Bioavailability: Why Eating a Nutrient Doesn't Mean Absorbing It

The nutrition label on a bag of spinach might say it contains 36mg of iron per 100g. That sounds impressive. But here’s what the label doesn’t tell you: your body might absorb less than 2% of that iron.

Meanwhile, a serving of beef contains less total iron. But you’ll absorb 20–30% of it.

Same nutrient. Dramatically different outcome. That gap is explained by bioavailability.

What Bioavailability Actually Measures

Bioavailability is a pharmacological term that nutrition scientists borrowed. It measures the fraction of an ingested substance that reaches systemic circulation in an active form, and more specifically, reaches the tissues where it can actually be used.

A nutrient’s bioavailability depends on two steps:

1. Absorption: How much crosses from the gut into the bloodstream
2. Utilization: How much of what enters the bloodstream is actually used by cells

Both steps can limit how much benefit you get from what you eat.

For practical purposes, bioavailability is expressed as a percentage or fraction. If you eat 10mg of iron and absorb 2mg, the bioavailability is 20%.

Why the Same Nutrient Behaves Differently in Different Foods

The food matrix matters enormously. Nutrients don’t travel alone. They’re embedded in a complex structure of proteins, fats, fiber, and other compounds that either facilitate or obstruct absorption.

Iron is the textbook example. It exists in two forms:

• Heme iron: Found in animal products (meat, fish, poultry). Bound inside a protein ring. Absorbed directly by specialized intestinal receptors. Bioavailability: 15–35%.
• Non-heme iron: Found in plant foods and fortified products. Must be converted to a soluble form first. Much more sensitive to dietary factors. Bioavailability: 2–20%.

This is why vegans and vegetarians need to be more strategic about iron intake, and why iron-fortified foods often deliver less iron than their labels suggest.

Lycopene in tomatoes is another good example. Raw tomatoes have lower lycopene bioavailability than cooked tomatoes, and lycopene is significantly more bioavailable when eaten with fat. The heating breaks down the cell walls that contain the lycopene. The fat helps your gut absorb it (lycopene is fat-soluble). This is one case where processed food (tomato paste, marinara sauce cooked in olive oil) is genuinely more nutritious than the raw ingredient.

Factors That Affect Bioavailability

Enhancers

Vitamin C and iron: Ascorbic acid in vitamin C converts ferric iron (Fe³⁺) to ferrous iron (Fe²⁺), the more absorbable form. Eating iron-rich plant foods with a vitamin C source (citrus, bell peppers, tomatoes) can increase non-heme iron absorption by 2–6 times (Hallberg et al., 1987, American Journal of Clinical Nutrition).

Fat and fat-soluble vitamins: Vitamins A, D, E, and K are fat-soluble. They require dietary fat for absorption. Eating a salad with fat-free dressing absorbs far less of these vitamins than a salad with olive oil. One study found beta-carotene absorption from salads was nearly absent without fat, and substantially higher with avocado or full-fat dressing (Goltz et al., 2012, Molecular Nutrition & Food Research).

Fermentation and soaking: Both processes reduce levels of phytates in grains and legumes. Phytates bind minerals like zinc, iron, and calcium in the gut and prevent their absorption. Soaked and sprouted beans have significantly lower phytate levels than unsoaked beans.

Inhibitors

Phytates (phytic acid): Found in whole grains, legumes, nuts, and seeds. Phytates are strong mineral chelators. They bind zinc, iron, calcium, and magnesium and carry them out of the body unabsorbed. This is why mineral absorption from whole grain bread can be lower than from white bread, even though whole grain contains more minerals.

Oxalates: Found in spinach, rhubarb, beets, and some nuts. Bind calcium tightly. Spinach is technically calcium-rich, but its oxalate content reduces calcium bioavailability to roughly 5%. By comparison, calcium from milk is about 32% bioavailable, and from low-oxalate greens like kale or bok choy it’s 49–61% (Weaver et al., 1999, American Journal of Clinical Nutrition).

Tannins: Found in tea, coffee, red wine, and some legumes. Bind non-heme iron. Drinking tea with meals reduces iron absorption. This is a meaningful consideration for people with borderline iron status.

Calcium and iron competition: These two minerals compete for the same intestinal transport proteins. Taking calcium and iron supplements at the same time substantially reduces absorption of both. This applies to dairy consumed with iron-rich meals as well.

Food Preparation and Bioavailability

How you prepare food changes what your body can absorb from it.

Cooking increases:

• Lycopene bioavailability from tomatoes
• Beta-carotene from carrots and sweet potatoes
• Folate from some vegetables
• Protein digestibility (denaturation makes proteins more accessible to digestive enzymes)

Cooking decreases:

• Vitamin C (heat-labile, oxidizes during cooking)
• Thiamine (B1) and B6, especially with high heat or long cooking times
• Folate in some preparations

Mechanical processing (grinding, blending) breaks cell walls and generally increases bioavailability by giving digestive enzymes better access to nutrients. This is one reason smoothies with intact cell walls aren’t quite the same as whole fruit, but also why nut butter may deliver more fat-soluble nutrients than whole nuts for some people.

Individual Variation

Two people eating identical meals can absorb different amounts of the same nutrients.

Iron status: Iron absorption is regulated by a hormone called hepcidin. When iron stores are low, hepcidin levels drop, and intestinal absorption increases. When you’re iron-replete, absorption decreases. This is a protective mechanism that prevents iron overload.

Age: Calcium absorption efficiency declines with age. Vitamin B12 absorption decreases in older adults due to reduced stomach acid production and lower levels of intrinsic factor, the protein needed to absorb B12. This is why supplemental B12 (which doesn’t require intrinsic factor) becomes more important with age.

Gut health: An intact, healthy intestinal lining is essential for nutrient absorption. Conditions that damage the gut (celiac disease, Crohn’s disease, SIBO) can dramatically reduce bioavailability of multiple nutrients simultaneously.

Genetics: Genetic variants affect how well people absorb and metabolize specific nutrients. MTHFR variants affect folate metabolism. Variants in vitamin D receptor genes affect how efficiently people use vitamin D. This explains some of the wide individual variation researchers observe in nutrition studies.

Why This Matters for Reading Nutrition Research

Understanding bioavailability explains why many nutrition findings don’t translate straightforwardly to dietary advice.

A study showing that a nutrient is associated with a health outcome in population data doesn’t tell you:

• How much of that nutrient people were actually absorbing
• What food sources it came from
• What else they ate alongside it

Supplement studies are similarly complex. A supplement can provide high doses of a nutrient that still don’t translate to meaningful blood level changes if the bioavailability is poor.

Current evidence suggests that eating varied whole foods from multiple sources remains the most reliable strategy for nutrient adequacy. Partly because real foods deliver nutrients in matrices that often optimize absorption in ways we’re still working to fully understand.