It has been shown that too little or too much iron supplementation in the diet is linked with poor health in Chinese women. Too little leads to anemia but too much is associated with cardiovascular disease.
A lack of sufficient iron in the diet leads to anemia, a known health problem in the developing world. In response, many governments have addressed this concern by supplementing foods with iron. This has resulted in increased iron intake and higher levels of serum ferritin (bioavailable iron in the blood) and hemoglobin (iron-containing protein found in blood used to transport oxygen). Recent findings suggest that high levels of serum ferritin and heme iron (the iron-containing portion of hemoglobin) are associated with an increased risk of developing diabetes. Possible mechanisms for this include higher oxidative stress due to more bioavailable iron and associated oxygen, as well as increased insulin resistance due to excess iron interfering with insulin binding. Increased mortality has also been linked to increased iron intake, especially heme iron.
Researchers hypothesized that high levels of iron in the body increase the risk of cardiovascular disease. However, there are conflicting reports on the relationship between iron intake and mortality. A U-shaped association between non-heme iron intake and mortality has been reported. It describes the hazard to health as being greatest when levels of hemoglobin are low or high, outside the optimum range. Lower hemoglobin correlates with anemia while higher levels are associated with cardiovascular disease and diabetes.
A study published in the British Journal of Nutrition reports on the association between hemoglobin levels, serum ferritin, nutritional iron intake and mortality in Chinese adults. Initial data for the study was collected from a subset population in the Jiangsu province in the year 2002 and follow-up data was sampled 10 years later in 2012. Participants were tested for hemoglobin and serum ferritin levels after an overnight fast. Iron intake was assessed by keeping a 3-day weighted food diary. For those that died during the study, the underlying cause of mortality was assessed by a household visit and information from the death registry. Deaths by cardiovascular disease and cancer were noted. Other covariates such as cigarette smoking, alcohol consumption, education, occupation, diabetes, blood pressure, and BMI were factored into the analysis as well.
The U-shaped association between hemoglobin levels and mortality in both women and men revealed the lowest mortality at 125 g/L and 140 g/L, respectively. High hemoglobin levels were associated with cancer mortality in men. In women, low or high total iron intake was positively associated with mortality. More specifically, the mortality risk increased at the level of 130g of hemoglobin/L. In China, hemoglobin levels below 110 g/L for women and 120 g/L for men is considered anemic. According to the distribution of hemoglobin amongst the anemic participants, about 55% had levels between 110 and 120 g/L in women, and 120 and 130 g/L in men. According to the Chinese criteria, this would be considered normal, indicating that half of anemic patients would be overlooked. Also, when hemoglobin levels are above 150 g/L, the mortality risk is increased. The safe window for hemoglobin is relatively narrow for women.
Information on chronic kidney disease (CKD) was absent from the study. CKD has been found to have some interaction with anemia in relation to mortality. Secondly, only a third of the participant data had dietary information and thus the authors didn’t have enough statistical power to make further correlations between iron intake and mortality. Lastly, approximately 50% of the population from the initial measurements had relocated so the follow-up data for these participants was not used for the final analysis.
Written By: Kenneth Dominguez, PhD