SIGN 97: Risk estimation and the prevention of cardiovascular disease

Guideline Index Page | SIGN Methodology

4 Diet

Environmental factors, including diet, play an important role in the development of CHD. The diet of any individual is related to other lifestyle factors (smoking, exercise, etc). Randomised controlled trials of diet are able to eliminate such bias but are more difficult to conduct than those of drugs or supplements.

4.1 Altering dietary fat intake

There is more evidence about the role of fat in risk modification than of other dietary factors. Reduction of fat, in particular of saturated fat is one of the pillars of dietary advice to prevent CHD.46 Modifying the composition rather than the amount of fat in the diet may be a more effective strategy.

4.1.1 Saturated fat

A Cochrane review of 27 trials (18,196 participants) examined the effect of reduction or modification of dietary fats for at least six months on reducing serum cholesterol levels and on total and cardiovascular mortality and morbidity. The review included trials of high (seven), moderate (six) and low risk (14) participants. Trials involving high risk participants included men only. There was no significant effect on total mortality (rate ratio 0.98, 95% CI 0.86 to 1.12), a trend towards protection from cardiovascular mortality (rate ratio 0.91, 95% CI 0.77 to 1.07), and significant protection from cardiovascular events (rate ratio 0.84, 95% CI 0.72 to 0.99). This effect was non-significant if studies at high risk of bias were removed. Trials with at least two years? of follow up provided stronger evidence of protection against cardiovascular events (rate ratio 0.76, 95% CI 0.65 to 0.90). The reviewers concluded that there is a small but potentially important reduction in cardiovascular risk with a reduction or modification of dietary fat intake, seen particularly in trials of longer duration.47 Evidence level 1++

ADiets low in total and saturated fats should be recommended to all for the reduction of cardiovascular risk.

4.1.2 Omega 3 fats

There is conflicting evidence on the benefits associated with increased consumption of omega 3 fats. Some studies had suggested that omega 3 fatty acids were beneficial in preventing and treating CHD.48 A meta-analysis of 48 RCTs and 26 cohort studies does not support this.49 Analysis of the cohort studies alone did suggest that omega 3 fats would reduce total mortality, although insufficient adjustment for confounding life style was a common feature in many of the studies. The pooled results from the RCTs in patients with CHD showed omega 3 fats had no benefits on mortality or cardiovascular events. There was considerable heterogeneity among the RCTs which disappeared when studies at high risk of bias were removed from the analysis. Evidence level 1+

Relative risk for total mortality was 0.98 (95% CI 0.86 to 1.12). There were similar findings for cardiovascular events. There was no evidence of benefit from plant oil omega 3 (mainly a- linolenic acid) either. Nor did the results differ when considering whether the increased intake of omega 3 was from dietary advice or supplements. There is no current evidence of benefit from omega 3 fats, although confidence intervals do not exclude either a moderate benefit or harm.

In view of this uncertain effect and in order to avoid conflicting dietary advice, no change is recommended from the advice given in the current dietary guideline (two 140 g portions of 4 fish, one of which should be a fatty fish, per week).50 Evidence level 4

Fish consumption may help to reduce intake of (saturated) fat from meat.

[Good practice point] All individuals should eat at least two portions of fish per week, one of which should be a fatty fish.

4.2 Reducing dietary salt

A meta-analysis of 28 trials on the effect of moderate salt reduction on blood pressure demonstratedthatamodestreductioninsaltintakeforfourormoreweekshasasignificanteffect on blood pressure in both hypertensive and normotensive individuals. The pooled estimates of blood pressure fall were 4.96/2.73 ± 0.40/0.24 mm Hg in hypertensive patients (p<0.001 for both systolic and diastolic) and 2.03/0.97 ± 0.27/0.21 mm Hg in normotensive individuals (p<0.001 for both systolic and diastolic). A reduction of salt intake of 6 g per day (100 mmol or 2.3 g sodium per day) predicted a fall in blood pressure of 7.11/3.88 mm Hg (p<0.001 for both systolic and diastolic) in hypertensive patients and 3.57/1.66 mm Hg in normotensive individuals (systolic: p<0.001; diastolic: p<0.05).51 Evidence level 1+

A Cochrane review of salt restriction for the prevention of CHD cited too few cardiovascular events in the trials of at least six months duration to make a clear conclusion. It did report a small but significant reduction in systolic blood pressure in participants who had followed a salt-restricted diet and, reductions were greater in subgroups with hypertension.52 Another Cochrane review of advice to reduce salt intake lasting at least six months, also reported small but significant benefits to blood pressure. Long term maintenance of low sodium diets was difficult for individuals, even with considerable advice, support and encouragement (see section 10).53 Evidence level 1+

The Food Standards Agency has recommended that adults should consume no more than 6 g of salt per day (approximately equivalent to one teaspoonful).54 Evidence level 4

APeople with hypertension should be advised to reduce their salt intake as much as possible to lower blood pressure.

