Human nutrition: Key to health and development People and populations at risk Availability of food: How countries compare Staple foods: What do people eat? Who are the food producers? Impact of poverty on life Feeding the world: The search for food security

Food and people Dimensions of Need

To be healthy and active, our diet (what we regularly eat and drink) must be adequate in quantity and variety to meet our energy and nutrient needs. Most foods contain many different nutrients; no single food, except breast-milk, provides all the nutrients required. The best way, therefore, to ensure the body gets all the necessary nutrients is to eat a variety of foods.

Nutrients are classified as carbohydrates, fats, proteins, vitamins or minerals. Water and dietary fibre are also essential. Each nutrient has specific functions and is made available to the body tissues through the processes of digestion and absorption.

How well foods are digested and their nutrients absorbed can be influenced in several ways. For example, infections, particularly those that accelerate the passage of food through the digestive system, can reduce the body's capacity to absorb nutrients and accelerate the loss of water and body salts. The combination of foods eaten can also affect digestion and absorption. Iron from animal sources is usually well absorbed, but iron in plants is generally not, because of the presence of natural compounds, such as phytates in cereals and tannins in tea, that inhibit iron absorption. Other factors, such as an increased vitamin C intake, can enhance the absorption of iron from plant sources. Excessive dietary fibre can interfere with the absorption of some nutrients.

Nutrient requirements vary between individuals according to, for example, age, gender, level of activity and health. The ways in which people meet these requirements also vary. There is no ideal or universal dietary pattern and the body is wonderfully adaptive. From arctic tundra to tropical forest, and from big cities to remote islands, various populations demonstrate that human nutritional needs can be met by diverse ranges of foodstuffs and dietary habits.

What is a balanced diet?

The Food Guide Pyramid is one example of a national food guide. Prepared for consumers in the United States, the pyramid shows the recommended proportions of the various categories of food. It is easy to see that for most Americans, the daily food intake should include a high proportion of cereals, fruits and vegetables whereas fats, oils and sweets are best eaten in moderation.

Nutritional requirements vary from person to person. Everyone needs to pay attention to the quality, quantity and diversity of food sources to have a balanced diet. Many countries have set or suggest dietary guidelines to help people meet their nutritional needs. Since there is no ideal dietary pattern suitable for all people, these guidelines must be developed with a specific food supply and population in mind.

A daily food guide from the United States

Nutrients and where to find them

Carbohydrates are the basic source of energy. They range in complexity from simple sugars to complex starches. Sugars are found in sweet foods such as honey, and in milk and fruits. Major sources of starches include cereals, root vegetables, pulses (beans, lentils, peas) and some fruits such as plantains and bananas.

Dietary fats and oils are rich sources of energy and provide essential fatty acids. They can be obtained from both animals and plants. Animal sources include fatty meats, poultry such as duck and goose, butter, ghee and oily fish. Plant sources include oilseeds (sunflower, safflower, sesame), nuts and legumes (peanuts, soybeans).

Proteins, which are long chains of amino acids, form much of the basic structural material of the body; they are necessary for its growth, functioning and repair. The body can make many amino acids but some, called essential amino acids, must be obtained from food. Different foods contain varying quantities of these. Animal products are a prime source, but a mixture of vegetable sources can also satisfy the body's needs. Rich sources of proteins include meat, fish, dairy products, pulses, nuts and cereals.

Vitamins are essential to practically all the body's chemical processes and for maintaining the health and integrity of body tissue. They are usually required in small quantities, but must be consumed regularly because many are not stored well in the body. Vitamin A is found only in animal products, particularly liver, eggs and milk, but many fruits and vegetables such as carrots, mangoes and papaya contain carotenes, chemicals that the body can convert into vitamin A. Good sources of vitamin C are fruits and vegetables. The B complex is found in cereals, legumes, meat, poultry and dairy products.

Minerals are essential to structures such as bones and teeth (calcium) and processes such as energy transfer (iron) and functioning of the body and brain (iodine). We need comparatively large amounts of some minerals, such as calcium -found in peas and beans, milk, meat and cheese - and much smaller amounts of others, such as iron - found in meat, fish and shellfish, dark green leafy vegetables and nuts.

Because vitamins and minerals are usually needed in only small quantities they are called micronutrients.

