In a review by Garg (26) monounsaturated fat diets compared to high- carbohydrate diets reduced fasting TG and VLDL-cholesterol by 19% and 22% respectively, with a modest increase in HDL-cholesterol without adversely affecting LDL-cholesterol (25).
CARBOHYDRATE AND INSULIN SENSITIVITY
Daly et al. (28) have recently reviewed the evidence and clinical implications of dietary carbohydrates and insulin sensitivity.
This is a controversial area.
Extensive studies in animals show a detrimental effect of diets very high in fructose or sucrose, particularly in association with induction of hypertrigly- ceridaemia.
The more limited results in human studies show conflicting results, partly because of heterogeneity of design.
Certain groups of subjects such as the elderly, sedentary subjects, those with established coronary artery disease, males and hyperinsulinaemic subjects may be more sensitive to very high intakes of sucrose and fructose than others.
CARBOHYDRATE AND PLASMA LIPIDS
The elevation of blood lipid concentrations in response to large amounts of dietary sugars, particularly fructose and sucrose, has been recognised for many years.
There are also many other variables that can influence postprandial TG concentrations, such as obesity, excessive alcohol consumption, genetic background and renal failure.
High-carbohydrate diets are reported to increase TG, mainly in short-term studies (29).
However, most of these studies have been poorly controlled and have been very short term and thus the evidence is poor.
Turley et al. (30) recently demonstrated that free-living healthy subjects randomised to a high- carbohydrate diet (59%) had no detrimental effect on fasting TG concentra- tions over a six-week period.
The literature contains conflicting findings, particularly in studies that contain >20% of energy from sucrose or >5% from fructose, where both sugars have been shown to raise TG concentrations.
In studies containing amounts of sugars more typical of dietary habits in the Western world, elevated plasma TG concentrations are not usually observed (29).
Interestingly, the glycaemic index of carbohydrate was significantly related to serum HDL- cholesterol in a retrospective cross-sectional study of 2200 middle-aged adults, where a low glycaemic diet was the only dietary variable related to the CHD risk factors measured (31).
NUTRITIONAL MANAGEMENT OF CARDIAC RISK FACTORS 139
OTHER NUTRITIONAL FACTORS ASSOCIATED WITH REDUCED CARDIAC RISK
FISH AND OMEGA-3 FATTY ACIDS The cardioprotective benefits of the ‘Mediterranean’ diet and its reduction of mortality are strikingly evident in the results of studies such as the DART study (32) and the Lyon Diet Heart Study (33).
The results of these studies are not solely due to the regular inclusion of oily fish, but this did have a key role.
Oily fish such as mackerel, herring, sardines, trout and salmon are a rich source of the n-3 polyunsaturates.
Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), the long-chain omega-3 PUFAs, are thought to be beneficial due to their anti-thrombolytic and anti-inflammatory action as well as their triglyceride-lowering effects.
The UK Department of Health has recommended to the general population that they consume two portions of fish (100 g or 3, 4 oz portion) a week, one of which should be oily (34).
People with diabetes should be encouraged to include oily fish in the diet, ideally two to three times a week (35).
Fish oils are increasingly available in capsule or liquid form.
These should provide approximately 0.5, 1.0 g of n-3 fatty acids per day.
Vegetarians, or those allergic to fish, can optimise their n-3 intakes by using vegetable n-3 sources (rapeseed, canola, linseed and flax oils), but the conversion rate is low and other polyunsaturates can compete.
STANOLS AND STEROLS Foods enriched with plant stanols and sterols have recently been introduced to the market.
Plant sterols and stanols, which are structurally closely related to cholesterol, effectively inhibit the absorption of cholesterol.
Plant sterols occur naturally in vegetable oils such as soybean and rapeseed, whereas plant stanols are found in tall oil, a side-product of paper manufacture.
Manufactured products containing these products include spreads, cereal bars, cheeses, milk, ice cream and yoghurts.
The optimal dose appears to be 1.6, 2.0 g/day which equates to *20 g spread/day.
For these products to be effective the recommended intake should be consumed daily.
This results in a 9, 14% decrease in LDL-cholesterol (36).
Moreover, these agents complement the action of the statin drugs, particularly in poor responders to drug therapy who have high rates of cholesterol absorption from the gut but low rates of cholesterol synthesis in the liver.
However, concern exists regarding absorption of fat-soluble vitamins.
Randomised trials have shown that plant sterols and stanols lower the blood concentration of b-carotene by about 25%, the concentration of a-carotene by about 10% and the concentration of vitamin E by about 8%.
However, since these vitamins protect LDL-cholesterol from oxidation and stanols/sterols decrease the amount of LDL-cholesterol, these
140 NUTRITIONAL MANAGEMENT OF DIABETES MELLITUS
changes may self-adjust. Patients should be encouraged to eat foods containing stanols and/or sterols daily to reduce LDL-cholesterol.
