Our understanding of fats - including which ones are actually good for us - is evolving. We know for example that red meat and meat products, cakes and biscuits, which are rich sources of saturated fatty acids, are associated with an increased number of cardiovascular deaths. Conversely, nuts, oily fish and milk products, which are high in saturated fats, are associated with lower risk.
There are four main types of fats in our foods: polyunsaturated, monounsaturated, saturated and transfats. Each has different chemical and physical properties. Vegetable spreads and cooking oils - mainly rapeseed, sunflower, soybean and olive - usually contain the first two but relatively small amounts of saturated fat. But palm oil, which has a higher melting point and is now used in many products, is highly saturated.
Dietary advice, then, has moved away from the simplistic mantra that we should just eat less saturated fat, salt and sugar, towards a more discerning pattern that emphasises fruit, vegetables and low-fat dairy food, includes wholegrains, poultry, fish and nuts, and contains less red meat, sweets and sugar-containing beverages. But where do fats fit in? Here are ten things you may not know.
1. Fat is an energy food
Most of the energy in our diet comes from carbohydrates. But fat supplies between a quarter and two-fifths of an adult's energy intake and half for a newborn. In babies, a high fat intake promotes fat deposits which insulate against heat loss.
Adding fat to food can double its energy content. Removing fat, from products like meat and milk, can substantially reduce it. Fat provides 9kcal/g (kilocalories/gram) in energy compared with 3.75kcal/g, 4kcal/g and 7kcal/g for carbohydrates, protein and alcohol.
2. Less energy intake, bigger weight loss
Reducing energy intake rather than increasing physical activity is the most effective means of reducing body fat. This can be achieved by using lower fat versions of existing foods, trimming fat from meat and using oils sparingly. There is not much difference in fat content between grilled and fried meat. Restriction of energy intake also requires limiting the intake of carbohydrates and alcohol.
3. Where it is in the body matters
Excess accumulation of body fat is most harmful if it is in the abdominal cavity or liver and is causally linked to developing type 2 diabetes. The use of a waist measurement (more than 80cm for women 94cm for men) indicates central obesity and is useful for predicting risk of type 2 diabetes. Women have more subcutaneous fat stores than men, so men store this visceral fat around the mesenteric blood vessel in the abdomen. When energy stored in fat cells is released, the fat mobilisation process leads to fatty acids entering the bloodstream. Visceral fat is more rapidly mobilised than subcutaneous fat and can accumulate in the liver. Fat also accumulates in the liver if the intake of alcohol or sugar is high.
4. Body uses carbohydrate for fuel not fat
Obesity results from the excess accumulation of dietary fat in the body. Very little fat is made in the body from carbohydrates (including sugar) or alcohol because they are used as fuel in preference to fat. But if you have excess fuel on board you deposit it as fat because we have a limited capacity to store carbohydrates.
5. Women need fat for fertility
Body fat plays an important role in female fertility. Between 20-30% of a healthy mature woman's body weight is fat - twice as much as men. If the level drops below about 18%, ovulation stops but if it raises to very high levels - typically about 50% of her weight - it also results in infertility. A hormone called leptin is secreted by adipose (fat) tissue into the blood in proportion to the amount of fat it stores. The brain detects the blood leptin signal and this promotes ovulation when the level is high enough.
6. Some fatty acids are essential
We need certain polyunsaturated fatty acids, aptly named essential fatty acids (linoleic and linolenic acids), in our diet for healthy skin. These also contribute to maintaining cardiovascular health as well as brain and visual function. We mainly get these from vegetable oils, nuts and oily fish.
7. We need fat to absorb some vitamins
About 30g of fat is required every day to promote the absorption of fat-soluble vitamins A, D, E and K, which we also get from fatty foods. Vegetable oils are an important source of vitamin E and oily fish is the best dietary source of vitamin D. Provitamins are substances that can be converted within the body into vitamins. And adding a little oil to green vegetables and carrots actually improves the absorption of carotene (pro-vitamin A).
8. Big scale effect on blood cholesterol
A population's average blood cholesterol level is a major determinant of coronary heart disease risk. Trials show the replacement of saturated fatty acids with polyunsaturated fatty acids lowers blood cholesterol and reduces the incidence of disease but not mortality. These days high cholesterol levels are more effectively treated with statins, but the public health goal is to reduce average cholesterol levels.
9. Not all saturated fat is bad
Not all saturated fats increase blood cholesterol. The cholesterol raising effects are confined to lauric, myristic and palmitic acids (the latter is found in palm oil). These raise low-density lipoprotein cholesterol (LDL-C) in decreasing order of potency compared to carbohydrates (including all types of starches and sugars) or unsaturated fatty acids. It is generally more effective to lower cholesterol by replacing saturated fatty acids with oils rich in monounsaturated (olive, rapeseed) or polyunsaturated fatty acids (soybean, sunflower oil) than lowering carbohydrates. For example, replacing butter or lard with olive oil as your main source of fat can lower LDL-C by about 10%.
10. Saturated fat intake is stable
Food and nutrition policies have changed the food supply. In the UK, energy intakes of fat and saturated fatty acids respectively fell from 42% and 20% in the early 1970s to 35% and 12% by 2000, where they have remained since. Between 1987 and 2000, average blood cholesterol levels fell from 5.7mmol/L to 5.2mmol/L. Despite the continuing rise in obesity and diabetes, death from cardiovascular disease fell from 141 to 63/100,000 of the population between 1994-97 and 2009-11, owing mainly to better treatment and improvements in control of risk factors such as blood pressure, smoking and cholesterol.
Tom Sanders is the Head of Diabetes & Nutritional Sciences and Professor of Nutrition & Dietetics at King's College London. He does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.