For the longest time I was convinced that vegetables like spinach were a great source of iron. A big spinach salad has as much as 30% of your Daily Recommended Value of iron! I told myself, “It’s got be much healthier to get iron from spinach than it is from meat, less calories, fat, & cholesterol!” Little did I know, there were three big factors that were actually preventing me from absorbing the iron in spinach and arguably even worsening my health. 100g of spinach & 100g of red meat have the same value (~30%) of your DRV of iron on paper. So what’s the difference?
- Non-Heme Vs. Heme Iron – Many vitamins and minerals such as iron, magnesium, vitamin A, D, and K, all have different forms that are more or less bioavailable than the counterpart. As for iron, the two main forms are non-heme and heme. Non-heme is found in plant foods and heme iron is found in animal foods. Heme iron is the form that the body can better absorb, is more bioavailable, and is the preferred form for the body. Non-heme iron has a lower absorption rate and is more difficult for the body to convert to the ready form of heme iron. In fact, studies have shown that non-heme iron’s average absorption rate is between 2-20%. As for heme iron the absorption rate is anywhere between 15-35%. The absorption percentage is also greatly dependent on other factors in the diet such as vitamin C consumption (Vit. C helps iron absorption) but also the presence of antinutrients such as oxalates and phytic acid found in many plant foods.
- Oxalic Acid – Antinutrients are rarely ever discussed but bring important effects to the absorption of key minerals in our body. Spinach (and most other leafy greens) are high in an antinutrient known as Oxalic Acid. These antinutrients bind mainly to calcium and slightly to iron in the bloodstream and excretes it out of the body. Calcium, another key nutrient for the neuromuscular and bone structure of humans, is being wasted away from high consumptions of oxalic acid. Oxalic acid has also been shown to cause kidney stones. It can be reduced slightly when the plant is washed & cooked.
- Polyphenols – Polyphenols are often touted as a ‘powerhouse antioxidant’ but it’s not mentioned that they actually have some negative side affects. A 2010 study revealed that polyphenols actually inhibit the absorption of iron in the body. While this may be beneficial for those who deal with iron overload, (often in the rare case that their body has an inability to store it) most people, especially endurance athletes must be mindful of their iron consumption as it plays an important role in the proper function of oxygenating the body.
Summary – As for the topic of iron, animal sources are a far superior source for iron because of its bioavailability, digestibility, and absence of antinutrients. This is not to say that spinach is ‘bad’ for you, it is a good source of potassium and magnesium. It is just important that we understand the bigger picture of each food & realize many mainstream ideas in nutrition are not examining the whole picture.