We can’t live without iron in our diets but …
Iron is an essential micronutrient for humans, but excess iron intake predicts problems, possibly including increased risk of type 2 diabetes. Researchers in China conducted a systematic review and meta-analysis of 11 prospective studies that related intakes of total iron and the heme form of iron, as well as total body iron stores, to risk of developing type 2 diabetes during follow-up. Data pooled from 5 of the 11 studies showed that the highest compared to the lowest level of daily heme iron intake predicted a significant 33 percent higher risk of developing type 2 diabetes. The risk of type 2 diabetes increased by 16 percent for every 1 mg increase in daily heme iron intake. Total, non-heme, and supplemental iron intakes were not related to type 2 diabetes risk. Data pooled from 6 studies showed that highest compared to the lowest level of body iron stores, measured as ferritin, predicted a significant 70 higher risk of developing type 2 diabetes. All results reflect multi-variable adjustment for confounding factors. At least in countries where most residents consume plenty of iron-containing foods, further increasing iron intake may not be healthful.
We can’t survive without heme iron but …
Chronic inflammation, which arises in part from an unhealthy diet, underlies the metabolic syndrome, type 2 diabetes, and cardiovascular disease. Researchers used data from 8,285 participants with an average age of 62 years in the Multi-ethnic Study of Atherosclerosis to determine if iron, zinc, vitamin E, vitamin C, magnesium, and beta-carotene intakes were linked to risks of developing the metabolic syndrome, type 2 diabetes, and cardiovascular disease. Dietary data came from a 120-item food frequency questionnaire at baseline that inquired about usual food intakes over the previous year. Amounts of the nutrients in question were grouped into increasing quintiles of intake.
After accounting for confounding factors and compared to the lowest quintile of intake, the highest quintiles of heme iron from red meat and zinc from red meat predicted significant 25 and 42 percent higher risks, respectively, of developing the metabolic syndrome during 5 years of follow-up. For cardiovascular disease, the highest quintile of from heme iron from red meat increased risk by 57 percent, while zinc from red meat was not linked to cardiovascular disease risk. Neither heme iron from animal sources nor zinc from animal sources was significantly linked to risk of developing type 2 diabetes. After accounting for confounding factors and compared to the lowest quintiles of intake, the highest quintiles of vitamin C, magnesium, beta-carotene, and zinc and heme iron from non-red meat sources were not associated with the risk of developing any of the three chronic diseases during 5-6 years of follow-up. Higher levels of heme iron and zinc and/or something else in red meat or the way it’s cooked may increase risk of the metabolic syndrome.
Something about red meat
Increased intake of processed red meat predicts increased risk of type 2 diabetes. The link between unprocessed red meat and type 2 diabetes remains less certain. Red meat has relatively high concentrations of heme iron, which has been linked to increased risk of type 2 diabetes. Researchers used data from 45,411 participants with an average age of 55 years in the Singapore Chinese Health Study to determine if total dietary iron, dietary heme iron, and different types of meat affected the risk of developing type 2 diabetes. Baseline dietary data came from validated food frequency questionnaires.
After accounting for confounding factors, the risk of developing type 2 diabetes during 11 years of follow-up increased significantly from the first to the fourth quartiles of red meat intake. Compared to the lowest quartile, participants in the highest quartile of red meat intake had a significant 23 percent greater risk of developing type 2 diabetes during follow-up. Replacing one daily serving (1.8 ounces) of red meat with the same amount of fish or shellfish significantly reduced the risk of type 2 diabetes by 22 percent. Women—but not men—participants in the highest quartile of heme iron intake also had a significant 33 percent higher risk of developing type 2 diabetes compared to women participants in the lowest quartile. Further adjusting for red meat intake diminished to risk of 14 percent but remained statistically significant. Non-heme iron was not associated with risk of developing type 2 diabetes. Overall, some aspect(s) of red meat, including heme iron, but not other forms of iron, appear to increase the risk of type 2 diabetes in Chinese people living in Singapore.
Daily total iron intake for men
The prevalence of cardiovascular disease has exploded in China over the past few decades, increasing from 10 to 24 percent nationwide. One possible cause might be high levels of dietary iron intake, especially heme iron. Chinese residents have substantially higher daily intake of iron than residents of Western countries. High intakes of iron could lead to high levels of iron storage, which is linked to chronic inflammation and increased risk of the metabolic syndrome. Animal foods, especially red meat, contain relatively high levels of heme iron, which is preferentially absorbed by the body.
