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Ask the Nutrition Nerd: What makes foods more or less bioavailable?

by Denis Faye - The Nutrition Nerd | May 13, 2014

Popeye-SpinachBuenos días, el Nerdo Herdo! Today’s question comes to us direct from beautiful downtown Paraguay. Nutrition Nerd Facebook page mainstay Romina asks:

“What’s the deal with bioavailability, especially when it comes to the whole dairy-calcium situation?”

Muey excellent question, Romina. Thanks for asking. Bioavailability relates to how much of a drug, nutrient, or other substance you ingest actually absorbs into your system. In other words, contrary to Popeyeian lore, just because you down a can of spinach, it doesn’t mean you’re absorbing all that iron.

Speaking of iron…

In fact, before we dig into calcium, let’s talk iron. Iron plays an important role in transporting oxygen through our red blood cells which is why we need it in our diets. Unfortunately, iron bioavailability is pretty complex.

There are two forms of iron, heme and non-heme. Heme comes from animal products, especially liver. Non-heme iron comes from vegetable sources, particularly dark leafy greens like spinach, kale, and chard; pumpkin and sesame seeds; and beans. Heme is considerably more bioavailable than non-heme iron. You absorb about 15%-35% of heme versus less than 10% from non-heme.

But wait, there’s more! Bioavailability can also be influenced by other nutrients. In the case of iron, vitamin C and animal protein increase bioavailability. Various forms of fiber, as well as polyphenols, phytates and calcium decrease the bioavailability of iron.

If you’re a meat eater, you’re probably okay if you’re eating a diet filled with clean animal protein sources, plenty of leafy greens, and plenty of vitamin C. However, women and endurance athletes tend to lose more blood than, say, lazy men, so if you fall into one or both of those groups, be extra vigilant on your intake.

If you’re a vegetarian or vegan, you’re screwed. There’s nothing I can do for you. No, not really. You’ll be fine. If you’re an ovo-lacto vegetarian, make sure to eat leafy greens and eggs. Veggie omelets are your friends. Vegans should focus on leafy greens, beans, and seeds. Here’s a great list. Also, don’t forget about that vitamin C. You’ll need it to make up for the animal protein. Finally, you probably want to let your doctor know what’s up so they can check your iron levels during check ups.

Calcium consternation 

Calcium is another 7213112f9855164c331ab8d1a5e83d3csuper-important mineral. it plays roles on muscle contraction, nerve conduction, and intracellular communication. But the main thing it does is keep your skeleton strong. Our bones are constantly being broken down and rebuilt. It’s called bone metabolism. When you don’t get enough calcium, the breaking down keeps happening, but not so much with the rebuilding. This can lead to osteoporosis. 

Like iron, we don’t absorb all the calcium we consume–probably about 25%. However, we need vitamin D to hit that number. If you eat a massive amount of calcium, your body can do the job sans D, but for the levels most people get, if you’re low on vitamin D, that means you’re low on calcium.

Vitamins A, C, and K, as well as magnesium also aid in calcium absorption. Sodium and caffeine decrease calcium absorption, but I wouldn’t lose sleep over the caffeine (see what I did there?) since one cup of coffee only decreases your calcium by 2-3 mg.

As for how you should get your calcium, that’s a subject of some debate. Dairy, veggies, and fish are your options, with most experts seeming to think dairy is the way to go, given milk and yogurt are absolute calcium bombs. You’d need three servings of kale to get the same amount of calcium that you get in one glass of milk. What’s more, when it comes to calcium, lactose finally gets a chance to play the good guy because it can increase this mineral’s bioavailability.

On the other hand, dairy detractors point out that many people have lactose intolerance. (Poor lactose. We give you kudos, then we take them away.) Also, dairy contains saturated fat, which many experts feel to be problematic. Finally, excess protein can decrease calcium absorption.

Another big source of calcium are the same leafy greens and beans I mentioned for iron. Astute nerds may recall that I listed calcium as an inhibitor of iron absorption, so we’re not off to a great start. Also, produce tends to contain a number of other inhibitory substances, including oxalates and physic acid. Spinach and rhubarb are high in oxalates, so they aren’t great choices. Sweet potatoes fall in the middle. Broccoli, kale, and bok choy are low-oxalate, so they’re great. To confuse the issue, soy is high in oxalates and phytate, yet its calcium is very bioavailable.

A third source of calcium that doesn’t get a lot of press is fatty fish, especially sardines, anchovies, and salmon. They’re also a good source of vitamin D. Some experts get hung up on the mercury thing, but those smaller fish–anchovies and sardines–are low enough on the food chain that they don’t take on many heavy metals. And if you prepare them right, they’re damn delish. 

At this point, I hope you’re seeing a trend here. Nitpicking aside, there are two constants when it comes to bioavailability of nutrients.

1. Nutrients interact with other nutrients.
2. Most food sources for specific nutrients have pros and cons.

So, I’m happy to explain to you how bioavailability works and even give you a few pointers, but given the vast amount of interactions, it’s almost impossible to get everything right, every time. And the negative interactions are rarely an issue in the Western world. For example, some people get down on physic acid in legumes, claiming they block the absorption of mineral, which they do. However, it’s only ever been shown to be an issue in severe malnutrition circumstances, like people starving in the middle of Africa. You’re just not going to miss out on calcium or iron because you ate an extra serving of three-bean salad last night. Furthermore, you’re going to profit from all the other micronutrients, amino acids, and fiber that legumes tend to provide.

So instead of fretting over the minor points of interacting nutrients, your best bet is to take a shotgun approach. Eat a wide range of foods that contain those nutrients–and all other nutrients. You shouldn’t need to add extra vitamin C to help with iron and vitamin D to help with calcium because you should be consuming plenty of both already anyway.

This trick applies even if you have specific dietary standards. If you’re vegan, you avoid dairy and fish. If you’re Paleo, you avoid dairy and legumes. That’s fine. Just make sure to have plenty of all the other calcium and iron-rich foods available to you.

When all is said and done, bioavailability is complex, so don’t make it complicated.

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2 thoughts on “Ask the Nutrition Nerd: What makes foods more or less bioavailable?

  1. Matt G.

    Am I correct to assume that RDA/DV do not account for bioavailability?

    B12 is another nutrient where your body doesn’t use the whole volume. So when my yogurt says I’m getting X%, I know it’s false.

    Reply
  2. Denis Faye - The Nutrition Nerd Post author

    I think it does, Matt, in a rough sort of way. I know that the RDA for iron for vegetarians is 1.8 times higher because the NIH assumes vegetarians consume mostly non-heme iron. But the RDA is vague anyway. It’s considered the bare minimum for the average person. Already if you’re super active or stressed out, the RDA probably isn’t enough.

    And the B thing is why I tell people eating small amounts over the day is better. As you know, you pee out excess B vitamins, so by having smaller, more frequent amounts, it’s my hypothesis that you absorb it better.

    Reply

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