Are food and beverage purchases reflective of dietary intake? Validity of supermarket purchases as indicator of diet quality in the Supreme Nudge Trial.

Dietary intake assessment is often complicated by intrinsic bias. This study investigated whether food purchase data could constitute a valid indication of dietary intake, by evaluating the extent to which diet quality as measured by supermarket food purchases is correlated with diet quality as measured by reported dietary intake. We used data from the Supreme Nudge cluster-randomised controlled supermarket trial (n=227). Data were collected at baseline from supermarket purchases (loyalty cards) and a dietary questionnaire (short 40-item food frequency questionnaire (FFQ)) to compute two scores reflecting diet quality from purchasing data (purchased diet quality) and FFQs (consumed diet quality). Both scores constituted of 13 food groups and could theoretically range between 0 (low diet quality) to 130 (high diet quality). The relationship between purchased diet quality and consumed diet quality was assessed using correlation coefficients, and the Bland-Altman limits-of-agreement method. Multiple linear regression was fitted between purchased diet quality and consumed diet quality, adjusted for age, sex, waist circumference, educational level, and household size. Consumed and purchased diet qualities were modestly positively correlated (Pearson's ρ = 0·31, 95% CI: 0·18 - 0·42). A positive association from linear regression was found after confouding adjustments (β_baseline = 0·22, 95%CI: 0·10 - 0·34). Purchased diet quality was systematically lower than the consumed diet quality. This study found that diet quality as measured by supermarket purchases provided a reasonable indication of diet quality as reported by short-FFQs, albeit modest.