Chinese researchers used cross-sectional data from 3,699 participants in the Shanghai Diet and Health Survey to determine if iron intake and bodily iron stores would predict risk of the metabolic syndrome, which increases risk of cardiovascular disease. Dietary iron intake data, excluding supplements and medications, came from 24-hour diet records over 3 days. Dietary iron came largely from grains, potatoes, and vegetables, reflecting the participants’ plant-based diets. After adjusting for confounding factors and compared to men participants in the lowest quartile of daily total iron intake, men participants in the highest quartile had a significant 111 percent higher odds of having the metabolic syndrome. Total daily total iron intake was not associated with the metabolic syndrome for women. Daily intake of heme iron was not associated with the metabolic syndrome for men or women.
Total and non-heme iron intakes in women
Iron metabolism disorders can create oxidative stress, chronic inflammation, and insulin resistance, thereby increasing risk of type 2 diabetes. Would increased intake of dietary iron in persons with type 2 diabetes increase their risk of premature death? Researchers used data from 4,416 diabetic Americans with an average age of 61 years in the US National Health and Nutrition Examination Survey to find out. Dietary data came from 24-hour dietary recall surveys on 2 non-consecutive days. After adjusting for a range of potential confounders, including dietary factors, and compared to women in the lowest quartile of daily total iron and non-heme iron intake, women in the highest quartiles had significant 133 and 128 percent higher risks, respectively, of dying from diabetes. For men, compared to the lowest quartile of heme iron intake, men in the highest quartile had significant 61 and 42 percent higher risks, respectively, of dying from cardiovascular disease or of any cause. Diabetic women with higher levels of total iron and non-heme iron intake were more likely than diabetic men to die of a chronic disease. Diabetic people, especially women, might not want to eat huge quantities of iron-containing foods or to take supplements, such as multi-vitamins, with iron.
Iron deficiency and iron deficiency anemia
A recent review of dietary heme iron focused on safely reducing iron deficiency and iron deficiency anemia. An estimated 2 billion people globally suffer from iron deficiency anemia. About one-third of American 1-2 year-old children have iron deficiency. Addressing iron deficiency and iron deficiency anemia is not as simple as taking a supplement. Dietary iron come in two forms, heme iron (mainly from animal sources) and non-heme iron (from plant sources, especially grains, legumes, and vegetables). The human body absorbs heme iron much more readily than non-heme iron. Certain compounds, such as phytates in whole grains, beans and nuts, inhibit non-heme iron, zinc, and calcium absorption. Supplements of non-heme iron, such as ferrous gluconate, can create gastrointestinal problems and lead to high elemental iron concentrations in the gut, which may cause oxidative stress and intestinal damage.
Some evidence suggests that heme iron facilitates the absorption of non-heme iron. We Americans get most of our dietary iron in the form of heme iron from meat. Alas, higher intakes of meat, especially red meat, predicts increased risks of colorectal cancer. Preservatives, such as nitrates and nitrites, added processed meats appear to increase risks of certain cancers. Heme iron in meat can catalyze the formation of carcinogenic N-nitroso compounds and lipid peroxidase products. Carcinogenic heterocyclic amines and polycyclic aromatic hydrocarbon chemicals created in cooking (especially when cooked dry and at high temperatures) diminish the attractiveness of meat as a source of iron. One possible way to consume enough iron and avoid excess free iron in the gut involves eating modest amounts of meat (for example, one ounce a day). The heme iron will increase the absorption of non-heme iron in the whole grains, beans, and vegetables we eat and avoid excess elemental iron in the gut.
What to do
The current evidence relating daily heme iron and total iron intake to chronic diseases seems confusing. Men and women appear to react differently to high levels of heme iron and total iron intake. Those of us who eat plenty of vegetables (including beans) or eat meat frequently probably get enough iron in our diets and don’t need any more from a supplement, such as a multi-vitamin with minerals. Something about meat, especially red meat, perhaps the high concentration of heme iron and potentially carcinogenic compounds created during cooking may increase risks of chronic diseases. Big red meat eaters might want to cut back on their red meat consumption.
