Describe the functions and food sources of fat, distinguish saturated from unsaturated fat, and explain the effects of eating too much or too little

What this dot point is asking

SEAB wants you to describe what fat does in the body, name the foods that supply it, tell saturated and unsaturated fat apart, and explain the effects of too much or too little. The key idea is balance: some fat is essential for energy, warmth, protecting organs and carrying certain vitamins, but too much, especially saturated fat, raises the risk of heart disease. Fat is also the most energy-dense nutrient, which is why fatty foods are easy to over-eat.

The answer

What fat is made of

Fat (also called lipid) is made of fatty acids and glycerol. Whether a fat is solid or liquid at room temperature, and how it affects health, depends on the kind of fatty acids it contains.

Functions of fat

Fat does several important jobs:

• A concentrated source of energy, giving about $37\ \text{kJ}$ (around $9\ \text{kcal}$) per gram, more than twice as much as protein or carbohydrate.
• Insulation, as a layer under the skin that helps keep the body warm.
• Protection of delicate organs such as the kidneys, which sit in a cushion of fat.
• Carrying the fat-soluble vitamins A, D, E and K, which the body can only absorb in the presence of fat.

Saturated and unsaturated fat

• Saturated fat has no double bonds in its fatty acids, is usually solid at room temperature, and comes mainly from animal foods such as butter, lard and fatty meat, plus palm oil and coconut oil. Eating too much raises blood cholesterol.
• Unsaturated fat has one or more double bonds, is usually liquid at room temperature, and comes from plant and fish sources such as olive oil, canola oil, nuts and oily fish. It is the healthier choice.

Food sources of fat

Fat is found in obvious foods such as cooking oil, butter, margarine and the visible fat on meat, and hidden in foods such as fried snacks, pastries, cakes, cheese and full-cream milk.

Too much or too little fat

Too much fat supplies a lot of energy, so excess leads to weight gain and obesity. Too much saturated fat in particular raises blood cholesterol, which can build up in the arteries and increase the risk of heart disease and stroke. Too little fat leaves the body short of energy, less able to keep warm, and unable to absorb enough of the fat-soluble vitamins A, D, E and K.

Examples in context

Example 1. The oil in a fried hawker dish. A plate of char kway teow is cooked with a generous amount of oil and often includes lard and Chinese sausage, making it high in fat, including saturated fat. The fat carries flavour and energy, but because fat gives 9 kcal per gram the dish is very energy-dense, which is why it is best enjoyed in moderation rather than every day.

Example 2. A healthier fat in a salad. Drizzling olive oil over a vegetable salad adds mainly unsaturated fat, which helps the body absorb the fat-soluble vitamins in the vegetables (such as vitamin A from carrots) while being a healthier choice than a creamy, saturated-fat dressing. This shows fat doing its useful job of carrying vitamins.

Try this

Q1. State two functions of fat in the body. [2 marks]

• Cue. Any two of: concentrated source of energy, insulation, protection of organs, carrying fat-soluble vitamins A, D, E and K.

Q2. Explain the difference between saturated and unsaturated fat and give one example of each. [3 marks]

• Cue. Saturated = usually solid, mainly animal (e.g. butter), raises cholesterol; unsaturated = usually liquid, plant or fish (e.g. olive oil), healthier.

Q3. A serving of food contains 12 g of fat. Calculate the energy it provides in kilocalories. [2 marks]

• Cue. Fat gives about 9 kcal per gram, so $12 \times 9 = 108$ kcal.