Nutrition basics

Food supplies energy and the raw materials the body uses to build and repair itself. Energy in food is measured in kilocalories, commonly just called calories, or in kilojoules. Nutrients are grouped into macronutrients, needed in large amounts, and micronutrients, needed in small amounts.

Nutrition science is complicated because diets are hard to study precisely and individual foods interact. Headlines about single superfoods or miracle nutrients should be treated with caution; the strongest evidence concerns overall eating patterns.

Carbohydrates are the body's main source of energy. Digestion breaks them down into sugars, mostly glucose, which cells use as fuel. The brain relies heavily on glucose.

Not all carbohydrates are alike. Whole grains, beans, vegetables and fruit come packaged with fibre, which slows digestion, supports gut health and helps people feel full. Refined carbohydrates such as white bread and sugary drinks are digested quickly. Health authorities recommend limiting added sugars, which are linked to tooth decay and excess calorie intake.

Proteins are made of amino acids, which the body uses to build muscles, enzymes, hormones and antibodies. Of the twenty amino acids in human proteins, nine are essential, meaning they must come from food. Meat, fish, eggs and dairy provide all of them, and a varied mix of plant foods, such as beans, lentils, grains and nuts, can too.

Fats provide concentrated energy, help absorb vitamins A, D, E and K, and form part of every cell membrane. Unsaturated fats, found in olive oil, nuts, seeds and oily fish, are generally considered beneficial for heart health. Evidence links replacing saturated fats with unsaturated fats to lower cardiovascular risk, and artificial trans fats are harmful enough that many countries have restricted them.

• Vitamin C supports collagen formation; severe deficiency causes scurvy, once common among sailors.
• Vitamin D helps the body absorb calcium and is made in skin exposed to sunlight.
• Iron is needed to carry oxygen in red blood cells, and deficiency is one of the most common nutritional problems worldwide.
• Iodine is essential for thyroid hormones; adding it to salt has greatly reduced deficiency.
• Folate is especially important before and during early pregnancy to help prevent birth defects.
• Calcium builds bones and teeth and is involved in muscle and nerve function.

Despite disagreements over details, major health organisations broadly agree on the basics: eat plenty of vegetables, fruit, legumes, whole grains and nuts; choose unsaturated fats over saturated ones; limit added sugars, salt and highly processed meats; and drink water rather than sugary drinks.

Traditional eating patterns such as the Mediterranean diet, rich in vegetables, olive oil, fish and legumes, are associated with lower rates of heart disease in many studies. Different cultures achieve healthy diets with very different foods, and there is no single perfect diet for everyone.

Body weight is influenced by the balance between energy eaten and energy used, but that balance is shaped by many factors, including genetics, sleep, stress, hormones, medications, the food environment and physical activity. This is why weight management is harder than a simple calorie equation suggests.

Nutritional needs vary with age, sex, pregnancy, activity level and health conditions. People with specific medical conditions, allergies or restrictive diets should seek advice from a doctor or registered dietitian rather than relying on general guidance.