Energy requirements and expenditure

Energy requirements
	Energy requirements are determined by body size, activity level and physiological conditions such as illness, infection, pregnancy and lactation. As body size increases, so does the total energy requirement; however, per body unit, the energy requirement decreases.

Energy expenditure
	The pattern of energy expenditure tends to change with age. Elderly people tend to use less energy, primarily as a result of reduced physical activity.

	FAO projects to combat undernourishment and malnutrition: training nutrition agents, Lesotho
	Children tending their school vegetable garden, Ecuador

Undernourishment and malnutrition

Over 800 million people, mostly in the developing world, are chronically undernourished, eating too little to meet minimal energy requirements. Millions more suffer acute malnutrition during transitory or seasonal food insecurity. Over 200 million children suffer from protein-energy malnutrition (PEM) and each year nearly 13 million under fives die as a direct or indirect result of hunger and malnutrition.

Malnutrition usually results from diets lacking specific nutrients but can also be caused by so-called "diets of excess".

PEM is most common among young children and pregnant women in the developing world. It is usually caused by energy-deficient diets (that may also lack protein) coupled with infections that raise nutrient requirements while limiting the intake and utilization of food. It is the prime cause of low birth weight and poor growth in the developing world where mothers themselves may have a legacy of low birth weight, stunted growth and anaemia.

Malnutrition can have serious effects, right from conception. Vitamin A deficiency is associated with increased child mortality, and is a prime cause of child blindness. Iodine deficiency leads to slow growth and mental development and to goitre. Anaemia, largely due to iron deficiency, is the most widespread nutritional problem, affecting 2 000 million worldwide. It can impede learning and productivity and is a leading cause of maternal mortality in developing countries. Calcium deficiency is a leading risk factor for osteoporosis, a condition where bones become fragile and brittle. Inadequate vitamin C can lead to scurvy and has been linked to poor absorption of iron and an increased risk of certain noncommunicable diseases.

Weight and health

The body mass index (BMI) gives an easy guide to what body weight is compatible with good health. BMI is calculated by dividing body mass (weight in kilograms) by the square of the person's height in metres. In adults the BMI should fall between 18.5 and 25. Up to 30 is considered overweight. Values above 30 indicate obesity.

Estimate of chronically undernourished in developing regions 1990-92

"Chronically undernourished" is defined as those whose estimated annual energy intake falls below that required to maintain body weight and support light activity.
The data cover 93 developing countries, accounting for 98.5 percent of the total population of the developing world.

Average daily energy supply per caput

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The FAO-WHO Consultative Group on Nutrition has determined that, on average, a daily diet of around 2 200 calories is sufficient to meet basic nutrition needs. Like all averages this conceals important differences. Boundaries of nations formed since 1990 (in former USSR, in former Yugoslavia, in former Czechoslovakia, Eritrea) are shown in grey.

Diet-related non-communicable diseases

	Malnutrition is caused by bad diet as well as lack of food.
	More than 800 million people, ever 13 percent of the world's population, do not get enough to eat.

Non-communicable diseases such as cardiovascular ailments, cancer, stroke, hypertension and diabetes have become more common since advances in medicine have reduced the impact of infectious diseases and life expectancy has increased. In the developing world, infectious diseases remain the prime cause of death, but deaths related to diet, activity and lifestyle are on the increase. In the developed world, by far the most common causes of death are non-communicable diseases.

Foremost among factors which contribute to the greater incidence of these diseases is the widespread change towards increasingly sedentary lifestyles. This, combined with dietary change (more fat, less fruit, vegetables and whole grains), can contribute to a wide range of chronic diseases that often lead to permanent disability and premature death.

The relationship between dietary intake, exercise and heart disease is specially strong. Studies show a clear connection between diets extremely high in fats, especially animal fats, and low in fruits and vegetables and an increased risk of obstruction of blood flow and hardening of the walls of the arteries. With the arteries constricted, the heart must work harder to pump blood through them. This extra stress often results in coronary heart disease.

Studies also indicate a direct relationship between diets rich in complex carbohydrates and fibre and a reduced risk of cardiovascular disease and certain cancers, particularly of the lower bowel. Eating fibre-rich foods assists bowel function.

For a healthy lifestyle: avoid tobacco, do not exceed a moderate alcohol intake and take exercise - sufficient to raise heartbeat - for 20 minutes, three to five times a week.