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Myths and science of dietary fat and coronary heart disease

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VOL: 98, ISSUE: 34, PAGE NO: 49

Jennette Higgs, Bsc, Nutr SRD, RPHNutr, FRSH, is an independent nutritionist and consultant dietitian for the American Peanut Council

Malnutrition, in the form of undernutrition, preoccupied nutritionists’ concerns during the first half of the 20th century, and fat was seen as a valuable source of much needed energy for a population dominated by manual labourers with few cars. After the Second World War, however, a coronary heart disease epidemic was claiming the lives of middle-aged men. Keys (1957, 1970) first demonstrated that fat intake appeared to be the main dietary difference between countries such as Japan and Greece, where heart disease was not a problem, and Finland and the USA, where rates were rising rapidly. Furthermore, the finding that fat, specifically saturated fat, raises blood cholesterol precipitated the differentiation of fats into ‘good’ and ‘bad’.


Malnutrition, in the form of undernutrition, preoccupied nutritionists’ concerns during the first half of the 20th century, and fat was seen as a valuable source of much needed energy for a population dominated by manual labourers with few cars. After the Second World War, however, a coronary heart disease epidemic was claiming the lives of middle-aged men. Keys (1957, 1970) first demonstrated that fat intake appeared to be the main dietary difference between countries such as Japan and Greece, where heart disease was not a problem, and Finland and the USA, where rates were rising rapidly. Furthermore, the finding that fat, specifically saturated fat, raises blood cholesterol precipitated the differentiation of fats into ‘good’ and ‘bad’.



The low fat hypothesis
During the 1970s and early 1980s several major trials were undertaken to prove the low fat hypothesis (World Health Organization, 1982; Stamler, 1986; McGee et al, 1984; Kannel and Gordon, 1970). However, despite millions of pounds being spent on research into the dietary fat and heart disease equation, there remains no proof that reducing population-wide saturated fat intake or eating a low-fat diet will reduce mortality from heart disease.



Drug trials were able to demonstrate that lowering blood cholesterol using lipid-regulating drugs, such as colestyramine, was effective in those with the highest cholesterol levels and did reduce rates of heart disease. However, a big leap was made in assuming that such drug effects could be equated to diet and that heart disease could be effectively tackled by a dietary change to lower fat and saturated fat intakes within the general population (Taubes, 2001).



Types of dietary fat
Dietary fat is made up of three types - saturated fatty acids (SFAs), monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs) - and these fat types are present in foods in differing proportions. The type of fat that will dominate a given diet is dependent on both the choice and quantity of different fat sources in the diet and the proportion of different fatty acids present in the foods consumed. Different fat types have different health effects. In the early 1980s it was thought that saturated fat raised cholesterol, polyunsaturated fat reduced it and that monounsaturates were neutral in their effects on cholesterol.



Early studies measured total serum cholesterol and equated high cholesterol with increased risk of heart disease. However, once it was appreciated that cholesterol is involved with both delivering fat to the arterial tissues (low density lipoprotein, LDL) and also taking fat away to the liver (high density lipoprotein, HDL), then the more complex chapters of the fat story started to emerge.



Although saturated fats raise LDL fractions, they also raise HDL fractions, which are protective. In fact, break down the saturates group into individual fatty acids and it seems that some saturated fats, such as stearic acid - the main fatty acid in red meat and also in chocolate - raise the HDL fraction but have no effect on the LDL fraction, so they could be described as neutral. Monounsaturates raise HDL and lower LDL fractions, so are in fact protective against heart disease and not neutral as previously believed. Trans-fatty acids, the hydrogenated fatty acids found mainly in manufactured (baked and deep fried) foods, tend to raise LDL and lower HDL, hence concerns over their role in the diet.



Carbohydrate intake



A further aspect that deserves consideration is that, in reducing total fat in our diet, carbohydrate intakes have tended to increase, and usually this has been the more processed and fast food kinds, rather than fruits and vegetables. High carbohydrate diets can raise triglyceride levels and reduce HDL levels and this scenario, alongside ‘insulin resistance’, is collectively called ‘syndrome X’, a condition that increases risk of heart disease.



Cholesterol levels



The emphasis on reducing cholesterol levels was questioned when death from all causes was tracked against cholesterol level. This produced a U-shaped curve in men and was flat for women. Men with high cholesterol levels were dying from heart disease, but equally men with low cholesterol levels were dying prematurely from other diseases, such as cancer and other causes, (Jacobs et al, 1992).



