Bioavailability refers to the extent to which active substances (such as those found in medication and nutrients) reach their intended sites in the body. This process occurs through the absorption of active substances and nutrients in the digestive tract. When a medicine is injected into a blood vessel, its bioavailability is always 100 per cent. The bioavailability of the same medicine will always be lower when taken orally. This is due to incomplete absorption in the digestive system. The same applies to nutrients from food or dietary supplements.
This incomplete absorption is influenced and exacerbated by several factors and depends, for example, on the body’s physical condition, particularly that of the digestive system. Someone with a stomach or bowel condition will find it more difficult to absorb certain nutrients from food and dietary supplements than someone without such a condition. The small intestine, in particular, plays a key role in the absorption of a wide range of nutrients.
In addition to the physical condition of the digestive tract, other factors and nutrients from food and supplements can also influence the bioavailability of nutrients. One important factor to note is that when mineral requirements are high, such as during pregnancy, the body absorbs minerals more effectively. Nutrients can influence each other’s absorption; this influence can be either positive or negative. Examples of this include: increasing the absorption of iron by combining iron-rich meals and iron supplements with vitamin C; increasing calcium absorption by combining calcium-rich foods with vitamin D; and taking fat-soluble vitamins (A, D, E, K) at the same time as a high-fat meal.
Combinations that actually have a negative impact on bioavailability include the binding of minerals to phytates (phytic acid) found in dietary fibre, vegetables and nuts. Phytates are also known as antinutrients and inhibit the absorption of calcium, magnesium, zinc and iron in the small intestine. It is therefore not advisable to consume phytate-rich foods and supplements at the same time. Furthermore, vitamins and minerals can inhibit each other’s absorption. A good example of this is the reduced absorption of vitamin K when taken simultaneously with other fat-soluble vitamins (vitamins A, D and E). Another example is the inhibitory effect of magnesium on calcium absorption.
Form
Particularly in the case of micronutrients (minerals and trace elements), the form in which these nutrients are present plays a crucial role in their bioavailability within the human body. This is especially important for the form of nutrients added to dietary supplements. For example, minerals with an organic bond are absorbed much more effectively than the same minerals when bound in an inorganic form. This is particularly true for people with digestive disorders. Organic compounds are compounds that occur naturally in living organisms and are therefore recognised by the body as its own substances. Examples of easily absorbed organic compounds include: citrates, glycinates, gluconates and amino acid chelates. Inorganic compounds, by contrast, are found in ‘dead’ material and are therefore not endogenous. Examples of poorly absorbable inorganic compounds include: oxides, carbonates, chlorides, phosphates and sulphates. Choosing an organically bound form as an ingredient in a supplement is therefore more beneficial than adding less readily absorbable inorganically bound forms.
Natural and synthetic vitamins
Then there are also vitamins derived from natural and synthetic sources. Natural vitamins are of plant or animal origin. Synthetic vitamins are substances produced in a laboratory. A common misconception is that natural vitamins are always better absorbed than their synthetic counterparts and therefore have better bioavailability. However, this is by no means true for all vitamins. When synthetic vitamins have exactly the same chemical structure as naturally derived vitamins, the body cannot distinguish between these forms and absorption will generally be no different. Exceptions to the rule are vitamins that are better absorbed in their synthetic form than in their natural form. For example, vitamins D, B5, B6, folic acid and vitamin B12 are better absorbed by the body in their synthetic form than in their natural form from food. In the case of vitamin E, however, the natural form is better absorbed. It is therefore by no means always the case that natural variants are better than synthetic ones.
