Food, nutrients, health and fitness

Food, nutrition and health are like two sides of one coin: what you eat decides the nutrients your body gets, and those nutrients decide your health. This chapter defines the key words.

Food is anything, solid or liquid, that we eat and that, once digested and absorbed, gives the body nutrients. Food has four jobs:

• it supplies energy for work and play;
• it builds and repairs body tissues;
• it protects us from disease;
• it regulates body functions such as heartbeat and temperature.

Nutrition is the science of foods and nutrients and their action in the body: from ingestion (eating) through digestion, absorption and metabolism (use in the body) to excretion. Nutrition also has social, psychological and economic sides: a well-nourished person feels better, works and studies better, and can earn and contribute more.

Nutrients are the substances in food the body needs: carbohydrates, proteins, fats, minerals, vitamins, water and fibre.

• Macronutrients are needed in large amounts: carbohydrates, proteins and fats (with water and fibre also needed in quantity).
• Micronutrients are needed in small amounts: vitamins and minerals such as iron, iodine and calcium. Small does not mean unimportant: too little iodine, for example, causes goitre.
• Energy is measured in kilocalories (kcal). One gram of carbohydrate or protein gives 4 kcal; one gram of fat gives 9 kcal. So a snack with 30 g carbohydrate, 5 g protein and 10 g fat gives 120 + 20 + 90 = 230 kcal.

A balanced diet includes a variety of foods in adequate amounts and the correct proportions to meet the day's needs for all nutrients, with a little extra as a margin of safety for days when intake falls short, such as during illness or a fast. A balanced diet keeps body weight right for one's height and protects health.

RDA (Recommended Dietary Allowance) is the amount of a nutrient recommended per day for a particular group (by age, sex and activity). RDA = requirement + margin of safety.

Health. The World Health Organization defined it in 1948 as a state of complete physical, mental and social well-being, and not merely the absence of disease or infirmity. A person with no illness but who is always anxious and lonely is not fully healthy.

Mental health means being able to use one's thinking and emotional abilities, function in society and meet everyday demands. Signs of good mental health include feeling capable, handling normal stress, keeping satisfying relationships, living independently and being able to bounce back after setbacks. Simply not having a recognised mental disorder does not by itself prove good mental health.

Physical fitness comes from regular exercise, proper diet and adequate rest. It is the body's ability to work efficiently in work and leisure, resist disease and meet emergencies. It can be general (everyday health) or specific (fitness for a particular sport or job). Its five components are:

• aerobic fitness (heart and lungs supplying oxygen during sustained activity, like a long cycle ride);
• muscular strength (how much force a muscle can exert);
• muscular endurance (how long muscles can keep working);
• flexibility (how freely joints move);
• body composition (the proportion of fat to lean tissue).