[Validity of a food frequency questionnaire to access dietary intake of adults.]

The aim was to assess the validity of a food frequency questionnaire (FFQ).

Participants (n=84, aged 36-/+6) filled in a food frequency questionnaire, and the intake of nutrients and food items was estimated. Vitamin-C and beta-carotene blood concentration was measured as well as sodium (Na), potassium (K) and nitrogen (N) excretion in the urine. Correlation between results from the FFQ and biological measurements was assessed. PABA test (para-amino benzoic acid) was used to assess the completeness of the urine collection.

There was a correlation between plasma vitamin-C concentration and vitamin-C intake (r=0.294, P=0.008). A correlation was also seen between plasma vitamin-C concentration and intake of tomatoes, cucumber, peppers and green salat (r=0.231, P=0.039), as well as to the total consumption of vegetables (r=0.291, P=0.009). There was no correlation between beta-carotene concentration in the blood and in the diet. However, beta-carotene concentration in the blood correlated with intake of onion, leak, and garlic (r=0.240, P=0.032). There was a strong correlation between all the groups of fruits and vegetables (P<0.001). Potassium intake correlated with potassium excretion (r=0.452, P<0.001), but sodium intake was not associated with sodium excretion. There was no statistical difference between nitrogen intake and total nitrogen excretion in the urine.

FFQ developed by the Icelandic Nutrition Council is valid to assess intake of vitamin-C, potassium as well as vegetables. It also gives estimates of protein intake, but should not be used to assess sodium (salt) intake.