Spinach is one of those foods that are classically regarded as healthy. It made its way into the category of superfoods by being part of the “dark, leafy greens”, which are considered high in nutrients. You will have a hard time finding a green smoothie recipe that doesn’t include a heap of spinach. But, is it justified? Let’s check it out!
Claim: Spinach is high in vitamin A
Technically this isn’t correct since vitamin A (retinol) can only be found in animal foods. Spinach contains beta-carotene which is a provitamin. However, poor bioavailability and conversion rates put a big dampener on the effectiveness.
The gist is, that the bioavailability of beta-carotene is especially low in spinach resulting in a conversion ration of 21:1. 100 g of baby spinach contains about 3400 µg of beta-carotene, which would result in ~162 µg of retinol. This is however highly individual and depending on many factors and it could be much lower than you’d expect. I’ve made an in-depth post about beta-carotene if you want to check it out.
Going by the averages, 162 µg of retinol is a fine amount. But at the same time, it’s nothing special either. A single egg will give you 75 µg of highly bioavailable, pre-formed retinol.
Claim: Spinach is high in calcium
Spinach does indeed contain a relatively high amount of calcium. To be specific, 100 g spinach contains 100 mg of calcium. That would be an average 10% of the daily requirement. That’s pretty good, until you consider the meager 5% bioavailability. Just because the food contains it on paper, does not mean you actually absorb it. This leaves you with 0.5% of your daily requirement covered… riveting.
In spinach the culprit is mainly oxalates (also called oxalic acid) which binds to the calcium and prevents you from absorbing it. There’s more going on with oxalates than just calcium absorption, but I’ll have to write an extra article for that.
You know what doesn’t contain oxalates? Eggs (again). While eggs aren’t exactly calcium powerhouses either, a single one will still net you 25 mg of calcium. Unless you consider eating the shell which clocks in at ~400 mg of highly absorbable calcium per gram.
Claim: Spinach is high in iron
Historically spinach has been considered to be extremely rich in iron, which turned out to be simply a misplaced decimal point. This mistake has long been remedied, yet still it is considered a good source of iron. At 2.7 mg per 100g of spinach this fulfills 31% of the requirements for men and 18% for women.
But you probably already guessed it: Bioavailability is only between 2% and 7%. Firstly this is down to the iron being elemental iron, which is not well absorbed by humans compared to heme-iron in meat. Secondly, the oxalates I mentioned earlier also bind to iron, further decreasing the bioavailability.
Claim: Spinach is high in potassium
I actually struggled to find any definitive answers to the bioavailability of potassium in spinach, but it appears to be somewhere between 50-85%. This is actually a little bit surprising to me as I expected it to be similar to iron and calcium, since oxalates also bind to potassium. I thought I’d see a similar decrease in bioavailability here but this might be down to spinach also containing saponins which does not appear to interact with potassium. (More on saponins further down)
Either way, at 558 mg per 100 g and factoring in the bioavailability, spinach turns out to be comparable to meat in terms of potassium. Enjoy your potassium leaf-munchers.
Further claims: Folate, Vitamin C
Humans should take in between 400-600 μg of folate (Vitamin B9) per day and spinach contains 165 μg per 100 g. This is a reasonably high amount, and it’s actually quite bioavailable… sort of. Eating whole spinach leaves decreases absorbability since the fiber entraps the folate. Cooking also directly destroys ~50% of folate on average. So your best bet for folate from spinach are probably those nasty green smoothies.
Vitamin C is a similar story, as cooking will destroy Vitamin C. All things considered, spinach has a high amount of Vitamin C. Good, but I personally would rather eat a handful of berries for Vitamin C. Also, I’m not done with spinach just yet.
Protease inhibitors
Spinach isn’t exactly known for its high protein content. That’s probably because it isn’t high in protein (duh), at 3 g per 100 g. However, spinach contains a so-called protease inhibitor that strongly inhibits the enzyme trypsin. Trypsin starts the digestion of proteins in the first section of the small intestine and is required to absorb proteins and vital amino acids.
In other words, maybe you don’t want to do one of those eggs & spinach scrambles if you want to actually get the proteins from the egg.
Saponins
Spinach also contains saponins in a relatively high amount compared to other vegetables. Saponins are bitter tasting and generally toxic. The bitter taste is supposed to deter animals from eating the plant (antifeedant) by making it less palatable. While severe saponin toxicity is rare, it has some nasty effects on the body regardless.
They can lead to gastrointestinal distress, leaky gut and they have antinutrient properties (inhibits: magnesium, iron, zinc, Vitamin A, Vitamin E, calcium). They have even been implicated in potentially negatively impacting fertility.
Storage nutrient loss
Lastly, simply storing the spinach automatically loses nutrients over time, even when stored in refrigerators. After just 3 days, 80% of Vitamin C is gone. After 8 days of storage in a fridge, over 50% of folate is lost as well as over 50% of carotenoids. This means the praised nutritional benefits of spinach my even be worse depending on your source of spinach. If it’s stored at warmer temperatures, the loss is accelerated.
Actually fresh produce is harder to come by than you’d think. Spinach can be picked several weeks before it lands on supermarket shelves. It’s kept looking fresh through storage methods, but looking fresh is not an indication that it hasn’t lost nutrients. And after you stored it for a few more days in your fridge, the nutritional quality degraded even further.
In that regard, frozen or even canned spinach is supposedly better. I’m not so sure about the canned spinach though, since canning involves heating. While heating also destroys antinutrients, the same goes for nutrients.
Conclusion
I really don’t see a superfood here. The vitamin A content is nothing special and the amounts of bioavailable Calcium and Iron are greatly exaggerated. It also comes packaged with a good amount of straight up toxic substances that, among other things, inhibit protein absorption. Spinach doesn’t even provide any energy, it’s just a bunch of leaves. And maybe, if you’re lucky, you can extract some usable vitamins and minerals from it.
Spinach is certainly not “super” in any way and does not contain any nutrients that cannot be found in a different, much better type of food. The only thing I feel if you put this “superfood” on my plate is super disappointment.
References
- Conversion rate of beta-carotene in spinach
- Conversion rates of beta-carotene
- Bioavailability of beta-carotene in spinach
- Nutrition information about calcium
- Calcium from eggshells
- iron deficiency
- Effect of cooking on folate
- Spinach protease inhibitors
- Effect of saponins on the gut
- Saponins impacting fertility
- Nutrient degradation due to storage
- Vitamin C lost due to storage
- Factors that impact nutrient degradation in spinach
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