Fats and Health

Fats are a concentrated source of energy in the diet, supplying 9 kcal per gram no matter what the composition of the constituent fatty acids. Thus for many developed populations where food is plentiful and activity levels not as great as they might be, high-fat, energy-dense, palatable

foods may contribute to weight gain. However, at the opposite end of the scale low-fat diets (less than 10% energy from fat) may result in malnutrition if given to young children, as such diets are bulky and the child may simply not be able to eat enough to satisfy energy requirements.

A certain amount of fat is needed to supply the essential fatty acids and also to allow absorption of fat soluble vitamins A, D, and E – it has therefore been suggested that the levels of fat in the adult diet should not fall below 20% of the total energy intake. Epidemiological evidence has suggested that high fat intakes, especially saturated fatty acids, are associated with a higher incidence of atherosclerosis (hardening of the arteries) and coronary heart disease. In population

studies there is an association between serum cholesterol levels and coronary heart disease death rates. Cholesterol in the blood exists in several forms, the most important being carried in the blood on low-density lipoproteins and high-density lipoproteins....

These are therefore usually referred to as low-density lipoprotein cholesterol and high-density lipoprotein cholesterol, respectively. Total cholesterol is the sum of these two types (plus a little very low-density lipoprotein cholesterol). High saturated fat intakes correlate with higher serum cholesterol levels, especially the low-density lipoprotein type. In general saturated fatty acids raise serum low-density lipoprotein cholesterol, polyunsaturated fatty acids lower it, and

monounsaturated fatty acids have little effect. 

However, consumption of monounsaturated fatty acids appears to maintain levels of high-density lipoprotein cholesterol, which removes cholesterol from the arteries and helps protect against coronary heart disease. 

It is thought that the process of atherosclerosis begins with the oxidation of low-density lipoprotein cholesterol by free radicals; this is then taken into the lining of the arteries by scavenger cells which form lipid-loaded cells called ‘foam cells’ that accumulate cholesterol and form fatty streaks, narrowing the arteries. The role of polyunsaturated and monounsaturated fatty acids in the formation of these streaks is still controversial. 

However, recent research suggests that if polyunsaturated fatty acids form part of the cell membranes this may render the low-density lipoprotein cholesterol more susceptible to oxidation, whereas monounsaturated fatty acids appear to convey a protective effect. Fats containing monounsaturated fatty acids may also have beneficial effects by reducing platelet aggregation, which is important in the production of the blood clots that block the arteries in coronary heart disease.

Coronary heart disease is a complex multifactorial disease, and other factors besides dietary fat intake will be important. The presence of antioxidants in the diet and adequate intakes of certain B vitamins (folate and B12) may also be important, but at present most authorities advise reduction of total fat to less than 30% of energy,with 10% or less from saturated fatty acids and polyunsaturated fatty acids, and the remainder made up from foods supplying monounsaturated fatty acids. Fish oils containing very long chain n-3 fatty acids eicosapentaenoic acid and docosahexaenoic acid appear to have a protective effect against coronary heart disease. 

Recommendations to consume two portions per week of oily fish such as mackerel, salmon, and herring are therefore included in government guidelines to prevent coronary heart disease.

These fatty acids are also available as dietary supplements and will be discussed later. High dietary fat intakes have also been associated with some cancers, e.g. post-menopausal breast cancer, prostate cancer, and bowel cancer, but it is difficult to dissociate the effects of fat from the effects of obesity.

However, the guidelines for the prevention of heart disease are probably also applicable to the prevention of cancer. Dietary cholesterol intake is much less important in raising cholesterol levels than saturated fat cholesterol is an essential part of cell membranes and is also important in the production of hormones and emulsifying agents in the body. Dietary intake represents about 10% of the amount produced daily in the body. However, when low saturated fat diets are recommended, these will result in lower consumption of cholesterol as the high-fat animal foods containing it will be restricted.