluses of potential energy. Energy expenditure, conversely, is continuous. The human body has, therefore, developed complex mechanisms directing nutrients into storage when in excess, and mobilizing these stores as they are needed, and it is essential that sufficient energy is always available to maintain the basal metabolic rate, which is the amount of energy expended while at rest in a neutrally temperate environment. This energy is sufficient only for the functioning of the vital organs, such as the heart, the lungs, the liver, the kidneys, and the CNS.
Radio-activity and Normal Physiological Function
