Summary
- Babies get their iron with milk, and they turn out okay.
- Limited studies show that milk does not affect iron absorption more than other foods.
- One study using iron(III), like we use, showed that milk did not affect absorption.
- Iron absorption studies cannot monitor ferritin, which is a major caveat to their conclusions.
- It is okay to take milk with iron, especially if you are sensitive to iron on an empty stomach.
Does science really say you shouldn't take iron with dairy?
It’s commonly advised to avoid mixing iron supplements and dairy. The consensus is that dairy inhibits iron absorption due to the presence of calcium and/or casein. However, almost every baby gets their iron along with dairy, and we’ve turned out just fine. In this post, we investigate the science behind whether dairy affects iron absorption.
Why is iron recommended on an empty stomach?
Let’s begin with the important context. Iron supplements are recommended on an empty stomach because certain nutrients can bind to the iron, and certain minerals can compete with iron to be absorbed. If these events occur, they reduce how much iron gets to your body.
How can I enhance iron absorption?
Many foods are known to reduce iron absorption. Only animal tissue and vitamin C are known to improve iron absorption. Even among these two absorption boosters, one recent trial of 440 anemic adults found that taking vitamin C with iron did not significantly improve hemoglobin or ferritin levels. The take-home message is that long-term iron regulation is complex.
How do people study iron absorption in humans?
Studies of iron absorption in humans typically follow a similar protocol. Iron supplements are modified to include radioactive iron tracers and are administered to subjects under varying conditions (e.g., with or without milk). Later, the subjects’ blood is measured for levels of the radioactive iron, which represent how much of the administered iron was absorbed. A crucial limit of this methodology is that it only tracks iron in the blood, leaving it blind to iron stored as ferritin. Given that 30% of the body’s iron is stored in ferritin, this is a major shortcoming.
Why does it matter that iron absorption studies do not track ferritin?
Ferritin is a protein that the body uses to store iron. Ferritin levels are often regarded as the best measure of iron status. In the body, about 70% of iron is in the blood and about 30% is stored as ferritin in the liver. When monitoring iron absorption, it would be very valuable to track ferritin; however, there is no good way to “follow” the iron (as ferritin) into the liver. Therefore, conclusions made from human studies on iron absorption are blind to whether the trends of iron absorption into blood apply to iron absorption into ferritin. It is conceivable that ingesting iron with other macronutrients leads to more storage as ferritin than ingesting iron pill on an empty stomach. If this were true, it would severely compromise our understanding of how to best resolve low iron.
How does milk affect iron absorption?
There are two questions when considering how milk affects iron absorption: How does milk influence iron compared to water, and how does milk influence iron absorption compared to food? Compared to water, taking iron with milk (or any non-meat food) reduces absorption, typically by a factor of 2-3x. However, taking iron with water causes bad reactions in at least one third of users, such that many of us have to take iron with food or avoid it altogether.
How does milk affect iron absorption compared to other foods?
Despite the consensus about not mixing milk and iron, there is not much human data to support this position. We screened over 200 studies in Google Scholar based on the keyword search {“Iron”, “Milk” OR “Dairy”, Absorption}. Of the 200, we determined nine to be relevant to this analysis, based on these studies including a clinical trial measuring iron absorption with/without milk. The table below lists these studies and their key features. Of the eligible studies, 4/9 (44%) found that milk significantly affected iron absorption, while 5/9 (56%) found no statistically significant effect. Based on these reports, we conclude that the evidence does not indicate milk to be worse than other foods at inhibiting iron absorption.
Does the type of iron affect how milk influences absorption?
In addition to the limit of not tracking ferritin, iron absorption studies are limited due to comparing different types and doses of iron. For example, in our list of relevant studies, four used iron from food, one used iron from cow/human milk, and the remaining four used four different types of iron supplements. When digested, both the molecule bound to the iron – e.g., sulfate or glycinate – and the charge of the iron – e.g., iron(II) or iron(III) – influence how other components affect iron absorption.
Are there any studies on how milk influences iron(III) absorption?
