Maintaining a healthy diet is the practice of making choices about what to eat with the intent of improving or maintaining good health. Usually this involves consuming necessary nutrients by eating the appropriate amounts from all of the food groups, including an adequate amount of water. Since human nutrition is complex, a healthy diet may vary widely, subject to an individual's genetic makeup, environment, and health. For around 20% of the planet's population, lack of food and malnutrition are the main impediments to healthy eating; people in developed countries have the opposite problem, and are more concerned about obesity.

Generally, a healthy diet is said to include:
Sufficient calories to maintain a person's metabolic and activity needs, but not so excessive as to result in fat storage greater than roughly 30% of body mass. 2,000 is the recommended daily allowance of calories. (see Body fat percentage)
Sufficient quantities of fat, including monounsaturated fat, polyunsaturated fat and saturated fat, with a balance of omega-6 and long-chain omega-3 lipids. 65 grams is the recommended daily allowance of fat.
Maintenance of a good ratio between carbohydrates and lipids (4:1): four grams of the first for one gram of the second.
Avoidance of saturated fat (although the "evidence" for this claim is forever in debate after the testimony of results provided by the Framingham Heart Study of 1948-1998)
Avoidance of trans fat.
Sufficient essential amino acids ("complete protein") to provide cellular replenishment and transport proteins. (All essential amino acids are present in both animal and plant protein sources.)
Essential micronutrients such as vitamins and certain minerals.
Avoiding directly poisonous (e.g. heavy metals) and carcinogenic (e.g. benzene) substances;
Avoiding foods contaminated by human pathogens (e.g. e.coli, tapeworm eggs);
Avoiding chronic high doses of certain foods that are benign or beneficial in small or occasional doses, such as
foods or substances with directly toxic properties at high chronic doses (e.g. ethyl alcohol);
foods that may interfere at high doses with other body processes (e.g. refined table salt);
foods that may burden or exhaust normal functions (e.g. refined carbohydrates without adequate dietary fibre).

[edit] Natural eating
The holistic approach to healthy eating focuses more on food as a whole than the individual nutrients within the food. It also emphasizes a lack of processing in the production of the food (see organic food.) Other aspects of healthy eating [under the alternative definition] are not using microwave ovens, using Kirlian photography to see the aura of the food, whole food supplements, coral calcium, sea salt, juicing fruits, and above all, "if man made it, don't eat it." Natural Cures "They" Don't Want You to Know About is a good source of additional information.

[edit] Governmental guidance
Although a healthy diet is based upon nutrition, people eat foods and not nutrients; as few people know which foods supply which nutrients, allowing people to self-regulate their diets means that they run the obvious risk of deficiency. Due to past difficulties of educating people about nutrient intake, governments have opted to counsel on what foods to eat rather than on what nutrients to ingest.
Most states set guidelines for a healthy diet -- these usually vary slightly from country to country based upon demographics. These guidelines do however usually share the same recommendations of eating less fried or fatty foods to reduce cholesterol. Many guidelines suggest replacing certain foods with healthier alternatives that supply an abundance of nutrients, for instance using legumes or beans within a salad or pasta.
As BMI and weight changes from person to person, the general Reference Nutrient Intakes (RNI) set by governmental institutions[citation needed] may be somewhat lacking for some people, despite the fact that the RNI is generally calculated as higher than the average nutrient intake. It is even thought[attribution needed] that some people may have needs above that of the RNI, meaning even if a person ate the recommended amount of nutrients, they would still suffer deficiency. The only real way to know the RNI for many people is to monitor the intake of nutrients and amount of exercise.
Examples of specific recommendations include:
The Dietary Reference Intake system, used to set recommended amounts of various nutrients on food labels in the United States and Canada.
MyPyramid, formerly food guide pyramid, a graphical recommendation from the United States Department of Agriculture.
Canada's Food Guide
Some groups have been critical of the U.S. pyramids, alleging poor scientific basis, and influence from food producers. Harvard School of Public Health researchers have proposed their own healthy eating pyramid.
A high-level summary of Government Guidance is:
Make sure that you eat five different types of fruit and vegetables every day. Every day also make sure that you eat at least one thing from each of the different food groups: Carbohydrates, Fruit and vegetables, Protein, Dairy, and Fats. Remember to eat the most fruit and vegetables, and the least fats.
Too much salt gives you high blood pressure; a high fat diet will give you hard and narrow arteries[citation needed] that could lead to heart attacks and strokes, possibly even death.
Fruit and vegetables contain antioxidants which will keep you healthy as long as you team them up with regular daily exercise. Your efforts to eat healthily will go to waste if you do not take part in regular daily exercise.
Having fibre in your diet will keep your digestive system going strong.

