Iron is an essential mineral that plays many important roles in the body. It is a key component of hemoglobin, the protein in red blood cells that carries oxygen from the lungs to tissues throughout the body. Iron is also part of myoglobin, a protein that provides oxygen to muscles, and is necessary for energy production and immune system function (1).
However, iron is also a finicky nutrient that can be difficult for the body to absorb properly. There are many factors that affect iron absorption, including other foods and nutrients consumed at the same time. One food that is commonly thought to inhibit iron absorption is milk and other dairy products. But why exactly is it not advised to take iron supplements or iron-rich foods with milk?
How Iron is Absorbed in the Body
To understand why milk and iron don’t mix well, it’s first important to understand how iron is absorbed in the body. Iron is absorbed primarily in the duodenum of the small intestine. There are two main types of dietary iron (2):
– Heme iron: This iron comes from hemoglobin and myoglobin in animal source foods. Heme iron is more readily absorbed by the body.
– Non-heme iron: This is the iron found in plant foods and iron supplements. Non-heme iron is not as easily absorbed as heme iron.
Absorption of both heme and non-heme iron in the intestine occurs via specific transporters. There are two key proteins responsible for iron absorption:
– DMT1 (divalent metal transporter 1): Transports ferrous iron (Fe2+) into intestinal cells
– Ferroportin: Transports iron out of intestinal cells and into circulation
The body carefully regulates these transport proteins to maintain iron homeostasis. Iron absorption is influenced by:
Iron status
People who are iron deficient absorb more iron than those with adequate stores.
Other compounds
The presence of certain food components and nutrients can either enhance or inhibit iron absorption.
For non-heme iron, absorption is greatly influenced by the meal content and other foods consumed at the same time. Compounds that improve non-heme iron absorption include vitamin C and heme iron from meat. Substances that inhibit non-heme iron absorption include polyphenols in tea and coffee, calcium in milk and antacids, and phytates in whole grains (2).
Why Milk May Inhibit Iron Absorption
Milk and other dairy products contain high amounts of calcium along with the milk protein casein. Both calcium and casein have been shown to inhibit iron absorption when consumed together (3, 4).
Calcium
Calcium and iron compete for absorption in the intestine. Iron and calcium use some of the same transporters, including DMT1, for uptake into intestinal cells (5). When calcium and iron are present at the same time, calcium is preferentially absorbed first. This interferes with iron absorption.
Studies show that taking iron supplements or iron-rich foods with a calcium-rich meal can decrease iron absorption by 50-60% (6, 7). Higher calcium intakes are associated with reduced iron status, especially when iron intakes are low (8).
Casein and Other Milk Proteins
Casein and whey, the two main proteins in milk, have also been found to inhibit iron absorption. Several studies indicate casein reduces both heme and non-heme iron absorption when milk is consumed with an iron-rich food or supplement (9, 10).
The mechanism is not fully understood, but casein appears to form complexes with iron in the intestinal lumen, preventing it from being absorbed (11). Milk proteins may also affect iron absorption by increasing gastrointestinal transit time and reducing interaction between iron and absorption sites (12).
How Different Types of Milk Impact Iron Absorption
All types of milk contain the compounds calcium and casein that can inhibit iron absorption. However, some varieties, such as raw versus pasteurized and whole versus skim, may affect absorption differently.
Raw vs Pasteurized Milk
A few studies have found raw milk has less of an inhibitory effect on iron absorption compared to pasteurized milk when consumed with foods or supplements containing iron (13, 14).
It’s thought the heat of pasteurization causes calcium to dissociate from casein micelles, making it more available to interact with iron (14). The proteins may also become more denatured with pasteurization.
One study saw a 54% decrease in iron absorption from pasteurized milk compared to 32% from raw milk when consumed after an iron-rich meal (14).
Whole vs Skim Milk
Some research indicates whole milk may inhibit iron absorption somewhat less than skim milk.
One study in children found that consuming whole milk with an iron supplement reduced iron absorption by about 35% while skim milk reduced absorption by 54% compared to water (15).
Another study saw no significant difference between whole and skim milk, though both reduced iron absorption from a test meal by about 60% compared to water (7).
The fat content of whole milk may provide a protective effect by forming complexes with calcium and casein, preventing their interaction with iron (16). However, both whole and skim milk contain enough calcium and casein to significantly inhibit iron absorption.
Fortified vs Non-Fortified Milk
Iron-fortified milk could theoretically improve iron status, rather than inhibit it, as the additional iron may counteract the inhibitory effects of the calcium and casein.
However, research indicates that the added iron has poor bioavailability. Several studies have found fat-free and whole milk fortified with iron do not improve iron status compared to unfortified milks (17, 18).