In addition to the limit of not tracking ferritin, iron absorption studies are limited due to comparing different types and doses of iron. For example, in our list of relevant studies, four used iron from food, one used iron from cow/human milk, and the remaining four used four different types of iron supplements. When digested, both the molecule bound to the iron – e.g., sulfate or glycinate – and the charge of the iron – e.g., iron(II) or iron(III) – influence how other components affect iron absorption. At Smart Eats, we use iron(III) glycinate. This choice is very important when considering that milk is primarily thought to inhibit absorption due to the presence of calcium. The theory is that when calcium(II) and iron(II) are both present in the body, calcium(II) is more preferentially absorbed through a specific ion channel (DMT1) that targets metal(II) ions. In this model, iron(III) is not subject to the same competition with calcium.
In fact, iron(III) is absorbed through a distinct pathway where receptors bind iron(III), convert it to iron(II) at the cellular membrane, and then push it through that DMT1 ion channel. Consistent with this model, the one study on our list that examined iron(III) absorption (using FeCl3) found that milk did not reduce iron absorption when added to the meal.
So, can I take iron with milk?
In conclusion, you can take iron with milk! If you can tolerate iron on an empty stomach, with water and vitamin C, that is still likely to be the best option. But if that bothers you, like it does to many others, milk is an acceptable way to improve tolerance. In particular, your absorption of iron(III) or “ferric” iron, as used in Iron Lift, is unlikely to be affected by milk.
List of References and Relevant Details
Listed below are the studies we reviewed. The italicized titles (4/9, 72 subjects) showed a significant effect of milk on iron absorption. The non-italicized titles (5/9, 96 subjects) showed an insignificant effect of milk on iron absorption. For each reference, the study design, number of subjects, and iron source are listed. With over half the studies and participants showing no significant effect of milk on iron absorption, it cannot be concluded that milk inhibits iron absorption.
| Study Title | Year | Journal | Study Design | Subjects | Iron Source |
| Iron absorption from the whole diet: comparison of the effect of two different distributions of daily calcium intake | 1995 | Am J Clin Nutr | 3 meals with labeled iron; consumed with even calcium distribution, or with no calcium midday | 21 | Ferrous Sulphate, 13.3 mg/day |
| Bioavailability in Man of Iron in Human Milk and Cow’s Milk in Relation to Their Calcium Content | 1992 | Pediatri Research | Human milk vs cow’s milk; human milk vs human milk with added calcium | 18 | From milk |
| Milk Inhibits and Ascorbic Acid Favors Ferrous Bis-Glycine Chelate Bioavailability in Humans | 1997 | Nutrient Requirements and Interactions | Iron with/without milk; Iron with/without milk + vitamin C | 14 | Ferrous bis-glycinate/Ferrous ascorbate, 3mg; |
| Effects of different calcium sources on iron absorption in postmenopausal women | 1990 | Am J Clin Nutr | Meal with iron and placebo or calcium | 19 | Ferrous Sulphate, 0.18 mg |
| Effect of calcium intake on nonheme-iron absorption from a complete diet | 1997 | Am J Clin Nutr | High/low calcium diets | 14 | Ferric Chloride, 0.1mg |
| Effects of high compared with low calcium intake on calcium absorption and incorporation of iron by red blood cells in small children | 1999 | Am J Clin Nutr | 5-week high/low calcium diets | 11 | From food, 9 mg/day |
| The Addition of Milk or Yogurt to a Plant-Based Diet Increases Zinc Bioavailability but Does Not Affect Iron Bioavailability in Women | 2005 | Community and International Nutrition | 3 groups: test meal; + milk; + yogurt | 48 | From food, 8.7 mg |
| Calcium from milk or calcium-fortified foods does not inhibit nonheme-iron absorption from a whole diet consumed over a 4-d period | 2004 | Am J Clin Nutr | 4 meals: no calcium + 3 calcium sources | 14 | From food, 13.2 mg/day |
| Effect of milk and fermented milk on iron absorption in ileostomy subjects | 1995 | Am J Clin Nutr | 4 diet periods: 2 soft drinks + 2 milk types | 9 | From food, 12.2 mg/day |
