Quantitative assessment of dietary supplement intake in 77,000 French adults: impact on nutritional intake inadequacy and excessive intake.

Background: Dietary supplements (DS) are largely consumed in Western countries without demonstrating their nutritional benefits and safety in the general population. The aims, in a large population-based study of French adults, were: (1) to compare the prevalence of nutrient intake inadequacy and the proportion of individuals exceeding tolerable upper intake levels (UL) between DS users and non-users, and (2) to quantify the extent of potentially "at-risk" DS use practices (e.g., DS/drugs contraindicated association or use of beta-carotene DS in smokers).

Methods: 76,925 participants, 47.6% men and 52.4% women, mean age 46.9 ± 16.3 years were enrolled to the NutriNet-Santé cohort and they completed a quantitative DS questionnaire and three 24 h dietary records. A composition database including > 8000 DS was developed. Variance reduction was applied to estimate usual intakes and analyses were weighted according to the French census data.

Results: Among DS users of the specific nutrient, DS contributed to 41% of total intake for vitamin D in men, 55% in women; and to 20% of total intake for pyridoxine in men, 21% in women. Compared to dietary intakes only, their prevalence of inadequacy was reduced by 11% for vitamin C, 9% for magnesium, 6% for pyridoxine in men, and 19% for calcium, 12% for iron, and 11% for magnesium in women (p < 0.0001). The proportion of subjects exceeding UL reached 6% for iron and 5% for magnesium in men, and 9% for iron in women. 6% of DS users had potentially "at-risk" practices.

Conclusion: While DS use contributed to decrease the prevalence of insufficient intake for several nutrients, it also conveyed excessive intake of iron and magnesium. Besides, a substantial proportion of potentially "at-risk" DS use practices was reported.