You can swallow a supplement containing 500mg of a nutrient and absorb almost none of it. Or you can swallow 200mg of the same nutrient in a different form and absorb the majority.
Same label claim. Dramatically different outcomes.
This is bioavailability — the proportion of a nutrient that actually reaches your bloodstream and gets used by your cells. It’s one of the most important concepts in supplementation, and one of the least discussed. Understanding it transforms how you choose and take every supplement you buy.
What Bioavailability Actually Means
Bioavailability measures what the body absorbs and uses — not what enters the mouth.
When you swallow a supplement, it travels through the digestive system. It survives stomach acid, gets broken down in the small intestine, crosses the gut wall into the bloodstream, and eventually reaches the cells and tissues that need it. Every stage of that journey involves losses.
Some nutrients survive the journey efficiently. Others don’t. The chemical form of the nutrient, the presence or absence of food and fat, competing nutrients in the gut, individual digestive health, and the supplement’s physical construction all influence how much makes it through.
A supplement with poor bioavailability isn’t just less effective — it’s potentially worthless at the dose stated on the label.
The Form of the Nutrient Makes the Biggest Difference
Nutrient form is the single most influential factor in bioavailability. Two supplements listing identical doses of the same nutrient can deliver vastly different amounts to the bloodstream based on form alone.
Magnesium illustrates this better than almost any other mineral. Magnesium oxide — the cheapest and most widely used form — has a bioavailability of approximately 4%. That means a 500mg magnesium oxide supplement delivers roughly 20mg of absorbable magnesium. Magnesium glycinate absorbs at rates closer to 80%. The same 500mg dose delivers 400mg to the bloodstream. The difference between these two products is not marginal. It’s the difference between a supplement that works and one that doesn’t.
Iron follows the same pattern. Iron sulphate absorbs moderately and causes significant digestive side effects — nausea, constipation, stomach cramps — because a large proportion of the dose remains unabsorbed and irritates the gut lining. Iron bisglycinate absorbs at nearly double the rate of iron sulphate and causes minimal digestive discomfort. For the same stated dose, it delivers more iron with fewer side effects.
Zinc behaves similarly. Zinc picolinate and zinc bisglycinate absorb well. Zinc oxide — found in many cheap multivitamins — absorbs poorly. A zinc oxide supplement hitting 100% of the reference intake on the label may deliver a fraction of that to the cells.
Folate vs. folic acid — covered in detail in our pregnancy article — represents another critical form distinction. Methylfolate (5-MTHF) is the bioactive form the body uses directly. Folic acid requires enzymatic conversion that a significant portion of the population cannot complete efficiently due to a common genetic variant. Same nutrient, same label claim, dramatically different cellular availability.
Food Changes Absorption — Significantly
Fat-soluble vitamins — A, D, E, and K — require dietary fat to absorb. Without fat present in the gut at the same time, absorption falls dramatically.
A 2015 study published in the American Journal of Clinical Nutrition found that Vitamin D absorbed significantly better when taken with a fat-containing meal compared to a fat-free meal. The difference wasn’t minor. Participants who took Vitamin D with a high-fat meal absorbed approximately 32% more than those who took it without food.
Fish oil absorption tells the same story. A high-fat meal can improve EPA and DHA absorption from fish oil by up to 300% compared to taking it on an empty stomach.
Water-soluble vitamins — Vitamin C and B vitamins — absorb more flexibly. Food helps with tolerance but isn’t strictly required for absorption. The exception is Vitamin B12, which requires a protein called intrinsic factor produced in the stomach lining to absorb properly — a factor that declines with age and explains why B12 deficiency becomes more common in older adults regardless of dietary intake.
What Else Impairs Absorption
Several factors consistently reduce how much of a supplement the body absorbs.
Competing nutrients — calcium and iron compete for the same transport proteins in the gut. Taking high-dose calcium alongside iron significantly reduces iron absorption. Zinc and iron compete similarly. Separating these supplements by at least two hours improves the absorption of both.
Digestive health — the gut wall is where absorption happens. Conditions that damage or inflame the gut lining — Crohn’s disease, coeliac disease, IBS, dysbiosis — reduce the efficiency of absorption across the board. People with digestive conditions often need higher doses to achieve the same blood levels as people with healthy gut function.
Age — digestive efficiency declines with age. Stomach acid production falls. Intrinsic factor production decreases. Enzyme activity slows. Older adults consistently show lower absorption rates for B12, magnesium, zinc, and calcium from equivalent dietary or supplemental doses. This is one reason why recommended intakes for several nutrients increase with age.
Medications — proton pump inhibitors reduce stomach acid, impairing the absorption of B12, magnesium, and iron. Metformin, commonly prescribed for type 2 diabetes, consistently depletes B12. Statins deplete CoQ10. These interactions don’t just affect drug effectiveness — they affect nutritional status.
The Bioavailability Checklist
When evaluating a supplement, run through these questions:
- What form does it use for each nutrient? Is that form well-absorbed?
- Does it require food or fat for optimal absorption?
- Does it compete with anything else you take at the same time?
- Does your digestive health or age affect how well you absorb this nutrient specifically?
Answering these questions turns supplement purchasing from guesswork into precision.
A supplement’s label dose tells you what went in. Bioavailability tells you what actually gets used. The gap between those two numbers is where most supplement decisions go wrong.
Body by Nature Supplements prioritises bioavailable nutrient forms across its entire range — because a supplement that absorbs properly is worth taking, and one that doesn’t is worth leaving on the shelf.