[Good practice point] All individuals should aim to consume less than 6 g of salt per day.

4.3 Fruit and vegetable intake

Diets with at least 400 g of fruit and vegetables per day are recommended in Scotland.47 Diets rich in fruit and vegetables tend also to be low in fat. Two systematic reviews of cohort studies examined the benefits of fruit and vegetable consumption for the reduction of CHD risk. There is evidence from cohort studies to support reduced CHD event rates from increased vegetable (risk ratio 0.77) and fruit (risk ratio 0.86) intake in one review,55 and 15% reduced relative risk of CHD in those consuming high levels of fruit and vegetables compared to those consuming low levels (equivalent to a four-fold increase in fruit and doubling of vegetables) in another.56 Evidence level 2++, 2+

CIncreased fruit and vegetable consumption is recommended to reduce cardiovascular risk for the entire population.

4.4 Effect of specific minor dietary components

4.4.1 Antioxidant vitamin supplementation

Several systematic reviews of RCTs were identified that investigated the association between vitamin supplementation and prevention of CHD. One systematic review of 84 RCTs found that neither supplements of vitamin E alone nor given with other agents yielded a statistically significant beneficial or adverse pooled relative risk for all-cause mortality, cardiovascular mortality, fatal or non-fatal myocardial infarction or reduction in blood lipids.57 Another meta- analysis of RCTs of vitamin supplementation identified a lack of any statistically significant or clinically important effects of vitamin E on cardiovascular disease.58 Evidence level 1++

A meta-analysis examining the effect of vitamin E dose on all cause mortality identified that high dose (=400 IU per day) vitamin E increased all cause mortality by 39 per 10,000 persons treated (95% CI: 3 to 74 per 10,000; p<0.035). Low dose trials did not significantly reduce all cause mortality.59 Evidence level 1++

The US Preventive Services Task Force guideline investigated the evidence on the role of antioxidant supplementation in reducing the incidence of or progression to CHD. The guideline found little evidence that any single vitamin supplementation (vitamin A, vitamin C, vitamin E, β-carotene), combined antioxidants or multivitamins had a benefit on primary or secondary prevention.60 Evidence level 4

AAntioxidant vitamin supplementation is not recommended for the prevention or treatment of coronary heart disease.

4.4.2 Folate supplementation

A general overview examined the association between vitamin deficiency and chronic disease. It suggested that folate and vitamins B6 and B12 are required for homocysteine metabolism and their deficiency may be associated with coronary heart disease risk.61 In contrast, two systematic reviews suggest that the link between hyperhomocysteinaemia and CHD may be overstated.62,63 In one review the summary odds ratios for a 5 micromol/l increase in homocysteine concentration ranged from 1.06 (95% CI 0.99 to 1.13) to 1.70 (95% CI: 1.50 to 1.93).62 Prospective cohort studies appear to offer weaker support than case control studies for an association between homocysteine concentration and cardiovascular disease. Further research using robust methodologies should be carried out in this area. Evidence level 2++, 4

4.4.3 Stanol esters and plant sterols

Stanol esters and plant sterols are present in small amounts in normal diets, and can be supplemented using dietary products such as certain margarines and yoghurt drinks. Two systematic reviews provide evidence that they can reduce LDL cholesterol.64,65 In the larger review of 41 RCTs of the effect on serum lipids, 2 g per day supplements of stanol esters and plant sterols led to 10% reductions in LDL cholesterol.64 There was no benefit from further dosage increases. The cost to the individual of this supplement has been estimated at ?70 per year.65 Evidence level 1+, 1-

As yet, there is no evidence on whether these reductions in cholesterol translate in the longer term into reduction in CVD, nor is there long term data (more than five years) on their safety.

4.4.4 Nuts

There is limited evidence from two RCTs that consuming certain nuts may improve lipid profiles, reducing serum cholesterol by up to 0.4 mmol/l.66,67 The trials were small with short term follow up only, and involved consuming large amounts of unsalted nuts, which may be unrealistic for the general population in Scotland - 20% of calorie intake was derived from nuts (averaging about 75 g/day). Evidence level 1+

More evidence is needed before recommendations can be made.

