Complete anthropometric data were available for 14,619 of the 15,028 (97.3%) individuals who attended both health checks. Compared with the highest social class, mean BMI was greatest in the lowest social class (Table ) for both men (p for trend <0.001) and women (p < 0.001). As expected the proportion of smokers was greatest in the lowest social class (p for χ2 test for heterogeneity <0.001 men, p < 0.005 women) and energy intake was highest in the lowest social class (p = 0.005 men, p < 0.001 women). At baseline there was a statistically significant interaction between sex and social class with BMI (p = 0.0001). Specifically, the positive association between social class and BMI was stronger in women.
Characteristics of the EPIC-Norfolk cohort by social class measured at baseline from 1993 – 1997
For the prospective analyses, sex stratified results (see Additional file 1
) were similar to combined results (men and women) for weight change and for weight gain (all p for interactions >0.3); we therefore present data for the combined analyses. Although baseline BMI was significantly negatively correlated with weight change (r = -0.05, p < 0.001) there was no statistically significant interaction between baseline BMI and social class with weight gain. Mean weight change was +1.42 kg (SE 0.08) for individuals in the lowest social class, whereas mean weight change was +0.97 kg (0.12) for those in the highest social class (p for trend = 0.016) (Table ), reflecting an absolute mean difference of 0.45 kg (p = 0.002). These associations remained statistically significant after adjustment for sex, age, baseline BMI, smoking, and follow up time (p for trend <0.001). The average follow up time for the whole study population was 3.66 years (SE 0.01).
Association between social class, baseline BMI and weight change from baseline to follow up of 6,423 men and 8,196 women in the EPIC-Norfolk cohort
Over the follow up period 5,064 people gained more than 2.5 kg (Table ). Compared with those in the highest social class, individuals in the lowest social class had a greater risk of gaining more than 2.5 kg (OR 1.29; 95% CI 1.11–1.51; p for trend = 0.002). This association remained statistically significant following adjustment for sex, age, baseline BMI, smoking, and follow up time (OR 1.25; CI 1.07–1.46; p for trend <0.001).
Association between social class and risk of gaining more than 2.5 kg over the follow up period of 6,423 men and 8,196 women in the EPIC-Norfolk cohort
To assess whether certain lifestyle factors mediated the association between socioeconomic position and weight change or gain, we included plasma vitamin C level, total energy intake and physical activity as covariates in the adjusted models (see Additional file 2
). For weight change, based on a subset of 12,760 participants with complete data on confounders and potential mediators, there was no material difference between the unadjusted model and a model including all covariates (data not shown). Similarly, for weight gain there was no material difference between the unadjusted model (lowest social class compared to highest OR 1.35; CI 1.14–1.60; p for trend <0.001) and a model including all confounders and potential mediators (OR 1.33; CI 1.12–1.57; p for trend <0.001). Plasma vitamin C levels in this cohort positively correlate with fruit and vegetable intake [14
] Using vitamin C as a correlate of dietary intake, we found that this biomarker was not associated with subsequent weight gain (data not shown).