Nutrition

What nutrition is

Nutrients are the chemical compounds in food that the body uses to function and stay healthy.^[2] MedlinePlus, the US National Library of Medicine’s health site, groups them as proteins, carbohydrates, fats, vitamins, minerals and water, and says the body needs them to grow, repair tissues and stay healthy.^[1] The World Health Organization (WHO) links better nutrition with healthier mothers and children, stronger immune systems, a lower risk of diseases such as diabetes and heart disease, and longer lives.^[15]

Digestion is how the body breaks food down into nutrients, which it then uses for energy, growth and cell repair.^[2] Metabolism is the process of getting or making energy from food.^[2] The organs that do this work belong to the digestive system, part of Anatomy on the map.

Energy and calories

A calorie is a unit of energy in food, supplied by carbohydrates, fats, protein and alcohol.^[2] Carbohydrates, proteins and fats are called macronutrients, and the CDC (US) measures food energy in kilocalories (kcal), the unit usually just called calories.^[12]

The NHS (UK) gives average figures of around 2,000 calories (8,400 kilojoules) a day for women and 2,500 (10,500 kilojoules) for men, while noting that needs vary with things like how active a person is.^[8] MedlinePlus (US) stresses that no single calorie amount works for everyone.^[1]

The three macronutrients

Carbohydrates. The body breaks carbohydrates down into glucose (blood sugar), the main fuel for its cells, tissues and organs, and can use it at once or store it in the liver and muscles.^[3] MedlinePlus (US) gives an average of 45 to 65% of daily calories from carbohydrates.^[3] The WHO gives a different range: roughly 45 to 75% of daily energy, from a variety of unrefined carbohydrates.^[13]

Protein. The WHO explains that proteins build structures such as muscle and working molecules such as hormones and enzymes.^[13] The WHO considers 10–15% of daily energy from protein generally enough for adults, from a mix of animal and plant sources.^[13]

Fats. Fats provide energy and help the body absorb vitamins.^[2] The WHO recommends that no more than 10% of energy come from saturated fat and no more than 1% from trans fat.^[13] The NHS (UK) calls unsaturated fats, such as olive and sunflower oils, healthier fats, but notes that all fats are high in energy.^[8]

Fibre and water

Dietary fibre is a type of carbohydrate found in plants.^[4] The CDC (US) says fibre helps keep digestion healthy, helps people feel full for longer and helps control blood sugar.^[10] The WHO suggests at least 25 g a day of fibre from food for everyone over 10.^[13]

Fluid comes from both drinks and food, and how much a person needs depends on body size, activity and the weather.^[2] As a guide, the NHS reports that the UK government recommends 6 to 8 cups or glasses a day.^[8] The WHO recommends adults keep salt below 5 g a day (2 g of sodium).^[13]

Vitamins and minerals

Micronutrients are vitamins and minerals, needed only in very small amounts.^[16] Apart from vitamin D, the body cannot make them, so most must come from food.^[11] Examples of their roles, from the CDC (US):

• Iron helps make red blood cells that carry oxygen.^[11]
• Vitamin A helps protect eyesight and keep the immune system strong.^[11]
• Vitamin D helps the body absorb calcium, and the body can make it from sunlight.^[11]
• Iodine is needed to make thyroid hormones, which control metabolism.^[11]
• Folate is a B vitamin that helps the body make new cells.^[11]

What health bodies recommend

The NHS (UK) uses the Eatwell Guide, which shows how much of what people eat overall should come from each of 5 main food groups.^[8] It advises at least 5 portions of fruit and vegetables a day, and says starchy foods should make up just over a third of what people eat.^[8]

In the US, MedlinePlus lists eating a variety of foods, including plenty of vegetables, fruits and whole grains, and limiting salt, added sugars, alcohol and saturated fat.^[1] The CDC says most people in the US need more nutrient-dense foods and less highly processed food, added sugar, added sodium and refined carbohydrate.^[10]

The WHO, writing for all countries, bases a healthy diet on four principles: getting enough of every nutrient, balancing energy in and out, limiting foods that may harm health, and eating a wide variety of foods.^[13] It recommends that free sugars stay below 10% of daily energy and suggests over-10s eat at least 400 g of fruit and vegetables a day.^[13]

Malnutrition in all its forms

The WHO defines malnutrition as deficiencies, excesses or imbalances in what a person takes in, covering undernutrition, missing vitamins or minerals, overweight and obesity.^[14] Stunting means being too short for one’s age, the result of long-term or repeated undernutrition; wasting means being too thin for one’s height, usually after recent severe weight loss.^[14]

The WHO estimates that in 2022, 149 million children under 5 were stunted, 45 million were wasted and 37 million were overweight or living with obesity.^[14] In the same year, 2.5 billion adults were overweight, including 890 million with obesity, while 390 million were underweight.^[14] The WHO calls this mix of undernutrition and overweight a double burden, found especially in low- and middle-income countries.^[15]

The WHO classifies adult overweight and obesity using body mass index (BMI), a person’s weight in kilograms divided by the square of their height in metres: overweight is a BMI of 25 or more, obesity 30 or more.^[14]

Needs across life stages

MedlinePlus (US) says nutritional needs depend on age, sex, activity level and health.^[1] The WHO recommends feeding infants only breast milk for their first 6 months, then adding a variety of safe, nutrient-rich foods.^[13] In pregnancy, MedlinePlus explains that more of many nutrients is needed, and the CDC (US) recommends that all women who could become pregnant get 400 micrograms of folic acid daily.^{[7, 11]} MedlinePlus also notes that older adults may need fewer calories while still needing enough nutrients.^[6]

Common misconceptions

“Eating well means following a strict diet.” MedlinePlus (US) says a strict diet is not needed, and that healthy eating can look different depending on preferences, culture and budget.^[1]

“Enough calories means enough nutrition.” MedlinePlus explains that someone can get enough calories but still lack vitamins or minerals.^[5]

“Supplements can stand in for food.” MedlinePlus describes supplements as a complement to a healthy diet, not a replacement.^[1]

“Honey doesn’t count as sugar.” The CDC (US) counts honey as an added sugar, and the WHO includes the sugars in honey among free sugars.^{[10, 13]}

What this page does not cover

This is an overview: it does not cover diets for medical conditions, weight loss, sports nutrition, supplement doses, eating disorders or food safety, and it gives no dietary advice. People with special dietary needs are pointed by the NHS towards a doctor or registered dietitian.^[9]

Going further

See Anatomy for the digestive system, Immune System for how the body fights infection, Medicine for the wider world of health care, and Body & Health for the rest of this region.