[edit] Detrimental eating habits
In specific individuals, ingesting foods containing natural allergens (e.g. peanuts, shellfood) or drug-induced triggers (e.g. tyramine for a person taking an MAO inhibitor) may be life-threatening.
Some foods have low nutritional value, and if consumed on a regular basis will contribute to the decline of human health. This has been demonstrated by various epidemiological studies that have determined that foods such as processed and fast foods are linked to diabetes and various heart problems.
When improperly cut or prepared, a small number of foods (such as fugu) can result in death.
The ingredient usually cited as being most crucial to good health, water, has even been known to result in death when consumed in extraordinary quantities.

[edit] Cultural and psychological factors
From a psychological perspective, a new healthy diet may be difficult to achieve for a person with poor eating habits. This may be due to tastes acquired in early adolescence and preferences for fatty foods. It may be easier for such a person to transition to a healthy diet if treats such as chocolate are allowed; sweets may act as mood stabilizers, which could help reinforce correct nutrient intake.
It is known that the experiences we have in childhood relating to consumption of food affect our perspective on food consumption in later life. From this, we are able to determine ourselves our limits of how much we will eat, as well as foods we will not eat - which can develop into eating disorders, such as anorexia, bulimia, or orthorexia This is also true with how we perceive the sizes of the meals or amounts of food we consume daily; people have different interpretations of small and large meals based on upbringing.
While plants, vegetables, and fruits are known to help reduce the incidence of chronic disease, the benefits on health posed by plant-based foods, as well as the percentage of which a diet needs to be plant based in order to have health benefits is unknown. Nevertheless, plant-based food diets in society and between nutritionist circles are linked to health and longevity, as well as contributing to lowering cholesterol, weight loss, and in some cases, stress reduction.
Indeed, ideas of what counts as "healthy eating" have varied in different times and places, according to scientific advances in the field of nutrition, cultural fashions, religious proscriptions, or personal considerations.

[edit] Public policy issues
Fears of high cholesterol were frequently voiced up until the mid-1990s. However, more recent research has shown that the distinction between high- and low-density lipoprotein ('good' and 'bad' cholesterol, respectively) must be addressed when speaking of the potential ill effects of cholesterol. Low-density lipoprotein is often prevalent in animal products, such as bacon and egg yolks, whereas high-density lipoprotein is more common in plant and fish tissues, such as olive oil and salmon.
Media coverage of mass-produced, processed, "snack" or "sweet" products directly marketed at children has worked to undermine policy efforts to improve eating habits. The main problem with such advertisements for foods is that alcohol and fast food are portrayed as offering excitement, escape and instant gratification.
Particularly within the last five years government agencies have attempted to combat the amount and method of media coverage lavished upon "junk" foods. Governments also put pressure on businesses to promote healthy food options, consider limiting the availability of junk food in state-run schools, and tax foods that are high in fat. Most recently, the United Kingdom removed the rights for McDonald's to advertise its products as the majority of the foods that were seen to have low nutrient values were aimed at children under the guise of the "Happy Meal". The British Heart Foundation released its own government-funded advertisements, labeled "Food4Thought," which were targeted at children and adults displaying the gory nature of how fast food is generally constituted.