The Cochrane reviewers looked at 38 studies in which randomly assigned healthy adults received dietary advice in some cases and no advice in others. The recommendations usually focused on reducing fat and salt intake while increasing the intake of fruit, vegetables and fibre.
People who were received advice on their diets increased their fruit and vegetable consumption by 1.25 servings over the course of follow-up. Fiber intake also increased while total dietary fats and saturated fatty acids fell. Researchers also noted what they called "modest" changes for the better in total cholesterol, LDL (or "bad" cholesterol) and blood pressure. In studies that separated effects by sex, women tended to have larger reductions in fat intake.
"Diet is a key health behaviour and a definitive review of the effects of dietary advice among healthy people was lacking," said lead author Eric Brunner, Ph.D., a reader in the Department of Epidemiology and Public Health at the University College London Medical School.
"The key question is whether the general population responds. The answer is yes, but the dietary and risk factor changes are modest. Our findings point to the fact that individuals with risk factor elevation — such as high blood pressure or cholesterol — respond better than those at 'average' levels of risk, even though average coronary heart disease risk in Western populations is very high compared to a country like Japan, where rates of CHD are relatively low," Brunner said.
Counselling that occurred in health care settings, such as doctors' offices, resulted in greater reductions in fat and increases in fruit and vegetable consumption. However, it was not clear that these changes translate into lowering of blood cholesterol.
"High-intensity interventions involving more than three personal contacts were associated with larger effects than those with fewer contacts. However, "there was a dearth of long-duration studies, meaning that we have little idea of the long-term effects of counselling," Brunner said.
"Our review suggests that the average changes in individual nutrients and related risk factors are likely to be relatively small. When taken across the entire diet, however, several small changes in food habits may lead to greater health gains than the estimates would suggest," said Brunner.
"The more effective, higher-intensity interventions uncovered by the dietary counselling review may not mesh with the current U.S. health care system," cautioned Nieca Goldberg, M.D., spokesperson for the American Heart Association and director of the Women's Heart Program at the New York University School of Medicine.
"With the exception of people with diabetes, health insurance does not pay for targeted nutrition counselling. This often turns into a barrier to proper education when the person cannot afford the costs," Goldberg said.