The major source of energy in our diets is carbohydrates; the other is lipids, which comprise fats and oils. Lipids are a group of substances made up of carbon, hydrogen and oxygen but in different ratios from those found in carbohydrates.
They have more carbon and hydrogen in proportion to their oxygen. The lower amount of oxygen in relation to carbon and hydrogen results in fats being a more concentrated source of energy than carbohydrates.
One gramme of fat supplies nine kilocalories of energy while one gramme of carbohydrates supplies four kilocalories.
A class of lipids called triglycerides provide most of the energy dietary fats. Triglycerides account for approximately 98 per cent of the lipids in food.
They are made up of two components, that is, glycerol and fatty acids. The fatty acids vary in length of their chain and in the degree of saturation.
It is for this reason that words like short, medium or long-chain fatty acids and saturated or unsaturated fats are used in the categorisation of dietary fats.
Short-chain fatty acids contain two to four carbon atoms and are very rare. Medium-chain fatty acids contain six to 12 carbon atoms and are more soluble than the long-chain fatty acids, hence they are more easily absorbed. Long-chain fatty acids contain 14 or more carbon atoms and are predominant in food.
Fats have for a long time been associated with a number of health problems. Thus, to make healthy food choices, it is important to know the different dietary fats and oil as well as their respective sources. Dietary fat/oils are classified as follows:
They are found mainly in foods of animal origin. However, coconut and palm oils also belong to this category. They are solid at room temperature with the exception of coconut and palm oils.
They adversely affect serum cholesterol levels. Foods containing saturated fat include high-fat dairy products such as full-fat cheese, ice cream, whole milk, milk and sour cream; high-fat meats include regular ground beef, bologna, hot dogs, sausage, bacon and spareribs; lard, butter, fatback and salt pork, cream sauces, gravy made with meat drippings, chocolate, palm oil and palm kernel oil, coconut and coconut oil and poultry (chicken and turkey) skin.
This is another classification of dietary fats. These fats are divided into trans-unsaturated fats, monounsaturated fats (MUFA) and polyunsaturated fats (PUFA).
Like saturated fat, trans fats tend to increase blood cholesterol levels. Trans fats are produced when liquid oil is made into a solid fat through the process called hydrogenation.
Hydrogenation refers to a chemical process by which hydrogen is added to unsaturated or polyunsaturated fats, making them more saturated (solids) and more resistant to oxidation (rancidity).
For example, when hydrogen is bubbled through a vegetable oil, the hydrogen converts the unsaturated oil to saturated fat.
Food sources of trans fats include snacks (crackers) and baked foods (muffins, cookies and cakes) with hydrogenated oil or partially hydrogenated oil, margarines, shortening and some fast food items such as French fries, locally known as chips.
Monounsaturated fats are usually called ‘good or healthy’ fats because they can lower the Low Density Lipoproteins (LDL), which are soluble proteins that combine with and transport fat in the blood stream. LDL are composed of cholesterol.
They circulate throughout the body, making their contents available to all cells. As the cells take the dietary triglycerides from them, they pick cholesterol.
To include more monounsaturated fats in the diet, use peanut butter instead of butter, margarine or shortening when cooking.
Sprinkling a few nuts or sesame seeds on a salad is also another easy way to eat more monounsaturated fats. Nuts and oils are, however, high in calories, like all fats. If trying to lose or maintain your weight, eat small portions of these.
The food sources of monounsaturated fats include avocado, canola, nuts like almonds, cashews, pecans, peanuts, olive oil and olives, peanut butter and peanut oil, and sesame seeds.
Polyunsaturated fats are also ‘healthy’ fats. It is recommended that you include these in your diet as well as monounsaturated fats.
Food sources of polyunsaturated fats are corn oil, cottonseed oil, safflower oil, soybean oil, sunflower oil, walnuts, pumpkin or sunflower seeds and mayonnaise salad dressings. A high-fat food, mayonnaise is often thought to be unhealthy.
It is mostly fat, and therefore calorie-dense, so it is easy for calories and fat to quickly add up when you are not paying attention to portion sizes.
However, while mayonnaise is made almost entirely of fat, it mostly unsaturated fat, which is a healthier fat. Fat is an essential nutrient.
In the body, fats provide insulation, protect internal organs from mechanical damage, are precursors of prostaglandins and they promote absorption of fat-soluble vitamins A, D, E and K. In the diet, fat is a source of energy. It also provides satiety and palatability in foods and is a carrier of fat-soluble vitamins.
Health problems associated with fat focus on mainly two main issues, that is, too much dietary fat and too much dietary fat coming from animal food sources.
Too much fat in the diet provides excessive kilocalories, more than required for immediate energy needs. Excess fat is stored as adipose tissue, thereby increasing body weight.
This increased body weight is associated with health problems such as diabetes, hypertension and heart diseases. On the other hand, diets containing excess saturated fat and cholesterol, which come from animal sources, have been associated with atherosclerosis.
People who consume liquid fat and oils have lower blood cholesterol levels than those who eat solid animal fats. Polyunsaturated fats lower blood cholesterol levels.
The general recommendation is a limited total fat intake to approximately 30 per cent of total calories or less and the intake of saturated fats should also be reduced to less than 10 per cent of total calories.
Ms Ndung’i is based at the Department of Human Nutrition, Egerton University.