Long term effects of advice to reduce dietary salt. Front cover was highly misleading

Some problems with Cochrane reviews of diet and chronic disease

The sum of evidence-based nutrition has to be more than a Cochrane-type meta-analysis of randomised controlled trials (RCTs). Most of the evidence base in nutrition is observational, especially cohort studies. RCTs of diet change through to disease outcome are uncommon and the change has usually been addition or removal of only a single food component. Trials with whole diets through to disease outcome are rare and dietary changes made by individuals are unlikely to be an exact copy of the prescription. It is hard to even imagine a trial in which half (randomised) of a large group of middle-aged people agree to avoid vegetables for 5 y and be followed up to see who will develop cancer. Most of the USA's health claims, permitted by the Food and Drug Administration, are not supported by RCTs. But where controlled trials of nutritional change and disease outcome have been achieved, they must be reviewed very carefully. Two Cochrane reviews on diet and cardiovascular disease (CVD), published in the widely read British Medical Journal (BMJ), were criticised after their publication and the conclusions have not been subsequently adopted by expert committees. The first of these reviews was 'Dietary fat and prevention of CVD: a systematic review'. The second was 'Systematic review of long term effects of advice to reduce dietary salt in adults'. A critique of these two Cochrane reviews is presented here as a contribution to our discussion of the potential of Cochrane methodology to the reliability of knowledge about diet and disease.

Blood pressure and urinary sodium in men and women: the Norfolk Cohort of the European Prospective Investigation into Cancer (EPIC-Norfolk)

BACKGROUND

Abundant evidence indicates that a high sodium intake is causally related to high blood pressure, but debate over recommendations to reduce dietary sodium in the general population continues. A key issue is whether differences in usual sodium intake within the range feasible in free-living populations have clinical or public health relevance.

OBJECTIVE

We examined the relation between blood pressure and urinary sodium as a marker of dietary intake.

DESIGN

This was a study of 23104 community-living adults aged 45-79 y.

RESULTS

Mean systolic and diastolic blood pressure increased as the ratio of urinary sodium to creatinine increased (as estimated from a casual urine sample), with differences of 7.2 mm Hg for systolic blood pressure and 3.0 mm Hg for diastolic blood pressure (P < 0.0001) between the top and bottom quintiles. This trend was independent of age, body mass index, urinary potassium:creatinine, and smoking and was consistent by sex and history of hypertension. The prevalence of those with systolic blood pressure >/= 160 mm Hg halved from 12% in the top quintile to 6% in the bottom quintile; the odds ratio for having systolic blood pressure >/= 160 mm Hg was 2.48 (95% CI: 1.90, 3.22) for men and 2.67 (95% CI: 2.08, 3.43) for women in the top compared with the bottom quintile of urinary sodium. Estimated mean sodium intakes in the lowest and highest quintiles were approximately 80 and 220 mmol/d, respectively.

CONCLUSIONS

Within the usual range found in a free-living population, differences in urinary sodium, an indicator of dietary sodium intake, are associated with blood pressure differences of clinical and public health relevance. Our findings reinforce recommendations to lower average sodium intakes in the general population.