Observations from different parts of the world show that low protein intakes, often accompanied by low calorie intakes, occur in adults in many developing countries. In the industrialized world, excessive consumption of lipids, carbohydrates, sugars and alcohol (empty calories) without essential nutrients given by a balanced diet, make dietary considerations a major factor in most diseases. The malnourished adult usually shows seasonal deficiencies, which are transient and do not cause obvious ill-health unless accompanied by acute or chronic disease. Dietary imbalance shows in reduced physical output in work among adults and developmental problems among children. An additional effect is a deterioration in the use of mental faculties – namely listlessness, lowering of initiative, lack of awareness, poor judgement, etc. At any age, poor nutrition means greatly decreased resistance to disease.
Caloric needs determine the limits of food that can healthily be ingested. In order to maintain desirable protective levels of nutrients in the diet consistent with the acceptable food habits of society, some 40-50% of the total diet must consist of high-nutrient density foodstuffs supplying needed quantities of the essential nutrients: amino acids, vitamins, minerals and trace elements. This means that 40-60% of the energy value of the diet must be derived from high nutrient density foodstuffs: products such as meat, milk, eggs, cheese, vegetables, fruits, and cereals. In order to further maintain nutrient density of the diet, selected enhancement of the nutrient content of some foodstuffs, such as cereals, may be desirable.
Undernutrition occurs almost exclusively in the context of poverty with its concomitants, which include: lack of opportunity to earn money; lack of land, or access only to land which yields little in relation to the labour and capital invested in it; lack of clean water; poor nutritional education; traditional food habits; low food quality; poor sanitation; limited access to medical and social services; and limited educational opportunities. All these problems are so closely linked that it is practically impossible to isolate the problem of undernutrition for an analysis of its causes and consequences and for the design of interventions which might improve the situation. In those cases where undernutrition is not a factor of poverty, depletion of vitamins is caused by taking drugs and antibiotics, smoking cigarettes, drinking alcohol and eating overly processed, faddish or vegetarian foods.
In industrialized countries, the intake of rich foods with little nutritional value tends to increase, whilst the intake of milk products, fruit and vegetables decreases. Though poor diets cannot be directly related to malnutrition, malnutrition might result from consumption of a poor diet over a period of months or years.
In the USA in 1965, for example, it was estimated that 20% of the families existed on a nutritionally poor diet. This included both high and low income families. Only 50% of USA families were consuming a good diet. (Previous figures for 1955 gave 15 and 60% respectively). A usual mixed diet in Scandinavia, the UK, or North America provides 6 mg or less of iron per 1000 calories daily; a woman ingesting 1800 or 2000 calories per day therefore obtains 10-12 mg of dietary iron. This amount is insufficient to meet current standards of allowances for women, especially during pregnancy and lactation. Its adequacy may be further compromised if the dietary composition is not such as to favour iron absorption, that is, if it is devoid of or low in vitamin C or high in bulk or phytate.
Diet consciousness, reaching proportions of hysteria in the western world, has ironically propelled a wave of vitamin and mineral deficiency among affluent young people, who may only recognize their dietary foolishness later in life.
Some kinds of junk food are not deleterious to health; it depends on age and on the general diet. A bag of potato crisps contains 140 calories and a lot of Vitamin C, and this may be a good snack for a child, who needs vitamins and energy, provided he eats a broad diet.