Bioavailability is a term that one encounters reasonably often. It is primarily a poor translation of the meaning of being available, and the term bioactivity is often used instead – which, on the other hand, is a slightly different matter.
Most often, it is approached like the absorption or utilization of different nutrients in dogs, and in principle, that is the question, but it is, however, a slightly broader definition where both are combined.
Bioavailability means
the amount of a nutrient that can be absorbed in digestion and made available for metabolism or stored in tissues
Mere absorption is not enough; the substance must also be in a usable form. Since it is known that many different factors, such as the amounts of other nutrients, the animal’s age, activity, etc., affect it, they should always be stated when talking about bioavailability.
In principle, in vitro or test tube digestibility tests, which are often used as a measure of a substance’s usability as food, do not tell anything about bioavailability. However, I have come across studies where bioavailability is happily discussed because something has been made to dissolve with the help of hydrochloric acid, enzymes, and heat.
The term is not set in stone, and sometimes the context of the research must be used to deduce what meaning the researcher has given to bioavailability.
Bioavailability has been adopted in order to have a more accurate measure that better describes the need and its satisfaction.
In practice, it would mean, for example, that when a puppy’s need for zinc is 20 mg, the food with 2% calcium must contain about 130 mg of zinc oxide because its bioactivity is 20% (in 130 milligrams of zinc oxide, there is 80% zinc, i.e., 104 mg, of which 20% is usable, i.e., about 20 mg) – but the situation changes if we talk about an elderly dog, for whom the bioavailability of zinc oxide is 4% because the animal’s age affects it.
Bioavailability always comes up when trying to interpret how much of a trace element, for example, is obtained. Absorption rates vary systematically from less than five percent for manganese, chromium, or non-heme iron (iron obtained from sources other than meat, which is not “blood-based”) to over 70 percent for copper and selenium.
But these are formulas built in a standardized situation as normal digestibility. When enough zinc is added, the absorption of iron may decrease significantly. But if vitamin C is given alongside, non-heme iron is absorbed almost as well as heme iron (Fairweather-Tait 1998).
Bioavailability cannot be used directly as an amount that needs to be increased to achieve a desired amount for the dog to utilize. It is a value that explains why a certain amount must be given, even if the actual need is less.
The schematic bioavailability of zinc is 5%. This does not mean that the dosage according to the need of 2 mg/kgME should be increased due to low bioactivity, but that the dog should be given 2 mg/kgME according to NRC because zinc has low bioactivity. The fact that often double the amount, 2 mg/kgBW, is given is due to other reasons.
The schematics come from the fact that, for example, with zinc, absorption, and therefore also bioactivity, actually varies between 2 – 40% depending on the food, intestinal function, and many other factors.
Calcium is an even more difficult matter because the body regulates its use and absorption so strongly that there are two different absorption regulations in digestion alone to ensure calcium intake. In the body, on the other hand, activity depends entirely on the need, which is hormonally controlled.
So, in general, regulation also affects whether the nutrient is needed at all or whether absorption needs to be enhanced. This is related to the absorption routes through the intestine, which vary for different substances (and sometimes it is not even known how something is absorbed).
It is claimed that food processing weakens bioavailability, especially in the case of trace elements.
This claim is encountered regularly in the teachings of more natural diets, especially when talking about organic minerals. This is not a straightforward question, but it depends on what is done and to what.
Sometimes preparation may indeed weaken absorption and usability, but sometimes it improves, such as with the phytates in plant foods, which improve as preparation destroys them. Therefore, the matter must always be weighed on a case-by-case basis.
