Neither coffee, milk or vitamin C affects heme-iron absorption from meats

Background

Peatarians frequently recommend drinking coffee and milk with a meal of meat to inhibit iron absorption. It is also recommended to not combine vitamin C rich juices with meats as this increases iron absorption. 

Review

The majority (70-90%) of iron in red meat is heme-iron (actually it is red because of it) (0). Heme-iron is a ferrous iron ion (Fe 2+) that's chelated in a large porphyrin ring. This form of iron is very efficiently absorbed by the body, more than non-heme iron (1). Unfortunately, uptake of this form of iron is also quite poorly regulated by the body. While there is a very efficient regulating mechanism for non-heme iron, that enhances iron uptake in deficiency states, while decreasing uptake in sufficient states, heme-iron absorption is only very marginally reduced in a sufficient iron state (2). Looking on a single meal, the maximum heme-iron absorption is saturable (3). So it might be wise to eat meat only on one occasion, instead of splitting it up throughout the day.

This difference might explain why higher consumption of (heme-iron rich) red meats (i.e. beef), but not (lower heme-iron) white meats (i.e. chicken), is associated with a higher risk for several diseases, such as type-2 diabetes, cancer and cardiovascular disease (4).

Given these data it would be great to have something that inhibits heme-iron intake. But there are bad news: Heme-iron absorption has been found to be highly resistant to dietary absorption inhibitors.

• I couldn't find a single clear-cut study that showed that coffee inhibits heme-iron absorption. All studies I found only looked at non-heme iron. In one study they even added it to hamburgers but didn't measured heme-iron absorption (duh) (5). A new york times article based on an older study said that coffee did not affect heme-iron absorption (6). Unfortunately, I don't have access to the fulltext of that study (7). It is however plausible that it doesn't affect it, since heme-iron is absorbed differently than iron-salts. Experts in the field have a similar view, stating:

"Heme iron is relatively well absorbed under all circumstances. Moreover, heme iron absorption is relatively independent of meal composition and little affected by the enhancers and inhibitors that alter nonheme absorption." (8)

• Calcium doesn't inhibit non-heme or heme-iron absorption in doses below 800-1000mg calcium per meal (9). So for milk to have an effect, you would have to drink roughly 800-900ml of it along with your meat. At these doses, calcium also hampers with zinc absorption (10).
• Phytates inhibited non-heme-iron, but didn't affect heme-iron absorption (in contrast even seemed to slightly increase it) (11).
• Meat itself appears to increase the absorption of heme-iron, compared to pure heme-iron (i.e. blood) (12).

Here are some studies that show that iron inhibiting/enhancing strategies have no or only very marginal effects on iron status in the long term. Maybe because the body itself is pretty good in increasing iron absorption when it is deprived of it and vice versa.

• The enhancing effect of vitamin C on non-heme iron absorption is far less pronounced in full meals (most studies on that were done with isolated iron + isolated vitamin C) http://ajcn.nutrition.org/content/73/1/93.full.pdf+html
• Adding 2g (!) of ascorbic acid with meals for 16 weeks only had a very negligible effect on iron status, especially when the diet contains a lot of meat. http://www.ncbi.nlm.nih.gov/pubmed/6466873
• The addition of 500mg of Calciumcarbonate daily for 2 years didn't effect iron stores in a randomized controlled trial http://ajcn.nutrition.org/content/82/1/98.full
• Adding 500mg calcium carbonate in lactating and non-lactating women didnt affect iron status in a randomized controlled trial http://www.ncbi.nlm.nih.gov/pubmed/9625100
• Adding 1200mg Ca/d with meals over 6 months didnt affect markers of iron status in healthy adult volunteers http://ajcn.nutrition.org/content/68/1/96.full.pdf
• Long-term supplementation with calcium (total intake: approximately 1500 mg/d) did not affect iron status in adolescent girls http://www.ncbi.nlm.nih.gov/pubmed/9771866
• Only meat, but not calcium, vitamin C and dairy intake was associated with iron status in 4-year old children http://ajcn.nutrition.org/content/87/4/839.full.pdf+html
• Calcium intake is weakly associated with a lower ferritin level, irrespective of whether the calcium is ingested together with iron. (Suggesting that calcium doesnt inhibit iron absorption, but high dairy consumption displaces the consumption of iron-rich meats as a calorie source). http://jn.nutrition.org/content/129/5/963.short
• Discontinuing coffee in young toddlers for 2-5 months didn't significantly affect iron status in a randomzed controlled trial http://ajcn.nutrition.org/content/66/1/168.short
• In elderly subjects, coffee, vitamin c and calcium intakes only had a very weak association with iron status. The biggest predictors of iron status were heme-iron, meat and and supplemental-iron intake. The biggest, albeit modest, negative association with iron status was regular coffee intake. Tea intake had no significant association (although it was found to be stronger inhibitor in short term studies than coffee). http://ajcn.nutrition.org/content/67/4/722.full.pdf+html
• Tea consumption was not associated with an altered iron metabolism in french adults http://www.nature.com/ejcn/journal/v61/n10/abs/1602634a.html

This all sounds depressing but there's also good news: 

• In contrast to its effect on non-heme iron, Vitamin C doesn't increase the absorption of heme-iron (11) .

Conclusions

• Coffee and milk at the usual dose don't have significant effects on heme-iron absorption. So don't expect both to protect you from iron toxicity of too much red meat. The only effective way is to limit heme-iron rich foods (i.e. by switching from red to white meat, or looking for other protein sources). At last, both coffee and milk have been found to hamper with zinc absorption. And red meat is a great source of zinc you don't want to miss out on.
• It may still make sense to drink coffee/milk with meat when you know that your iron stores are high, as it could chelate the non-heme iron portion of it. For the average healthy person, the body already has a pretty decent system to regulate its non-heme iron intake.
• Fruit or juice with meats will not enhance heme-iron absorption. So it can be combined occasionally.

At last, Ray Peat on this issue: 

• "I think that's true, that coffee affects mainly non-heme iron absorption. The heme has toxic effects, forming carbon monoxide, apart from the iron." - Ray Peat (E-Mail advice)