What is bioavailability? (Plus bioavailable sources of essential nutrients)

From the desk of
Robb Wolf
ScienceWhat is bioavailability? (Plus bioavailable sources of essential nutrients)

Knowing that a food is high in protein, calcium, or magnesium can be misleading. You should also consider how much of each nutrient is absorbed. That’s a question of bioavailability: What percentage of a given nutrient, once ingested, becomes available for use in your body?

We want to optimize our diets for both nutrient density and bioavailability. Put another way, we want to eat nutritious foods that are well-absorbed. Animal sources, such as liver and clams, tend to outperform plant sources in both nutrient density and bioavailability. I’ll dig into that later, after we lay a bit of groundwork.
We’ll cover lots of ground in this article: What determines bioavailability, how recommended daily allowances factor in, how foods and supplements compare, how cooking affects nutrition, and a few top sources of essential micronutrients.

Bioavailability and Recommended Dietary Allowances (RDAs)

Bioavailability is the percentage of a nutrient that’s absorbed and retained in bodily tissue after consumption. So when you read that “calcium from dairy is 30% bioavailable,” it means that 30% of that mineral gets absorbed in the small intestine, stored, and put to use. You excrete the other 70%.

This doesn’t mean you should triple your dairy intake to hit the recommended daily allowance (RDA) for calcium. RDAs account for bioavailability, albeit imperfectly. It must be imperfect, considering that:

  • People absorb nutrients at different rates.
  • Nutrients’ bioavailability varies in different foods. For instance, calcium from spinach is 5 to 12 times less bioavailable than calcium from milk or broccoli.
  • Cooking affects nutrient absorption.
  • Other factors I’ll cover soon.

Bioavailability is complex. We’ll never have a perfect system to assess it, but knowing the fundamentals can help you make wiser dietary choices. You can choose foods that are, on average, better sources of particular nutrients.

5 Determinants of Bioavailability

To understand bioavailability, you need to know what determines it. Below are 5 fundamental factors.

#1: The presence of antinutrients

I’ve been writing about antinutrients (things that impair nutrient absorption) for over a decade. Limiting them is a central theme of my book, The Paleo Solution.

Antinutrients are ubiquitous in plant foods—especially legumes, nuts, seeds, and grains. Phytic acid, lectins, oxalates, and other antinutrients are why plants typically lose to animals on bioavailability.

I’m most concerned about an antinutrient called phytic acid, also known as phytate. Phytate, which is commonly found in grains, legumes, seeds, and nuts, binds to minerals and inhibits their absorption through the gut. I’m not saying you should never eat phytate-rich foods. I enjoy a handful of cashews as much as the next guy. Instead, understand that meals high in phytate will be poor functional sources of zinc, iron, magnesium, and other minerals.

#2: The presence of complementary nutrients

Ever heard of food synergy? It’s the idea that vitamins, minerals, polyphenols, and fatty acids are absorbed better in food form than in isolated supplements. Consider that:

  • Vitamin C enhances iron absorption (broccoli contains both, for example)
  • Vitamin D enhances calcium absorption (salmon contains vitamin D, and when consumed with its bones, you’ll get calcium too)
  • Consuming zinc and copper together helps prevent deficiencies in either (clams contain both)
  • Eating vitamins A, D, K, and E with fat enhances their absorption (eggs contain all of these)

Later, I’ll share a list of bioavailable food sources so you can harness food synergy without turning every bite into a research project.

#3: The form of the nutrient

When it comes to bioavailability, form matters. For example, heme iron (iron that comes from animals) is better-absorbed than non-heme iron (iron that comes from plants). And if you’re going to bust out the supplements, magnesium malate is better-absorbed than magnesium oxide. There are many other examples, but iron is the main one to consider regarding food-based forms of nutrients.

#4: Cooking

Cooking can both increase and decrease bioavailability. For instance, cooked starch (from sweet potatoes, yuca, etc.) tends to be better-absorbed than raw starch. And cooked vegetables are generally easier to digest than raw vegetables.

But on the other hand, boiling spinach cuts its folate content in half. Folate helps you repair DNA, form blood cells, and produce cellular energy as ATP, so it’s a good idea to use gentler methods of cooking (like steaming) to preserve this essential B vitamin.

#5: Individual factors

We’re not exactly lab animals in sterile environments being fed standardized chow. Our bodies and surroundings vary, so everyone will absorb a given nutrient differently. For instance, people with gut issues have difficulty absorbing vitamin B12 from food.

Bioavailability in Food vs. Supplements

Are nutrients more bioavailable in foods or supplements? Well, it depends on the slew of factors we just covered. Does your meal contain antinutrients? What other synergistic nutrients are present? What form of the nutrient are you consuming? How’s the food cooked? How does your gut absorb this compound?

You won’t be able to answer all these questions perfectly. Nobody can. In the absence of perfect data, we just take what we know and approximate the right course. That means eating a nutrient-dense whole foods diet and supplementing to make up for any shortfalls. If your diet regularly contains a few of the following foods, you’ll be doing better than most. Simple as that!

Bioavailable Sources of Nutrients

Besides Granny’s vitamin meatloaf (yuck!), which foods score best for nutrient density and bioavailability? Let’s review data published in Frontiers in Nutrition in 2022.

In the paper, researchers Ty Beal and Flaminia Ortenzi rated foods based on their density and bioavailability of six “priority micronutrients” that folks tend to be deficient in: zinc, iron, folate, vitamin A, calcium, and vitamin B12. Foods that scored “very high” included organ meats, shellfish, small fish, dark leafy greens, goat, beef, eggs, cow milk, and cheese.

A few things to say about the data:

  • Organ meats (especially liver) and bivalves were the highest-scoring foods.
  • The only plant-based winner was dark leafy greens.
  • Many foods commonly touted as nutrient-dense (fruits, nuts, seeds, whole grains, many vegetables) weren’t actually great sources of priority micronutrients.

To be clear, those six micros aren’t the only nutrients that matter. For instance, protein is a crucial macronutrient for muscular and hormonal health. The general story with protein remains the same, though. Animal sources (whey, egg, beef) outperform plant sources (beans, peanuts).

The analysis also doesn’t capture polyphenols, essential fatty acids, fiber, and many other vitamins and minerals like vitamin C, vitamin K, copper, magnesium, and potassium. But again, the best sources of these nutrients are whole foods like meat, fish, eggs, liver, shellfish, and leafy greens.

Avoid processed junk and center your diet around the aforementioned foods. When you eat these foods regularly, you don’t need to stress much about bioavailability. You’ll be getting—and absorbing—the nutrients you need to thrive.

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