Between the extremes of optimal health and death from starvation or malnutrition, there is an array of disease states that can be caused or alleviated by changes in diet. Deficiencies, excesses and imbalances in diet can produce negative impacts on health, which may lead to diseases such as scurvy, obesity or osteoporosis. Moreover, excessive ingestion of elements that have no apparent role in health, (e.g. lead, mercury, PCBs, dioxins), may incur toxic and potentially lethal effects, depending on the dose.

The science of nutrition attempts to understand how and why specific dietary aspects influence health.

As of 2005, twelve vitamins and about the same number of minerals are recognized as 'essential nutrients', meaning that they must be consumed and absorbed - or, in the case of vitamin D, alternatively synthesized via UVB radiation - to prevent deficiency symptoms and death. Certain vitamin-like substances found in foods, such as carnitine, have also been found essential to survival and health, but these are not strictly 'essential' to eat because the body can produce them from other compounds. Moreover, thousands of different phytochemicals have recently been discovered in food (particularly in fresh vegetables), which have many known and yet to be explored properties including antioxidant activity (see below). Other essential nutrients include essential amino acids, choline and the essential fatty acids.

The rate of conversions of omega-6 DGLA to AA largely determines the production of the respective prostaglandins PGE1 and PGE2. Omega-3 EPA prevents AA from being released from membranes, thereby skewing prostaglandin balance away from pro-inflammatory PGE2 made from AA toward anti-inflammatory PGE1 made from DGLA. Moreover, the conversion (desaturation) of DGLA to AA is controlled by the enzyme delta-5-desaturase, which in turn is controlled by hormones such as insulin (up-regulation) and glucagon (down-regulation). Because different types and amounts of food eaten/absorbed affect insulin, glucagon and other hormones to varying degrees, not only the amount of omega-3 versus omega-6 eaten but also the general composition of the diet therefore determine health implications in relation to essential fatty acids, inflammation (e.g. immune function) and mitosis (i.e. cell division).

The state of obesity clearly contributes to insulin resistance, which in turn can cause type 2 diabetes. Virtually all obese and most type 2 diabetic individuals have marked insulin resistance. Although the association between overfatness and insulin resistance is clear, the exact (likely multifarious) causes of insulin resistance remain less clear. Importantly, it has been demonstrated that appropriate exercise, more regular food intake and reducing glycemic load (see below) all can reverse insulin resistance in overfat individuals (and thereby lower blood sugar levels in those who have type 2 diabetes).

Overfatness can unfavourably alter hormonal and metabolic status via resistance to the hormone leptin, and a vicious cycle may occur in which insulin/leptin resistance and overfatness aggravate one another. The vicious cycle is putatively fuelled by continuously high insulin/leptin stimulation and fat storage, as a result of high intake of strongly insulin/leptin stimulating foods and energy. Both insulin and leptin normally function as satiety signals to the hypothalamus in the brain; however, insulin/leptin resistance may reduce this signal and therefore allow continued overfeeding despite large bodyfat stores. In addition, reduced leptin signalling to the brain may reduce leptin's normal effect to maintain an appropriately high metabolic rate.

There is debate about how and to what extent different dietary factors - e.g. intake of processed carbohydrates, total protein, fat, and carbohydrate intake, intake of saturated and trans fatty acids, and low intake of vitamins/minerals - contribute to the development of insulin- and leptin resistance. In any case, analogous to the way modern man-made pollution may potentially overwhelm the environment's ability to maintain 'homeostasis', the recent explosive introduction of high Glycemic Index- and processed foods into the human diet may potentially overwhelm the body's ability to maintain homeostasis and health (as evidenced by the metabolic syndrome epidemic).

Antioxidants are another recent discovery. As cellular metabolism/energy production requires oxygen, potentially damaging (e.g. mutation causing) compounds known as radical oxygen species or free radicals form as a result. For normal cellular maintenance, growth, and division, these free radicals must be sufficiently neutralized by antioxidant compounds, some produced by the body with adequate precursors (glutathione, Vitamin C in most animals) and those that the body cannot be produced can only be obtained through the diet through direct sources (Vitamin C in humans, Vitamin A, Vitamin K) or produced by the body from other compounds (Beta-carotene converted to Vitamin A by the body, Vitamin D synthesized from cholesterol by sunlight). Different antioxidants are now known to function in a cooperative network, e.g. vitamin C can reactivate free radical-containing glutathione or vitamin E by accepting the free radical itself, and so on. Some antioxidants are more effective than others at neutralizing different free radicals. Some cannot neutralize certain free radicals. Some cannot be present in certain areas of free radical development (Vitamin A is fat-soluble and protects fat areas, Vitamin C is water- soluble and protects those areas). When interacting with a free radical, some antioxidants produce a different free radical compound that is less dangerous or more dangerous than the previous compound. Having a variety of antioxidants allows any byproducts to be safely dealt with by more efficient antioxidants in neutralizing a free radical's butterfly effect.

The most widely accepted definition is that of the World Health Organization (WHO). It states that "health is a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity" (WHO, 1946). In more recent years, this statement has been modified to include the ability to lead a "socially and economically productive life." The WHO definition is not without criticism, as some argue that health cannot be defined as a state at all, but must be seen as a process of continuous adjustment to the changing demands of living and of the changing meanings we give to life. The WHO definition is therefore considered by many as an idealistic goal rather than a realistic proposition.

Health is maintained through the science of medicine, but can also be improved by individual effort. Physical fitness, healthy eating, stress management training and stopping smoking and other substance abuse are examples of steps to improve one's health. Workplace programs are recognized by an increasingly large number of companies for their value in improving health and well-being of their employees, and increasing morale, loyalty and productivity at work. A company may provide a gym with exercise equipment, start smoking cessation programs, provide nutrition, weight or stress management training. Other programs may include health risk assessments and health screenings.

Alternative medicine can sometimes be used to improve health. However, with the lack of scientific proof through double blind testing, the placebo effect should be assumed to provide the health improvement in the case of successful alternative treatments until such testing can provide proof of any effects besides placebo. This is because as someone who feels well from their (possibly subconcious) belief in the therapies may lower their stress levels, resulting in beneficial effects on numerous factors, including blood pressure, gastrointestinal functioning, and immune response. The field of psychoneuroimmunology explores these links.