The Importance of Bioavailability
The bioavailability of a supplement refers to how effectively the nutrients it contains are absorbed and used by the body. If a supplement has poor bioavailability, it means its nutrients are either being eliminated unabsorbed or the nutrients it contains are not in a form easily utilised by the body. Therefore, the quantities of ingredients and relative nutritional values are not always a true indication of how much of each nutrient your body will actually absorb and use.
The bioavailability of nutrients contained in a supplement depends upon a number of factors, including:
- age and health of consumer (gut health tends to decline as we age, reducing our ability to absorb and utilise nutrients);
- presence of co-factors or inhibitors;
- nutrient form.
For good bioavailability, nutrients must not be degraded before we take them. Some nutrients breakdown quickly when exposed to air or light, including vitamins A, B2, B6, C, D and fish oils. Supplements should, therefore, be stored in dark glass containers for best storage.
Nutrients in supplements enter the blood stream via the small intestine, so in order to be available they must firstly survive stomach acid intact. Some enzymes, probiotics, herbal extracts, potent antioxidants, such as L-glutathione and alpha lipoic acid, are susceptible to stomach acid and, therefore, have low bioavailability.
For optimum bioavailability, the supplement must disintegrate easily and rapidly so that the ingredients are available for absorption. Most supplements come in tablet format containing binding ingredients to make them hold together. These binders, together with the compression of tablets during manufacturing, are a major factor affecting the speed and extent of disintegration.
In the worse cases, tablets may disintegrate too slowly and fail to provide the specific nutrient to the right intestinal sites, decreasing absorption and potentially causing gastrointestinal upset, or they may not disintegrate at all, passing straight through you.
Presence of co-factors or Inhibitors
Another important consideration regarding bioavailability is whether the nutrients are water-soluble (eg. vitamins B and C) or fat-soluble (eg. vitamins A, D, E, K, coenzyme q10). Supplements almost always carry the instruction to take with a meal in order to increase the probability of the nutrients being emulsified in fat and liquid.
The bioavailability of each nutrient in a supplement also depends on the mix and whether the supplement contains a team of nutrients that facilitates absorption and has synergy of function, or whether nutrients within one formula compete for absorption. For example, both magnesium and zinc compete for absorption with calcium, phosphorus and iron, so, if integrated in a supplement also containing these minerals, their absorption will be inhibited. Vitamin C increases the absorption of chromium and iron, vitamin D increases utilisation of calcium in the body, magnesium improves absorption of fat-soluble nutrients and vitamin E has a synergistic relationship with selenium where the effect of both working together is greater than the sum of their individual functions.
When it comes to supplements, nutrients extracted from whole food sources can sometimes prove problematic due to low nutrient potency, fluctuating nutrient levels, possible lead contamination and limited shelf life as these nutrients are easily degraded by heat, light and oxygen.
Furthermore, certain acids in plant sources can actually inhibit the bioavailability of nutrients, such as phytic and oxalic acids that inhibit the absorption of zinc and iron respectively, in which case, these foods as a natural source of minerals are inferior to supplements.
It is important to understand that the biological activity of a substance is a function of its structure and not its origin. In supplements, the nutrients are usually produced in a laboratory and not harvested from a field, but they are often derived from natural processes, such as fermentation, and can have exactly the same chemical composition and function as their naturally-harvested counterpart.
Natural vitamin E (d-alpha-tocopherol) is preferable to synthetic vitamin E (dl-alpha-tocopherol) as the synthetic composition is different and has lower bioavailability. However, natural and synthetic vitamin C (both ascorbic acid), being identical chemically, perform equally well in the body.
So, in summary, the bioavailability of a supplement makes the difference between value for money and flushing money down the drain.