Title: Fat Quality Is More Important Than Fat Quantity

 

Key words: dietary fat, fatty acids, phospholipids, cell membranes, obesity, CHD, coronary heart disease, smoking, alcohol, stress, exercise, triglyceride, hypertension, cholesterol, saturated fats, polyunsaturated fats, low density lipoprotein LDL, high density lipoprotein, HDL, fish oil, atherogenesis, eicosanoids, thrombosis, platelet, thromboxane, polyunsaturated fatty acid, PUFA, prostacyclin,  

 

Date: Sept 2006

 

Category: Macronutrients

 

Nutrimed Module: Type: Article

 

Author: Morgan, G

 

Fat Quality Is More Important Than Fat Quantity

Dietary fat is an essential nutrient, providing energy, essential fatty acids and fat-soluble vitamins which play an important structural and functional role within the body. As essential fatty acids and phospholipids they are integral constituents of cell membranes and fulfil many other vital roles, such as in the normal development and function of the nervous system and the skin. In spite of this it has become clear that the increased fat intake characteristic of populations in developed countries has been associated with a near epidemic of obesity and fat related health problems.

 

Foremost amongst these is the steep rise in cardiovascular heart disease (CHD) which now accounts for a third of all deaths in the UK. Epidemiological studies have highlighted several fat related nutritional markers predisposing to CHD, factors which potentiate the effects of smoking, alcohol, lack of exercise and stress. Raised blood cholesterol is the single most predictive factor between populations, other factors being the blood triglyceride level and hypertension (Mann 2000).

 

In an early study, Keys (Keys 1980) showed that a high percentage of saturated fats in the diet was a strong indicator of CHD. The same study also showed a cardioprotective effect of polyunsaturated fatty acids (PUFA), a finding repeatedly confirmed in other surveys (Thorogood 1987). As a result of his work Keys was able to develop a formula that described the cholesterol level as a function of the relative amount of saturated and polyunsaturated fats in the diet, thus enabling the risk of CHD to be quantified from a nutritional point of view. Alterations in the ratio of saturated and polyunsaturated fatty acids in the diet have repeatedly been shown to lower the markers of CHD such as cholesterol, triglyceride, low density lipoprotein (LDL) levels and elevate cardio-protective markers such as high density lipoprotein (HDL). Roche, for example, has demonstrated how long chain n-3 fatty acids from fish oil are able to markedly lower circulating triglyceride levels (Roche 1999). Intolerance to circulating fats in the late post-prandial period is characteristic of subjects going on to develop CHD: the association with diabetes and the pathology of atheroma has been explicated by Griffin (Griffin 1999) and others.

 

Fish oils help to suppress this process by lowering blood triglycerides and by helping the liver to convert very low density lipoprotein to large LDL particles rather than the small, dense LDL particles that produce dysfunctional and atherogenic effects on coming into contact with vascular endothelium. Fish oils, in aiding fat clearance, also reduce oxidative damage to LDL, thought to play an important role in atherogenesis. Elucidation of the genesis and mode of action of eicosanoids has, more recently, increased our understanding of the important role played by the essential fatty acids in platelet aggregation and thrombosis.

 

Arachidonic acid, the progenitor of thromboxane A2, is largely derived from animal PUFA. Its thrombogenic action is balanced by n-3 and n-6 PUFA, one of the most important of which is the eicosanoid prostacyclin PGI3. This is present in fish oil and explains part of the ability of fish oils to reduce the incidence of thrombotic episodes such as myocardial infarction (Rao 1983).

 

Numerous surveys charting the beneficial effects of dietary fat manipulation have confirmed initial reports of the beneficial effects of reducing saturated fats and increasing consumption of PUFA (see WHO report 1982). An extensive meta-analysis of the literature on the subject by Truswell has shown that cholesterol levels could be lowered by 30% and cardiovascular events by 13% (Truswell 1994). These are encouraging findings that justify the policy, for example, to encourage consumption of at least two portions of fatty fish per week. 

 

References

1. Mann J (2000) In: Human Nutrition and Dietetics, 10th edn. Churchill, London

2. Keys A (1980) Seven countries: a multivariate analysis of death and coronary heart disease. Harvard University Press, Cambridge, Mass USA

3. Thorogood M, et al. (1987) Plasma lipids and lipoprotein cholesterol concentrations in people with different diets in Britain. BMJ 295: 351-3

4. Roche HM (1999) Unsaturated fatty acids. Proc Nutr Soc 58: 397-401

5. Griffin BA (1999) Small, dense atherogenic LDL a silent risk factor. CVDL/Lipids Dialogue June 1999, Issue 9

6. Rao GHR, Radha E, White JG (1983) Effect of docosohexaenoic acid on arachidonic acid metabolism and platelet function. Biochemical and Biophysical Research Communication 117: 549-55 7. World Health Organisation Expert Committee (1982) Prevention of coronary heart disease. Technical Report Series. WHO, Geneva

8. Truswell AS (1994) Review of dietary intervention studies: effect on coronary events and on total mortality. Austr New Zeal J Med 24: 98-106