How to Take Iron if Consuming Milk and Dairy
If milk and other dairy products are part of your diet, there are some tips to maximize iron absorption:
1. Take iron supplements between meals.
Consuming iron supplements with water between meals can help enhance absorption. Taking iron at least 2 hours apart from dairy and calcium-rich foods is recommended.
2. Enjoy calcium-rich foods later in the meal.
If you consume dairy and iron-containing foods at the same meal, have the dairy food towards the end to allow iron to absorb first.
3. Eat iron-rich foods with vitamin C.
Pairing iron-containing foods with vitamin C sources, like citrus fruits and juices, tomatoes, and bell peppers, can counteract the effects of milk and improve absorption.
4. Avoid tea, coffee, and calcium supplements with iron-rich meals.
Inhibiting substances like polyphenols in tea and coffee and calcium supplements are best avoided when consuming iron-containing foods.
5. Use iron cooking ware.
Cooking acidic foods in cast iron pots can increase the iron content significantly. The acidic environment helps release iron from the cookware into the food.
Recommended Calcium Intakes for Different Ages
Calcium is still an essential nutrient, so you don’t want to avoid calcium entirely when trying to improve iron absorption. Getting adequate amounts of calcium is important for bone health.
The recommended daily intakes for calcium are (19):
| Age | Calcium RDA |
|---|---|
| 0 to 6 months | 200 mg/day |
| 6 to 12 months | 260 mg/day |
| 1 to 3 years | 700 mg/day |
| 4 to 8 years | 1,000 mg/day |
| 9 to 13 years | 1,300 mg/day |
| 14 to 18 years | 1,300 mg/day |
| 19 to 50 years | 1,000 mg/day |
| 51 to 70 years (men) | 1,000 mg/day |
| 51 to 70 years (women) | 1,200 mg/day |
| Over 70 years | 1,200 mg/day |
Aim to meet but not greatly exceed calcium recommendations to get enough for bone health while not inhibiting iron absorption.
Recommended Iron Intakes for Different Ages and Sexes
Iron needs vary based on age, sex, and life stage due to differences in growth, menstruation losses, and pregnancy demands. The daily recommended intakes for iron are (20):
| Age | Males | Females |
|---|---|---|
| 0 to 6 months | 0.27 mg/day | 0.27 mg/day |
| 7 to 12 months | 11 mg/day | 11 mg/day |
| 1 to 3 years | 7 mg/day | 7 mg/day |
| 4 to 8 years | 10 mg/day | 10 mg/day |
| 9 to 13 years | 8 mg/day | 8 mg/day |
| 14 to 18 years | 11 mg/day | 15 mg/day |
| 19 to 50 years | 8 mg/day | 18 mg/day |
| 51 years and older | 8 mg/day | 8 mg/day |
Women of childbearing age have higher needs to replace iron losses from menstruation. Getting adequate iron is important for all populations at risk of deficiency.
Food Sources of Iron
Dietary iron comes in two forms: heme iron found in meat and non-heme iron found in plant foods and supplements. Here are some top food sources of iron (21):
Heme iron foods:
– Clams, oysters, mussels
– Red meats (beef, lamb, liver)
– Fish (tuna, salmon, sardines)
– Poultry (chicken, turkey)
Non-heme iron foods:
– Lentils, beans (kidney, lima, navy)
– Tofu
– Spinach
– Nuts and seeds (cashews, sesame)
– Dried fruits (prunes, raisins)
– Iron-fortified cereals and grains
Pairing non-heme iron sources with vitamin C foods can help increase absorption. Getting adequate iron from dietary sources should be prioritized over supplements whenever possible.
Signs of Iron Deficiency
Iron deficiency develops gradually in stages (22):
Stage 1: Iron depletion
Iron stores are decreased but no functional changes in hemoglobin levels. Most common in menstruating women.
Stage 2: Iron deficient erythropoiesis
Transport iron is decreased and production of iron-containing proteins like hemoglobin declines. Common symptoms include fatigue, headaches, irritability, and reduced work capacity.
Stage 3: Iron deficiency anemia
Hemoglobin levels fall below the normal range. Symptoms are fatigue, weakness, pale skin, chest pain, dizziness, and immune dysfunction.
Women of childbearing age, pregnant women, infants, children, and teens are at highest risk of iron deficiency. Paying attention to iron intake and absorption can help prevent deficiency.
Conclusion
Iron and calcium can compete for absorption in the body. The calcium and casein in milk and other dairy products have been shown to substantially inhibit iron absorption from foods and supplements. To maximize iron absorption, avoid taking iron supplements with milk or consuming milk with iron-rich meals.
However, calcium is still an essential nutrient, so include dairy in the diet away from iron-containing foods or supplements. Those at risk of iron deficiency should aim to meet increased iron needs through dietary heme and non-heme iron sources paired with vitamin C foods. Being mindful of recommended intakes for both calcium and iron can help optimize intake and absorption of these two important minerals.