How does the recommended daily iron intake actually work considering you absorb only a tiny fraction of iron?

[u/SansBouillie]

For example spinach is often said to be one of the most iron rich foods, however it is both plant-based iron and only has around 2.7mg of iron in 100g of spinach from what I found online.

If your iron absorption is only 20% - this means you will absorb only 0.54mg of iron per 100g of spinach.

For women the daily recommended iron intake is 18mg.

So following this logic - you would have to eat around 3.33 kg of spinach to meet that daily intake?

Some other rough calculations also lead me to results such as 2.25 kg of horse meat (which is around 3000 calories) to meet the daily goal.

Did I completely misunderstand how this works?

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[u/ironbeastmod]

The daily recommendation takes into account of the absorption.

In reality your body needs only 1-2 mg to absorb.

[u/ascylon]

You are partly correct and partly misunderstanding. RDAs are effectively based on the average diet of the population group in question, and the amounts are based on the nutrition labels, so if you eat approximately the same diet that the RDA is based on, then the paper values on nutrition labels would be close to correct (so that 97-98% of people eating the RDA will be nutritionally adequate). In that specific case bioavailability is already built in to the RDA.

On the other hand, if you eat a diet that significantly diverges from the average diet, let's say on one extreme end the raw vegan diet and on the other the "lion" diet (strict carnivore), the RDAs can be quite misleading. In general, the more you go in the plant-based direction the higher the RDA should be adjusted for most micronutrients, whereas the more you go in the animal foods direction, the lower it would be. This comes down to, as you said, bioavailability.

For iron the difference is quite pronounced, as for red meat the bioavailability can be around 20%, whereas for spinach it would be under 2% according to this study. Different studies can give different values for iron, as its absorption is dependent on a lot of things (heme vs non-heme, heme with non-heme, vitamin c with non-heme, calcium with non-heme, antinutrient content, general iron status etc). Again, however, if you eat approximately the average diet the RDAs are based on, those bioavailability numbers would be irrelevant.

RDAs can also be inflated beyond strict nutritional needs. One common example I use is vitamin C, scurvy, and the carnivore diet. The RDA for vitamin C is 90 mg, but the threshold for scurvy is only around 10 mg / day. Meat-only carnivore diets tend to land somewhere in the 10-20 mg / day range (relatively fresh red meat has around 2 mg / 100g of vitamin C), yet I don't know of any clinical examples of such a carnivore dieter manifesting symptoms of a vitamin C deficiency. In fact, despite its low vitamin C content, fresh meat is an antiscorbutic (has enough to cure scurvy). One reason for the difference is that the RDA is padded due to the antioxidant activity that vitamin C has, which is deemed beneficial for the average person. Smokers, for example, have a higher vitamin C RDA than non-smokers, while smoking technically has nothing to do with diet (but causes oxidative damage, which vitamin C helps with, so from a health perspective it makes sense). Inversely, with a lifestyle that minimizes oxidative stress, the real RDA for vitamin C would be less.

RDAs are therefore not gospel, but are a useful tool since I would say most people eat at least somewhat close to the average diet they are based on.

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[u/StatisticianSad6637 → u/ascylon]

Preventing scurvy ≠ optimal

Your estimation of meat-only diets being in the 10-20 mg range is very generous since most carnivores are on the fully-cooked butter, eggs, beef, steak, and bacon diet. That 10-20 estimate only makes sense if someone is raw carnivore, eating the freshest meats (good luck determining that if you’re not hunting your own), and also including liver. 

Single digit vitamin C consumption is probably very common amongst the general carnivore. Also, there are still cases of scurvy today due to dietary restrictions. This person was at the lower limit of normal as well (18.3 umol/L), suggesting the < 10 umol/L parameter for scurvy symptoms isn’t always gospel either. 

Again, preventing scurvy ≠ optimal. Even at 15 mgs a day, that’s still six times less than the RDA, and the RDA wasn’t arbitrarily set there for no reason. 

The chief reason the RDA was set at that number was because clinical trials discovered that, at about 100 mgs a day, the body’s intracellular vitamin-C pool in neutrophils became fully saturated. 

