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CHD: is dietary cholesterol reduction necessary?

Juliet Gray
BSc PhD SRD RPHNutr
Consultant Nutritionist and Dietitian Registered Public Health Nutritionist

The prevention of cardiovascular disease (CVD) continues to be a major public health issue in the UK. CVD mortality rates in the UK remain among the highest in the world, responsible for just under 238,000 deaths in 2002.(1) Coronary heart disease (CHD) is still the most common cause of death in the UK (117,000 deaths in 2001) and an important cause of much illness and disability.(1) Although deaths from CHD have been declining since the 1970s, more than one in five men and one in six women currently die from the disease.(1)
Since the late 1980s, it has been generally accepted that the key dietary recommendation for reducing CHD risk is the reduction in saturated fatty acids (saturates) intake in order to reduce circulating cholesterol levels, and that for the majority of people a reduction in dietary cholesterol per se is unnecessary.(2) However, there is still a commonly held belief, among both the public and some health professionals, that a reduction in dietary cholesterol (and therefore specifically a restriction on egg intake) is necessary to reduce blood cholesterol levels and prevent CHD. The reason for this confusion is threefold. First, in the early years of research into lipids and heart disease, the focus was very much on the role of both dietary saturates and dietary cholesterol in the aetiology of the disorder, and for many people this message has stuck.(3) Secondly, because cholesterol has become synonymous with risk of heart disease, there is confusion in the minds of the public concerning the relative roles of blood and dietary cholesterol. Thirdly, there are differences between the advice given as public health recommendations for the prevention of CVD/CHD and the more specific advice tailored for patients with familial dyslipidaemias and diabetes, some of whom may indeed need to restrict their dietary cholesterol intake. This article will discuss dietary cholesterol intake in relation to other dietary approaches to CVD risk reduction in a public health context.

Cardiovascular risk factors
It is well established that CVD risk is determined by a plethora of interacting factors, some of which are nonmodifiable, such as genetic predisposition, age and gender, and others that are modifiable, including abnormal blood lipid levels, hypertension, obesity and type 2 diabetes.(4) These factors are recognised as being associated with aspects of lifestyle and behaviour, including smoking, physical activity, alcohol intake and diet. A raised serum cholesterol level was the first evidence-based risk factor for CHD development, and modification of this factor by both dietary and pharmacological means has been and continues to be an important focus of both primary and secondary prevention strategies. However, a recently published report by a British Nutrition Foundation (BNF) task force serves to highlight not only the wide range of other factors that are emerging as potentially modifiable risk factors for CVD but also the complexities of their interactions.(5) Interestingly, although the new BNF report mentions dietary cholesterol in the context of the recommendations of the UK government's 1994 COMA report(2) and recommendations from the USA, it makes no further reference to dietary cholesterol reduction.

Dietary cholesterol and CVD risk
In both cross-population and prospective epidemiological studies there is evidence of an association between the cholesterol in foods (dietary cholesterol) and CHD mortality, but the association is not causal.(6) In fact, when the data are subjected to multiple correlation analyses, the majority of epidemiological studies have found no significant and independent relationships between the incidence of CVD and either dietary cholesterol or egg intake.(3) Furthermore, although a small increase in plasma cholesterol occurs in response to increases in dietary cholesterol intake (average 0.000062mmol/l for 100mg cholesterol), this involves an increase in both the so-called "good", anti-atherogenic, high-density lipoprotein (HDL) as well as in the "bad" atherogenic low-density lipoprotein (LDL) particles. There is therefore little effect on the ratio of the two components, now an accepted determinant of CVD risk.(7) The lack of effect of dietary cholesterol on CVD risk is further supported by the recently published results of a small, short-term study investigating the effects of egg and oats consumption on endothelial function (one of the newer indices of CVD risk) in healthy middle-aged adults.(8) In this study the ingestion of two eggs per day for two weeks had no adverse effects on endothelial function and no effect on total serum cholesterol or LDL concentrations.
The relatively small increases in serum cholesterol that have been observed explain why the early feeding studies used extremely high and unphysiological amounts of cholesterol to elicit a response (up to 4,000mg per day) and why dietary cholesterol intakes of 200-400mg per day (equivalent to 1-2 eggs) are associated with little or no change in plasma total cholesterol levels.(7) On the other hand, it is important to recognise that there is evidence suggesting that people with diabetes may be more sensitive to dietary cholesterol intake.(9)

Dietary approaches to risk reduction
There has been a tendency to focus on simplified dietary messages in the prevention of CVD and CHD, and this in part reflects the early single focus on cholesterol modification as the target for CVD risk reduction. However, it is increasingly evident that more appropriate dietary strategies should advocate ways or patterns of eating that include a wide range of recognised protective factors, including vegetables and fruit, oily fish and whole grains.(10,11) Importantly, evidence suggests that this approach may also offer protection against other diseases, such as cancer, and degenerative eye diseases such as cataract and age-related macular degeneration.(10,12)
Whereas earlier recommendations focused on reducing blood cholesterol levels by cutting down on total fat and saturates and increasing n-6 polyunsaturates, there is now more information available concerning the influence of the type or quality of dietary fat (fatty acid profile) on blood lipids, blood haemostasis, endothelial function and insulin sensitivity. A reduced sensitivity of the peripheral tissues to insulin (insulin resistance) is now considered to be a risk marker for CVD.(5) The potential importance in CVD prevention of a greater proportion of both monounsaturates (such as those predominating in olive oil) and n-3 fatty acids (derived from oily fish) is increasingly recognised.
Plant foods are important in CVD risk reduction for a variety of reasons, including their content of antioxidant nutrients, folate, phytochemicals, potassium and dietary fibre.(12) Folate is an important nutrient in the metabolic pathway of methionine and homocysteine. Plasma folate and homocysteine are inversely related, and high circulating homocysteine is an emerging risk factor for CVD.(5) The additive effect of moderate amounts of low-fat dairy products (as a source of calcium), in combination with a diet rich in fruit and vegetables (to increase potassium intake), and a reduction in sodium intake, has been shown to have beneficial effects on blood pressure, another important CVD risk factor, by the Dietary Approaches to Stop Hypertension (DASH) studies.(13) However, there is no strong evidence to support the promotion of individual fruits or vegetables for CVD risk reduction; eating a wide variety of different types of plant foods frequently forms the basis of current recommendations.
 
