Vitamins: Protecting and Regulating the Body

A child who stumbles at dusk because she cannot see in dim light, a teenager whose gums bleed when he brushes, a baby born with a damaged spine: each of these can be caused by the lack of a single vitamin, needed in amounts smaller than a grain of salt. Vitamins give no energy, but without them the body cannot use the energy, protein and minerals it takes in.

Vitamins are substances the body needs in very small amounts for growth, protection against disease and the control of body processes. Most cannot be made by the body, so they must come from food. In this lesson you will learn the two groups of vitamins, the job, sources and deficiency disease of each important vitamin, and how to keep vitamins in food when you store and cook it.

1Two groups of vitamins

Thirteen vitamins are recognised. They are grouped by what they dissolve in:

• Fat-soluble vitamins: A, D, E and K. They dissolve in fat, are absorbed with the fat in a meal, and are stored in the liver and body fat. Because they are stored, very large doses from supplements can build up and become poisonous.
• Water-soluble vitamins: the eight B vitamins and vitamin C. They dissolve in water, are not stored in large amounts, and extra is lost in urine, so they are needed every day. They are easily lost into cooking water.

2The main vitamins

3Keeping vitamins in food

Vitamins B and C are the easiest to lose. They are damaged by:

• Water: they dissolve into the cooking water, which is often thrown away. Cooking can reduce the vitamin C in vegetables by around 60%.
• Heat and time: long boiling and keeping food hot for hours destroy them.
• Air: vitamin C is lost from cut surfaces, so cut fruit and vegetables just before use.
• Alkali: adding baking soda (or other cooking soda) to beans or greens to soften them destroys thiamine and vitamin C.
• Storage: fruit and vegetables lose vitamin C as they wilt, so buy them fresh and use them soon.

Good practice: wash vegetables before cutting; cut them just before cooking; cook in a little water, with a lid, for a short time; use the cooking water in the sauce or soup; do not add soda. Fat-soluble vitamins A, D, E and K survive normal cooking better, but they need a little fat in the meal to be absorbed, so cook dark green leaves with a spoon of oil or serve orange sweet potato with a groundnut sauce.

Worked example: vitamin A from orange-fleshed sweet potato

Task: 100 g of raw orange-fleshed sweet potato contains about 709 µg (micrograms) of vitamin A activity. Cassava root contains about 1 µg per 100 g, and white-fleshed sweet potato very little. The recommended amount for a girl of 14 to 18 is about 700 µg a day. (a) Roughly how much orange-fleshed sweet potato gives this amount? (b) Explain why a family that swaps white sweet potato for the orange kind improves its diet at no extra cost.

Solution:

(a) Amount = 700 ÷ 709 × 100 = 98.7 g, so about 100 g, a small piece. (Cooking lowers the amount a little, so a normal serving easily covers it.)

(b) Both varieties give the same energy and cost about the same, but only the orange one supplies beta-carotene, which the body turns into vitamin A. The swap protects the family against night blindness and infection without extra spending. Eating it with a little fat, such as groundnut sauce or avocado, helps absorption.

Practice