Bioavailability, Nutrients and Supplements: Why the Effect Varies | Hardworkout Magazine

Key takeaways

- Bioavailability describes how much of a nutrient the body can actually absorb and use, not how much a product contains.

- Two supplements with identical amounts of a nutrient can have different effects because the chemical form determines solubility and uptake.

- Non-heme iron absorption is inhibited by coffee, tea and calcium, while vitamin C increases it; heme iron is far less affected.

- Fat-soluble vitamins A, D, E and K need fat in the same meal to be absorbed effectively.

- Improving the conditions for absorption often gives better results than raising the dose.

Many think that the amount of vitamins and minerals is the most important factor when choosing supplements. This is not always the case. The body does not absorb everything we eat, and absorption can vary more than one might think.

In this article, we will look at what determines bioavailability – and why it determines how much the body can actually use.

What is bioavailability, and why does it matter?

Bioavailability is about how much of a nutrient the body can actually absorb.

When you eat or take a supplement, the nutrient must first be broken down. Then it must be absorbed through the intestinal cells so that the body can use it further.

How well this works depends on both the form the nutrient comes in, the conditions in the intestine, and how quickly food passes through the digestive system.

This is why two products can contain the same amount of nutrients but have different effects.

Why does the same nutrient come in different chemical forms?

Many vitamins and minerals exist in several chemical forms, and the body handles these differently.

How the form is structured affects how easily it dissolves, how easily it passes through the intestinal wall, and how much the body can utilize. This applies to minerals, vitamins, and certain types of fatty acids.

Examples that often create differences in absorption:

- Magnesium exists as citrate, oxide, and glycinate, which vary in solubility and how easily the body absorbs them.

- Folate can come as folic acid or methylfolate, where the former must be converted before the body can use it.

- Iron exists as heme iron and non-heme iron, which have completely different absorption mechanisms.

The form is not about one product being "good" and another being "bad," but about how well the body can utilize the nutrient.


Which nutrients block each other in the gut?

The body uses its own transport mechanisms in the gut to allow nutrients to enter the bloodstream.

Several micronutrients use the same transport pathways, and can thus inhibit each other when taken simultaneously. This is an important part of bioavailability, and explains why timing and combinations matter for certain nutrients.

Iron is a good example:

- Non-heme iron, which is the most common form in iron supplements, is inhibited by coffee, tea, and calcium.

- Heme iron, found in animal foods, is affected much less, but can also be inhibited to some extent.

Vitamin C can increase the absorption of non-heme iron, but has less effect on heme iron.

Zinc and iron can also reduce each other's absorption if the doses are high and taken simultaneously.

Such interactions mean that even if the dose is the same, bioavailability can vary significantly.

What should you eat alongside a supplement?

What you eat alongside a nutrient greatly affects how well the body absorbs it.

Fat-soluble vitamins – such as vitamins A, D, E, and K – require fat in the same meal to be absorbed effectively. Without fat, a larger portion passes through without being absorbed.

Foods high in fiber can bind certain minerals and reduce absorption.

Calcium-rich foods can inhibit the absorption of non-heme iron, while vitamin C-rich foods can contribute to the opposite.

This shows that bioavailability is not just about the product itself, but about the situation in which it is consumed.

Two people can therefore take the same supplement but experience different effects depending on what they eat at the same time.

Can a sensitive stomach reduce how much you absorb?

Digestion plays a significant role in how well the body can utilize nutrients.

Rapid passage through the gut provides less time for absorption. Irritated mucous membranes, imbalance in the gut, or a sensitive stomach can also reduce absorption.

Certain forms of minerals can cause loose stools, which in turn can shorten the time the nutrient is in contact with the gut cells.

This is why a form that generally provides good absorption may still work poorly for individuals with a sensitive stomach. The body simply does not have the right conditions for effective absorption.

How can you improve absorption in practice?

When you understand the mechanisms behind bioavailability, it becomes easier to make choices that enhance the effectiveness of both food and supplements.

Useful measures include:

- Take fat-soluble vitamins with food that contains some fat

- Avoid coffee and tea around the intake of non-heme iron

- Avoid taking multiple minerals that use the same absorption pathway at the same time

- Choose forms that are known for good solubility and tolerance, especially with a sensitive stomach

- Test one supplement at a time to see how the body reacts

Such small adjustments can often yield better results than increasing the dosage, as the body gets better conditions to utilize the nutrients that are already present.


Sources:

Nutrient Bioavailability – [Science Direct](https://www.sciencedirect.com/topics/medicine-and-dentistry/nutrient-bioavailability)

Heme Iron – [Helseforskning](https://helseforskning.no/hemjern/)