LONDON — Our understanding of fats — including which ones are actually good for us — is evolving.
There are four main types of fats in our foods: polyunsaturated, monounsaturated, saturated and transfats. Each has different chemical and physical properties.
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, high fat intake promotes fat deposits that 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.
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. 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 32.5 inches for women, 37 inches 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-mobilization process leads to fatty acids entering the bloodstream. Visceral fat is more rapidly mobilized 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 and 30 percent of a healthy mature woman’s body weight is fat — twice as much as men. If the level drops below about 18 percent, ovulation stops, but if it raises to very high levels — typically about 50 percent of her weight — it also results in infertility.
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 30 grams of fat are 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. 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. These raise low-density lipoprotein cholesterol in decreasing order of potency compared to carbohydrates 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.
10) Saturated fat intake is stable.
Food and nutrition policies have changed the food supply. In Britain, energy intakes of fat and saturated fatty acids respectively fell from 42 percent and 20 percent in the early 1970s to 35 percent and 12 percent 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 per 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.