SOYA PROTEIN, FLAVONOIDS AND PHYTO-OESTROGENS The efficacy of soya and soya derivatives in lowering total cholesterol and LDL-cholesterol was recently supported by the US Food and Drug Administration (FDA) approving a health claim about the role of soya protein in reducing the risk of CHD.
In 1999 the FDA finalised a rule that authorises the use on food labels and in food packages under FDA jurisdiction of the health claims concerning the association between soya protein and reduced risk of CHD: ‘25 g of soya protein a day, as part of a diet low in saturated fat and cholesterol may reduce the risk of heart disease’ (37).
Serum total cholesterol and LDL-cholesterol concentrations can be lowered by about 13%, plasma TG by 10% and HDL-cholesterol goes up by about 2% (38), and these beneficial effects are also seen in people with Type 2 diabetes (39).
It is unclear if the benefits come from the main phyto-oestrogens found in soya, diadzein and genistein or from the soy protein itself.
Epidemiological evidence suggests high intakes of flavonols such as onions, broccoli, apples and tea may reduce the risk of CHD or certain cancers , however, as yet, prospective data in disease prevention is lacking.
EGGS In 2001 the American Heart Association relaxed its recommendations concerning the restriction of eggs and other high-cholesterol foods for the general healthy population, allowing people on plasma cholesterol-lowering diets five to six eggs a week. However, in a recent meta-analysis concern remains in the diabetic population where higher egg consumption was associated with an apparent increased risk of CHD (40). Further research is called for and continuing egg restriction for people with diabetes should be maintained.
GARLIC Garlic shows some promise for improving some cardiovascular risk factors.
Studies suggest small short-term benefits of garlic on some lipid and antiplatelet factors (41).
However, conclusions about the true effects of garlic are limited by the marginal quality and short duration of many studies.
Debate also continues regarding the quantity of garlic needed to see an effect, with some studies reporting an intake as high as three bulbs per day.
The implication for clinical practice is that this may not be the most effective way to reduce lipid levels, but if the patient enjoys garlic then do not dissuade the use of it.
NUTRITIONAL MANAGEMENT OF CARDIAC RISK FACTORS 141
HOMOCYSTEINE AND FOLIC ACID
Elevated plasma homocysteine has been shown in many studies to be an independent marker for an increased risk of cardiovascular disease (42).
The mean plasma homocysteine level is usually low or normal in DM patients except when nephropathy is present.
Levels in that case tend to be higher than in people without diabetes.
For people with diabetes an independent association with homocysteine and CVD has been shown in retrospective studies.
Prospective studies showed an association between elevated homo- cysteine and all-cause mortality in DM patients.
In general, the association between elevated levels of homocysteine and an adverse outcome was stronger for people with diabetes compared with non-diabetics.
Homocysteine is formed as a result of the breakdown of the dietary amino acid methionine.
This is dependent on the presence of four B vitamins (vitamin B12, vitamin B6, folate and riboflavin).
Homocysteine reference ranges still need to be clarified, as does the mechanism by which homocysteine damages the vasculature.
To try and prevent a raised homocysteine level, patients should be encouraged to have five portions of fruit and vegetables each day to ensure that adequate amounts of vitamins and minerals are consumed.
The combination of vitamins that most effectively lowers homocysteine levels has yet to be discovered.
If supplementation with B vitamins is going to form part of a public health strategy to prevent vascular disease, it is important, to avoid toxicity, that the lowest effective dose is found.
In the USA, grain products are fortified with folic acid at a level of 1.4 mg/g of product.
Although this is primarily to prevent neural tube defects, it is also hoped that it may be beneficial for vascular disease prevention.
Promotion of foods high in the appropriate B vitamins should therefore be encouraged within a healthy diet.
There continues to be doubt as to whether supplementation should be encouraged until further research has been completed, and specific information on target groups such as people with diabetes is lacking.
COFFEE
The evidence remains ambiguous regarding the effect of coffee on CHD (43). The association between CHD and coffee consumption has been weakened by long-term follow-up. These findings may possibly be explained by a change in the type of coffee consumed. A lipid-rich fraction from boiled coffee seems to increase serum cholesterol concentration. When boiled coffee is filtered the lipid-rich factor is retained in the filter paper and the effect on cholesterol is reduced substantially (44).
142 NUTRITIONAL MANAGEMENT OF DIABETES MELLITUS
An elevated plasma concentration of total homocysteine is considered to be a risk factor for cardiovascular disease. Heavy coffee drinking has been related to high homocysteine concentrations in epidemiologic studies and one experiment in which healthy subjects drank unfiltered boiled coffee (45).
VITAMIN E Several observational studies have suggested that a high intake of vitamin E may slow the development and progression of atherosclerosis. Some clinical trials have also reported beneficial effects of vitamin E supplementation in the secondary prevention of cardiovascular events. However, the results of a recent large, multi-centre clinical trial reported that vitamin E supplementation was not effective in reducing the incidence of cardiovascular events in high-risk patients (46).