Balanced diet
It has been suggested that, although the impact of individual dietary modifications on CVD risk is small, the combined influences may be significant.(5) This serves to reinforce the idea of the importance of advocating overall ways of eating or dietary patterns that emphasise variety and that incorporate a wide range of foods to provide the nutritional factors highlighted above, rather than focusing on individual foods. Such dietary patterns should include plenty of plant foods such as wholegrain cereal products, vegetables and fruit, and moderate amounts of low-fat dairy products and lean meat, poultry and fish or alternatives such as eggs, and avoid too much salt.
There is active ongoing debate as to whether a moderate-fat (predominately monounsaturates) and moderate-carbohydrate diet has advantages over a low-fat, high-complex-carbohydrate diet in relation to blood lipid profiles.(5) However, overall fat intake should provide no more than 30-35% of energy (approximately 90g per day and 70g per day for men and women, respectively), and foods rich in saturates should be replaced by foods providing fats derived from monounsaturates and n-3 fatty acids. Within the context of this type of dietary pattern, for the majority of people, eggs are a useful and economical alternative to other sources of protein such as meat. They are relatively low in saturates and a source of monounsaturates and provide a wide range of vitamins and minerals.
The aetiology of CVD is highly complex. The new and exciting area of nutrient-gene research suggests that in the longer term we may be in a position to refine the advice that we give to individuals for CVD risk reduction, by targeting advice on the basis of genetic susceptibility and responsiveness to nutritional factors.(14) In the meantime, the appropriate public health strategy should be to promote the broad dietary patterns discussed above, within the overall context of a lifestyle that is physically active and that encourages people to maintain a healthy weight, to drink only moderate amounts of alcohol and to avoid smoking. Although within the realms of public health nutrition such strategies are paramount, it must be acknowledged that people are still reluctant to change their diet(5) and rates of obesity are rising.(15) Therefore the approach of the food industry in providing suitable functional products, and of the agricultural industry in research into breeding programmes that may improve the nutrient composition of foods such as meat, milk, grains and vegetables, is vital and should also be encouraged.

References

  1. British Heart Foundation. Heart statistics. Available from URL: http://www.heartstats.org
  2. Committee on Medical aspects of Food Policy. Nutritional aspects of cardiovascular disease. Department of Health Report on Health and Social Subjects No 46. London: HMSO; 1994.
  3. Leeds AR, Gray J, editors. Dietary cholesterol as a cardiovascular risk factor: myth or reality? London:Smith-Gordon; 2001.
  4. British Nutrition Foundation Diet and Heart Disease. A round table of factors. London: Chapman and Hall; 1997.
  5. Stanner S, editor. Cardiovascular disease: diet, nutrition and emerging risk factors. The report of a British Nutrition Foundation task force. Oxford: Blackwell Publishing; 2005.
  6. Mann J. Dietary cholesterol: a review of research and practice over 30 years. In: Leeds AR, Gray J, editors. Dietary cholesterol as a cardiovascular risk factor: myth or reality? London:Smith-Gordon; 2001.
  7. McNamara DJ. Eggs, dietary cholesterol and cardiac risk - a US perspective. In: Leeds AR, Gray J, editors. Dietary cholesterol as a cardiovascular risk factor: myth or reality? London: Smith-Gordon; 2001.
  8. Katz DL, et al. Egg consumption and endothelial function: a randomised controlled crossover trial. Int J Cardiol 2005;99:65-70.
  9. Hu FB, et al. A prospective study of egg consumption and risk of cardiovascular disease in men and women. J Am Med Assoc 1999;281:1387-94.
  10. Slavin J. Whole grains and human health. Nutr Res Rev 2004;17:99-100.
  11. Franco OH, et al. The polymeal: a more natural, safer, and probably tastier (than the Polypill) strategy to reduce cardiovascular disease by more than 75%. BMJ 2004;329:1447-50.
  12. Goldberg G. Plants: diet and health. The Report of a British Nutrition Foundation task force. Oxford: Blackwell Science; 2003.
  13. Sacks FM, et al. DASH - Sodium Collaborative Research Group. Effects on blood pressure of reduced dietary sodium and the Dietary Approaches to Stop Hypertension (DASH) diet. N Engl J Med 2001;344:3-10.
  14. Williams CM. Chips with everything? Nutritional genomics and the application of diet in disease prevention. Nutr Bull 2003;28:139-46.
  15. House of Commons Health Committee. Obesity. London: Stationery Office; 2004.

Resource
British Nutrition Foundation
W:www.nutrition.org.uk