A study on the association between iron status and diabetes in adults in North China has recently been reported by Luan et al. (1). The results partly confirmed our previous finding from South China (2). In Luan et al.’s study, only heme iron was found to be associated with risk of diabetes. However, the intake of heme iron in their dataset is very low: mean intake ± SD in first quartile only 0.3 ± 0.3 mg/day. Thus, the reference group is mainly composed of vegetarians. It may not be appropriate to examine the association between heme iron and diabetes without adjusting for socioeconomic status because those eating less heme iron may be the poor, who have a lower risk of diabetes in China.
We reanalyzed our data and examined the association between heme and nonheme iron intake and diabetes. Although the subjects in our study had a similar heme iron intake (mean ± SD 2.27 ± 3.3 mg/day), we found opposite results. Among women but not men, only nonheme iron intake was associated with diabetes. In an age-, sex-, and energy- adjusted model, heme iron intake was positively associated with diabetes: the odds ratios (ORs) across heme iron intake quartiles were 1.00, 1.19, 1.27, and 2.13 (P for trend 0.023). In a fully-adjusted model, the ORs across nonheme iron intake quartiles were 1.00, 3.69 (95% CI 1.35–10.10), 4.52 (1.39–14.76), and 5.01 (1.25–19.98) (P for trend 0.017) in women and 1.00, 0.34 (0.12–1.00), 0.97 (0.36–2.62), and 1.15 (0.35–3.80) (P for trend 0.557) in men; ORs across heme iron intake quartiles were 1.00, 0.88 (0.32–0.45), 0.70 (0.20–2.40), and 1.52 (0.41–5.61) (P for trend 0.612) in women and 1.00, 0.68 (0.21–2.19), 0.77 (0.25–2.40), and 0.81 (0.23–2.86) (P for trend 0.816) in men. The above association as well as that between total iron intake and diabetes did not change after adjustment of blood lipids, which is also different from Luan et al.’s study.
Four studies among the Chinese population found a cross-sectional association between iron and diabetes or insulin resistance (1–4). Current evidence suggests that further longitudinal study is needed. When we finish the 5-year follow-up of the previous participants, by the end of this year, we can examine the association with a longitudinal approach. If a positive association between iron intake and diabetes is established, it will challenge the current practice of anemia prevention in China: using iron-fortified soy sauce.