a1 Department of Nutrition, School of Public Health, Loma Linda University, Loma Linda, CA, USA
a2 Department of Public Health, School of Graduate Studies, Adventist International Institute of Advanced Studies, PO Box 038, Silang 4118, Cavite, Philippines
a3 Department of Epidemiology and Biostatistics, School of Public Health, Loma Linda University, Loma Linda, CA, USA
a4 The Adventist Health Study II, School of Public Health, Loma Linda University, Loma Linda, CA, USA
Objective To validate a 171-item semi-quantitative food-frequency questionnaire (FFQ) for measurement of nutrient intake in an intervention trial based on walnut supplementation.
Design and setting Free-living adults from Southern California were randomly assigned to either an intervention (walnut-supplemented) or a control diet. The prescribed 6-month intervention was ≥ 28 g of walnuts per day for the walnut-supplemented group and ≤ 2 g of walnuts per day for the control group. Participants provided at least six 24-hour dietary recalls and completed a self-administered FFQ.
Subjects Eighty-seven adults aged 30–72 years (48 females, 39 males).
Results Our findings from validation (by correlation with six diet recall measures) of the measurement of 32 nutrients by the FFQ are as follows. We found significant positive correlations (corrected for measurement error) between the FFQ and diet recalls for total energy (r = 0.34), total carbohydrate (r = 0.42), vegetable protein (r = 0.43), total fat (r = 0.51), polyunsaturated fat (r = 0.77), total fibre (r = 0.60), linoleic acid (r = 0.78) and α-linolenic acid (r = 0.79) – the last nutrient being an excellent nutrient biomarker of the intervention (walnut supplementation). Significant positive correlations were also found for vitamin C (r = 0.96) and certain minerals (r = 0.46–0.80 for calcium, phosphorus, magnesium, iron and potassium). Uncorrected correlations were also high (r>0.40) for retinol, β-carotene, folate and alcohol. Both diet recalls and FFQ showed a similar significant difference in α-linolenic acid content between the walnut-supplemented and control diets.
Conclusions The FFQ demonstrated good relative validity in the estimation intake of some of the major nutrients in a dietary intervention trial and was a particularly valid estimate of an important nutrient biomarker of walnut supplementation.
(Received December 23 2005)
(Accepted May 23 2006)