Human Nutrition

Diet and Nutrients

The body needs seven kinds of dietary component: carbohydrates, fats and oils, proteins, vitamins, mineral ions, fibre and water. This page covers the three that give energy or build the body.

What makes a diet balanced

The amounts a person needs change with age, sex and activity. A growing child needs more protein for each kilogram of body mass than an adult, because the body is building new tissue. A person who is very active needs more energy-giving food than someone who sits all day.

A diet with too little of a nutrient, or too much of it, is not balanced. Eating more energy than the body uses is stored as fat.

Why each nutrient is needed

Carbohydrates are the main source of energy. Fats and oils also supply energy, and a layer of fat under the skin reduces heat loss. Proteins are the building material for growth and repair, because tissues such as muscle are made of protein.

Worked example (matching nutrient to need): A teenager is growing fast and has torn a muscle. Which nutrient is most important for growth and repair, and name one source. Step 1. Growth and repair of tissues is the job of protein. Step 2. Sources include meat, fish, eggs, beans and milk. Step 3. Carbohydrate and fat mainly supply energy, so they are not the answer. Answer: Protein, e.g. eggs.

Vitamins, Minerals, Fibre and Water

Vitamins and mineral ions are needed in small amounts, but a shortage causes disease. Fibre and water are not nutrients that give energy, yet the body cannot work without them.

Deficiency diseases

A deficiency disease is caused by a lack of a nutrient in the diet. Scurvy is caused by too little vitamin C, so a diet with no fresh fruit or vegetables leads to bleeding gums and slow healing of wounds. Rickets is caused by too little vitamin D, or calcium, so children’s bones do not harden properly.

Worked example (naming the cause): A child who spends little time outdoors and eats no oily fish or eggs develops soft, bent leg bones. Name the disease and the cause. Step 1. Soft, bent bones in a child are the signs of rickets. Step 2. Rickets is caused by a lack of vitamin D, which helps absorb calcium for hard bones. Answer: Rickets; lack of vitamin D.

The Digestive System

The alimentary canal is a long tube from mouth to anus. Other organs, joined to it, add juices that help digestion.

Worked example (naming the process): Name the process in which cells take up glucose and use it for respiration. Step 1. Uptake and use of nutrients by cells is assimilation. Step 2. Absorption only moves nutrients into the blood. Answer: Assimilation.

Physical Digestion and Teeth

Teeth and the stomach cut and crush food. This changes the size of the pieces, not the molecules in them.

Structure of a tooth

A tooth is covered by hard enamel. Beneath it is dentine, and in the centre is the pulp, which holds the nerves and blood vessels. Cement fixes the root in the bone of the jaw, and the teeth are embedded in bone and the gums.

The stomach

The muscular wall of the stomach churns the food, which is physical digestion.

ExtendedBile and emulsification

Bile, made in the liver, emulsifies fats and oils: it breaks large drops into many tiny droplets. This increases the surface area for lipase, so chemical digestion is faster.

Chemical Digestion

Enzymes in the digestive juices break large molecules into small ones. Each type of food molecule has its own enzyme.

Hydrochloric acid

The stomach makes hydrochloric acid, part of gastric juice. It kills harmful microorganisms in food and provides an acidic pH in which the stomach enzymes work best.

Worked example (naming the products): A meal of chips is rich in starch. Name the enzyme that digests it in the mouth and the type of molecule it makes. Step 1. Starch is digested by amylase, which is in saliva. Step 2. Amylase breaks starch down into simple reducing sugars. Answer: Amylase; simple sugars.

Digestion in Detail

This page is Extended content: the digestion of starch and protein in more detail, and the second job of bile.

ExtendedDigestion of starch

Amylase breaks starch down to maltose. Maltase, on the membranes of the cells lining the small intestine, then breaks maltose down to glucose, the sugar that is absorbed.

ExtendedDigestion of protein

Pepsin breaks down protein in the acidic conditions of the stomach. Trypsin breaks down protein in the alkaline conditions of the small intestine. Both are proteases.

ExtendedBile neutralises stomach acid

Bile is an alkaline mixture. It neutralises the acidic mixture of food and gastric juices entering the duodenum from the stomach, giving a suitable pH for the enzymes of the small intestine.

Worked example (why bile is needed): Food leaving the stomach is acidic, but trypsin works in alkaline conditions. How does the duodenum solve this? Step 1. The mixture entering the duodenum is acidic. Step 2. Bile is alkaline, so it neutralises the acid. Step 3. This gives a suitable pH for trypsin and the other enzymes. Answer: Alkaline bile neutralises acid.

Absorption

Once food is digested, the small molecules must pass into the blood. This happens in the small intestine.

Where absorption happens

The small intestine is the region where nutrients are absorbed. Most water is absorbed here too, but some is also absorbed from the colon. What cannot be digested or absorbed passes to the rectum and is egested as faeces.

ExtendedVilli and microvilli

The lining of the small intestine is folded into tiny finger-like villi, and each lining cell has microvilli. These greatly increase the internal surface area for absorption.

The wall of a villus is one cell thick, so absorption is fast. Capillaries carry glucose and amino acids away in the blood, and a lacteal carries fatty acids and glycerol.

Worked example (where is it absorbed?): Name the part of the alimentary canal where most nutrients are absorbed, and the part that absorbs the remaining water. Step 1. Nutrients are absorbed mainly in the small intestine. Step 2. Most water is absorbed there as well. Step 3. The remaining water is absorbed in the colon. Answer: Small intestine; colon.

Following a Meal

Follow a cheese sandwich through the body to see where each process happens.

The nutrients that reach the blood are then carried to the cells, where they are assimilated: glucose is used in respiration, amino acids build proteins, and fatty acids and glycerol build fats.

Worked example (tracing a nutrient): In which organs is the starch in bread digested? Step 1. Amylase from the salivary glands acts in the mouth. Step 2. Amylase from the pancreas acts in the small intestine. Answer: Mouth and small intestine.