Distinguish fat-soluble and water-soluble vitamins, state the functions and sources of key vitamins, and explain deficiency effects

What this dot point is asking

The syllabus wants you to group vitamins by whether they dissolve in fat or water, to know the function, source and deficiency of the key ones, and to explain why some vitamins are easily lost when food is prepared and cooked. The central idea is that vitamins are needed only in tiny amounts but are essential, and that water-soluble vitamins are far more fragile than fat-soluble ones.

The answer

Two groups: fat-soluble and water-soluble

Fat-soluble vitamins are A, D, E and K. They dissolve in fat, are carried in fatty foods, and can be stored in the body (mainly the liver). Because they are stored, a daily supply is not essential, but a large excess can build up and be harmful.

Water-soluble vitamins are the B group and vitamin C. They dissolve in water, are not stored well, and any excess is excreted in urine. This means a regular supply is needed, and they are easily lost into cooking water and destroyed by heat.

Key vitamins to know

Vitamin A keeps eyes healthy (especially night vision) and skin healthy. Sources: liver, carrots, dark green vegetables, milk. Deficiency causes poor night vision.

Vitamin D helps the body absorb calcium for strong bones and teeth. Sources: oily fish, egg yolk, and the action of sunlight on the skin. Deficiency causes rickets in children (soft, bent bones).

Vitamin C helps absorb iron and keeps skin, gums and blood vessels healthy; it aids wound healing. Sources: citrus fruit, guava, peppers, green vegetables. Deficiency causes scurvy (bleeding gums, slow healing).

The B group helps release energy from carbohydrate in the cells and keeps the nervous system healthy. Sources: wholegrains, meat, eggs, milk, green vegetables.

Why cooking destroys some vitamins

Water-soluble vitamins are the most vulnerable. They dissolve out of food into cooking water (leaching), and vitamin C in particular is destroyed by prolonged heat, by exposure to air, and by bicarbonate of soda. So boiling vegetables for a long time in lots of water and then discarding the water loses a large share of their vitamin C and B vitamins. Fat-soluble vitamins are more stable and survive normal cooking better.

Keeping vitamins in

To conserve vitamins: prepare vegetables just before cooking, cook them quickly in a small amount of water (steam or stir-fry), serve them straight away, and avoid keeping food hot for a long time. Using the cooking water in a soup or sauce recovers some of the leached vitamins.

Examples in context

Example 1. Fresh guava as a vitamin-C source. Guava, widely eaten in Singapore, is exceptionally high in vitamin C, more than many citrus fruits. Eaten raw it delivers its full vitamin C, helping iron absorption from a meal, which is useful for someone at risk of iron-deficiency anaemia.

Example 2. Steamed versus long-boiled kailan. Kailan steamed briefly keeps most of its vitamin C and stays bright green, while the same kailan boiled hard for ten minutes loses much of its vitamin C into the discarded water and turns dull. The science of water-soluble vitamins directly guides the better cooking choice.

Try this

• Cue. Name the four fat-soluble vitamins and explain why a daily supply is not essential. Recall A, D, E, K, and that they are stored in the body, mainly the liver.
• Cue. State the function, a source and the deficiency disease for vitamin D. Recall it aids calcium absorption for bones, from oily fish, egg yolk or sunlight, with rickets as the deficiency.
• Cue. Explain two reasons boiling vegetables for a long time lowers their vitamin C. Link it to vitamin C leaching into the water and being destroyed by prolonged heat.