4.4.5 Soya intake

Soya based foods are an important constituent in many vegetarian diets and have been investigated for possible beneficial effects on lipid profiles. Two small randomised trials, have suggested that substitution of moderate to large amounts of soya based foods in the diet may have a small impact in lipid profiles.68,69 Consuming 50 g soya protein a day (in the form of burgers) was reported to reduce total cholesterol by 0.4 mmol/l. Evidence level 1-

More evidence is needed before recommendations can be made.

4.5 Giving dietary advice

Randomised trials have shown that dietary advice can have effects on self reported dietary intake andobjectiveriskfactors.Mostevidenceonbeneficialeffectsisforpatientswithcardiovascular disease. These effects reduce with time,70 although, in one study a measurable effect persisted for six to nine years.71

4.5.1 Who should give dietary advice?

In one systematic review dietitians were better than doctors at lowering cholesterol through dietary advice alone, but there were no significant differences between dietitians and nurses 1++ or self help resources.72 Evidence level 1++

4.5.2 How should dietary advice be given?

A variety of methods have been attempted varying from brief advice to comprehensive multifactorial lifestyle interventions. In one RCT, up to two hours of counselling achieved greater effects than 10 minutes counselling, but the differences were small.73 In another RCT, 14 group sessions (90 minutes each) during one year increased self reported fruit and vegetable intake and reduced self reported fat intake, but without significant changes to lipid profiles.74 One RCT found that telephone “coaching” led to a 10% reduction in total and LDL cholesterol.75 The intervention involved five telephone calls over 24 weeks and included assessment to establish knowledge, explanation, assertiveness training, goal setting, and reassessment. Length of telephone calls varied, but median times were 20 minutes for the first call and 10 minutes for subsequent calls. Evidence level 1++, 1+

The SIGN guideline on cardiac rehabilitation reported that interventions to improve lifestyle were more successful if founded on the established education principles of relevance, individualisation, feedback, reinforcement, and facilitation.76 Evidence level 4

[Good practice point] Interventions to improve diet should be based on educational competencies (improved knowledge, relevance, individualisation, feedback, reinforcement and facilitation).

4.6 Weight reduction and cardiovascular risk

One systematic review of RCTs of diet to reduce weight which evaluated the effect on blood pressure was identified. Only small numbers of patients were included in the trials (six trials including 361 participants).77 Dietary interventions to reduce weight were moderately effective at reducing blood pressure. Diets producing weight loss in the range 3% to 9% body weight were partially associated with blood pressure reductions of about 3 mm Hg systolic and diastolic. The review had insufficient power to detect differences in morbidity or mortality outcomes. Evidence level 1+

Other studies have shown that improvements in blood pressure,78 lipid profile79 and glucose handling80,81 are produced by maintained weight loss, and it is possible to extrapolate these to the reduction of the cardiac events that would be predicted by risk analysis. Evidence level 1++, 4

BPatients, and individuals at risk of cardiovascular disease, who are overweight, should be targeted with interventions designed to reduce weight, and to maintain this reduction.

4.7 Managing metabolic syndrome

The metabolic syndrome is characterised by insulin resistance and visceral obesity and is associated with hypertension, impaired glucose handling, lipid abnormalities and a variety of moresubtlemetabolicandthromboticanomalies.Thelipidprofilemirrorsthatofdiabetes,with small, dense LDL, low HDL, and raised triglycerides, and is highly atherogenic.

Individuals with the metabolic syndrome have a cardiovascular risk approaching that of full diabetes and should be treated accordingly.82-84 The natural progression of untreated metabolic 2+ syndrome is to develop overt type 2 diabetes. Evidence level 2+

The diagnostic criteria for metabolic syndrome vary, with different definitions available from the World Health Organisation (WHO), International Diabetes Federation85 (IDF), and the US National Cholesterol Education Program Adult Treatment Panel86 (ATP). The ATP definitions were updated in 2005 by the American Heart Association/National Heart, Lung, and Blood Institute87 (AHA/NHLBI). The AHA/NHLBI and IDF definitions are most recent and are very similar, identifying many of the same individuals.

The AHA/NHLBI and IDF define metabolic syndrome as any three of the following:

Asians have a genetic predisposition to the syndrome. Action to prevent or reverse excess weight gain will prevent or sometimes even reverse the metabolic abnormalities and hypertension.88 Weight reduction often requires an exercise programme as well as dietary intervention, since 4 these individuals commonly have a low basal metabolic rate. Insulin sensitising drugs (eg, metformin, glitazones) are known to be effective in centrally obese patients with overt diabetes, and may also be useful in patients with metabolic syndrome and at high risk. Evidence level 4

[Good practice point] All patients with the metabolic syndrome should be identified and offered professional advice in relation to a cardioprotective diet, exercise and weight monitoring. They should be followed up regularly according to the progress they are making in reducing their total cardiovascular risk.

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