Role of dietary fat
Over the years the advice to eat less fat has become more widely recognised and adopted than even the high-fibre recommendation. Some fat in our diet is essential for energy production, brain function, essential fatty acids and absorption of fat-soluble vitamins (A, D, E, K). Since some fat is found in many important foods, becoming ‘fat phobic’ may lead to avoidance of key foods for health, such as protein sources, so reducing the variety of foods in the diet. This consequence of fat phobia may actually lead to nutrient deficiencies and so returns the malnutrition that characterised the early 20th century (Hu et al, 2001).



The Mediterranean diet



The Lyon Diet Heart Study (de Lorgeril et al, 1999) caused a rethink when it showed lower cardiac recurrence rates, after first myocardial infarction, in a group following a Mediterranean-type diet than the group following the typical western diet. Surprisingly, however, both groups produced similar HDL, LDL and total cholesterol levels, a fact that encouraged scientists to look beyond lipids to the other potentially protective components in the Mediterranean diet.



The Mediterranean diet has long been associated with lifelong good health. This is the diet associated with traditional food patterns typical of southern Mediterranean countries. It is characterised by an abundance of plant foods (fruit, vegetables, cereals, potatoes, beans, nuts and seeds), low to moderate amounts of dairy, meat, fish and wine, with total fat providing 25-35% of total calories, mainly from monounsaturated fatty acids (olive oil, seeds and nuts). Added to this is a lifestyle that includes regular physical activity.



Which types of fat and how much?
It is now widely recognised and supported by both metabolic and epidemiological studies that the type of fat is key to influencing cholesterol levels, not total fat intake. Replacing saturated fat with unsaturated fat is more effective at lowering risk of CHD than simply reducing total fat intake. Additionally, secondary prevention trials have effectively demonstrated a strong protective effect for adding n-3 fatty acids from fish or plant sources to the diet, without reducing the total fat intake.



Fish oils are rich in the long chain n-3 PUFAs, eicosapentaenoic acid and docosahexaenoic acid, and plants are rich in α-linolenic acid, which can be elongated in the body to form the long chain n-3 PUFAS. Until now, data on the protective effects of fish have been mainly undertaken in men, with limited data available for women.



Dietary consumption and long-term follow-up data from almost 85,000 female nurses (34-59 years) have just concluded that, among women, higher consumption of fish and separately omega-3 fatty acids is associated with a lower risk of coronary heart disease (Hu et al, 2002).



Several scientific studies have demonstrated that moderate fat diets, where the fat is supplied by MUFAs and where SFAs are kept low, will reduce total cholesterol levels and, more significantly, reduce levels of the harmful ‘LDL’ cholesterol fraction in much the same way as diets low in total fat. In fact, low fat diets can reduce levels of the protective ‘HDL’ cholesterol fraction in the blood. Diets where saturated fat is replaced with MUFAs and PUFAs help to reduce levels of the harmful ‘LDL’ cholesterol without adversely affecting the ‘HDL’ fraction, so helping to maintain normal blood cholesterol levels.



In more recent years, studies have looked at types of food to ascertain their effects, rather than focus on nutrients. Given that we obtain our nutrition from whole foods, with their complex physical and chemical properties, this makes sense. The protective effects of olive oil, a significant source of oleic acid - the main MUFA in the diet - are now well publicised (Berry et al, 1995; Bonanome et al, 1992).



Furthermore, over the past decade it has been recognised from scientific studies that diets which include nuts are also cardioprotective. Nuts are rich sources of unsaturated fat, particularly monounsaturated, and they are low in saturated fat. Additionally, nuts have other potentially protective components, including magnesium, vitamin E, fibre and a range of phytochemicals including resveratrol.



The latest study to emerge has begun to unravel the mechanisms by which nuts offer protection. The finding that regular consumption of two or more portions of nuts (2oz per week) by middle-aged men reduces risk specifically of sudden cardiac death points to nuts playing a role in reducing fatal ventricular arrhythmias (Albert et al, 2002).



What does this mean in terms of food choices? A moderate fat intake can now be enjoyed as part of a heart-healthy diet, with the fat from foods that will supply more unsaturated fatty acids, especially long-chain fatty acids found in oily fish, and the monounsaturated type, found in olive oil, avocados, rapeseed oil, tree nuts, peanuts and seeds.