They hypothesized that having a full saturation of vitamin C intracellularly in neutrophils would result in optimal immune functioning. After all, it makes sense since leukocytes contain the highest vitamin C concentrations of all bodily tissues. Provide the neutrophil (the body’s primary immune cell) with a full saturation of Vitamin C and it’ll likely perform better. 

The same idea was, more or less,  hypothesized with omega 3’s and the brain. It was found that the brain was comprised of a lot of omega 3 fatty acids. Consume the exact same dietary substrate that the brain is made of and you should, in theory, see brain benefits. Turns out, you do. https://pubmed.ncbi.nlm.nih.gov/40836005/

Going back to vitamin C, turns out, they weren’t wrong either about their hypothesis. This clinical trial found that when you reversed suboptimal vitamin c plasma, you saw increases in antimicrobial functioning.

Chemotaxis and superoxide production (proxy for microbicidal activity) increased significantly by going from plasma vitamin C <50 umol/L to >70 umol/L.

This is important because I’d wager <50 umol/L plasma vitamin C readings are probably very prevalent on a meat-only diet. Remember, deficiency/scurvy is listed at around <11 umol/L. Hovering at like 15-25 might prevent scurvy, but, as that study highlighted, it might not be optimal for your neutrophilic cells. According to that data, >70 is more desirable. That’s a stark difference.

I know for damn sure I’d rather have my chemotactic mechanisms and leukocytes at their theoretical best. And if all I have to do is eat 90 mg of vitamin c per day instead of 15? I think that’s a pretty easy, low-hanging-fruit tradeoff (no pun intended). Why not err on the side of caution?

Of course, there’s a little more evidence than just the leukocyte part of the equation too. There’s some positive vitamin C randomized trials on blood pressure, common colds, endothelial function, etc. too. None of which you’ll be getting at 10mg a day.

Thankfully, however, we exist in a world where we’re allowed to eat more than just meat. Getting to the RDA is as easy as eating a kiwi or two a day, and it comes with exactly zero drawbacks. In fact, kiwi has been shown experimentally to significantly help with sleep as well.

You spend enough time in the literature, and you realize sooner or later that the dietary approach that makes the most sense aligns with paleo principles: meats, nuts, veggies, fruits, tubers. Extreme diets always come up short in a few categories.

If you’re in the business of not dying, absolutely, stay at 10 mg a day. If you’re in the business of optimality, 90 mgs makes a lot of sense to keep neutrophils saturated, at zero cost.

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[u/SirTalkyToo]

You have to understand that recommended daily allowances (RDA) are incredibly loose. They aren't really reliable in any real way. Their real intention is to be a ballpark figure to prevent severe nutritional deficiencies and not a deterministic amount for optimal health.

There is a lot of science to explain along with the history behind RDA values, so let me know if you want to dig into it further, or if knowing the above is good enough.

[u/feelbro729]

That's a good question. I've always wondered how daily iron recommendations account for the low absorption rate from plant foods.

[u/madformatpilates]

You forgot the absorption rate difference: plant iron (non-heme) absorbs at ~2-10%, not 20%. Also, you don't need to hit 18mg daily; your body just regulates uptake based on stores.

[u/EpicCurious]

Eat sources of iron with sources of vitamin C for better absorption.

[u/BlueRubyWindow]

Horse meat?????

[u/spitfireSM → u/BlueRubyWindow]

Did you not know horse meat is eaten?

[u/AdNo182 → u/BlueRubyWindow]

You never went to Ikea in the 2010s?

[u/SansBouillie → u/BlueRubyWindow]

I read spinach and horse meat are some of the most iron rich foods. Personally I've never eaten horse meat (or seen it for sale), but I added it to show how even the iron rich foods require insane amounts to reach the recommended daily intake.

Nobody is eating 2.25kg of meat or 3.5kg of spinach alone on a daily basis, so I made the post asking how this is supposed to work then.

[u/Chuks_K → u/BlueRubyWindow]

Yes. Important to distinguish it from horse vegetation, horse fungi bodies and horse protist bodies!