Does fat put on weight?
It is now recognised that obesity itself is a risk factor for long-term health, including coronary heart disease, and it is assumed that the low-fat diet campaign would be beneficial for obesity prevention on the basis that fat calories are more concentrated (9kcal/g) than carbohydrate or protein (4kcal/g). However, long-term trials have failed to demonstrate convincingly that low fat diets will lead to successful weight loss.



To control weight, total calorie intake relative to calorie output is crucial. There is a need for people to both increase physical activity and watch calorie intake. Fat is a concentrated source of calories. However, the quality of fat is still important and fat itself can have a positive role in weight management. Fat is an important influencer of satiety: low-fat diets do not work in the long term because they deprive people of a large number of foods. This brings into play the psychological effects associated with following restrictive diets that all too often result in failure of the diet.



A recent study from the Harvard School of Public Health (McManus et al, 2001) compared the traditional low fat diet with a Mediterranean- style, high MUFA diet for weight loss. It was demonstrated, quite dramatically, that not only did those on the moderate fat diet lose more weight over the 18 month study period, but they actually improved the quality of their diet.



Those following the moderate fat diet were actively encouraged to include a mix of olive oil, avocados, peanut butter and nuts in their diet. The result was an increase in vegetables and fibre intake of one portion per day by those following the moderate fat diet, whereas the low fat diet caused a reduction in vegetables and fibre. It seems that the moderate fat, high MUFA diet improved satiety (that feeling of fullness and satisfaction) which helped adherence to a more varied and nutritious diet all round (McManus et al, 2001).



Practical advice for healthy enjoyable diets
Coronary heart disease remains a major cause of death and disability, and risk factors include age, sex, hypertension, smoking, diabetes, elevated serum LDL cholesterol and low HDL cholesterol.



There appears to be much conflicting information on the most appropriate diet to guard against coronary heart disease. However, our understanding of the role of fats in terms of health benefits is now more sophisticated than just the advice ‘eat less fat’, although for most of the population doing so is probably beneficial, as today’s sedentary lifestyles mean people need fewer calories overall.



Emerging scientific opinion stresses the importance of maintaining a normal body weight and avoiding obesity as an important focus for reducing the risk of heart disease, type II diabetes and other chronic diseases of the developed world. Recent evidence, for instance, indicates that the Mediterranean diet is also protective against cancer (de Lorgeril et al, 1998). It has also been reported that frequent consumption of nuts is associated with lower risk of type II diabetes in women (Jiang et al, 2002).



Nurses need to keep abreast of the latest thinking on healthy eating advice (Table 1). Since we eat foods and not nutrients or fatty acids per se, food-based dietary guidelines are likely to be more effective in the long term than more specific messages such as ‘eat less saturated fat’.



A key message for obesity prevention is portion control - portion sizes are increasing (following the American trend) and more snacks are replacing the ‘meat and two veg’ meals of old. Add to this the greater choice of tempting foods available from supermarkets, cafe bars and fast food outlets on every street corner, and it is not difficult to see how easy it is for people to consume more calories than they expend.



Body mass index
Body mass index is a useful tool for determining if a patient’s size is in the normal, overweight or obese range, and an additional tool is now available to help determine whether the overweight patient might be at increased risk of coronary heart disease, dependant on where they carry excess weight.



It is now well documented that waist circumference is a good indicator of overall health risk. Excess fat around the stomach tends to produce a large waist circumference and an ‘apple’ shape. This is often associated with risk factors for serious conditions, such as heart disease, raised blood pressure, diabetes and some types of cancer. Excess fat deposited under the skin, around the bottom, hips and thighs clearly is not concentrated around the waist and tends to produce a ‘pear’ shape. This is generally accepted to be less harmful to health.The Ashwell Shape Chart ( shape.htm), suitable for men and women, is useful in working out the health risk associated with overweight and waist circumference.



With ever increasing concerns for our nation’s long-term health due to escalating rates of cardiovascular disease, type II diabetes, obesity and cancer, the need to encourage permanent lifestyle changes has never been greater. While nutrition scientists continue to grapple with unravelling further chapters of the nutrition story for healthy hearts, the simple advice to pursue a physically active lifestyle and avoid smoking is as important as any dietary changes and pertinent to addressing all these health concerns.



The broad Mediterranean-style diet approach of increasing plant food intake and maintaining a normal body weight through eating a varied, portion-controlled diet may be a more effective message than attempts to give specific, complex dietary messages that have appeared to change over the years.

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