If anyone looks into the absorption of anything, from drugs to nutrients, it is imperative to understand bioavailability. Just because something is consumed does not mean it will be absorbed. The absorption of nutrients is dependent on several factors that I will outline for you here, as well as what impact that could have on your health and diet. Rev your engines, let’s get started.
We might want to know what bioavailability means before we go on with things. Simply put, bioavailability is the amount of a nutrient that is absorbed by the body and actively used. For example, if you ingest X amount of a vitamin and your body absorbs 95% of what you ingested, that is a vitamin that has a high bioavailability. On the other hand, if your body absorbs only 12% of what you ingested, it has low bioavailability.
What is the bioavailability of nutrients?
I will have separate articles on the matter of individual nutrients, but in general, macronutrients (protein, fat, carbohydrates) have a high bioavailability of over 90% (1). However, the bioavailability of micronutrients is highly dependent on many factors (1).
What impacts bioavailability?
There are many factors that affect bioavailability ranging from vitally important to relatively minor. Here we will examine several of these factors and better understand their overall impact, as well as how to mitigate limitations.
The bioavailability of nutrients can be impacted by mal-absorption, however, this is not the case as we age. Because of the highly flexible nature of the small intestine and pancreas, mal-absorption is not seen as we age (7). So, age itself is not a limiting factor to bioavailability as previously thought.
Raw/High Fiber Diets
Although it has been touted that raw foods are superior in many ways, the research seems to be quite conclusive that certain nutrients are absorbed and used less readily by the body if consumed raw compared to processed or heated (2)(5)(8). I would venture a guess that most of these nutrients that have reduced bioavailability in raw diets tend to be fat soluble vitamins and minerals, and as such, are due more to a lower prevalence of fat intake during meal time.
Meanwhile, in similar token, fiber is absolutely important as it offers many benefits, but although it offers many benefits, there may be some drawback on bioavailability of certain nutrients (9)(10)(11).
Our nutritional health is dependent on the absorbance of many different nutrients in varying quantities. Luckily, our body is intelligent enough to make this as efficient as possible. However, even so, several nutrients depend on one another for increased absorption. There are several examples, but a classic one is vitamin C and iron (1)(13)(14). Intaking iron and not intaking enough vitamin C can lead to lower absorption of iron as nonheme (a particular type of iron) iron is heavily dependent on vitamin C consumption for efficient utilization (14).
So, although not the case for all nutrients, some are dependent on others for their continual bioavailability (12). Interestingly, it is possible to consume two nutrients that impact another nutrient’s absorption; one nutrient offering enhancement of absorption while another inhibiting absorption, leading to a nil effect (1).
Competition Between Nutrients
When we are discussing the competitiveness of nutrients, we are not talking about nutrients having their own Olympics (although, that’d be interesting to see); we are, however, talking about various ways in which nutrients compete for absorption by the body. According to the European Food Information Council, there are three ways by which the body can miss out on key nutrients due to this issue (1):
1. Nutrients binding together in a way that intestinal cells do not recognize.
2. Rendering a nutrient insoluble and unable to break down to its chemical components.
3. Nutrients competing for the same uptake system into the body.
Again, while this is all true, the real world impact is likely not strong enough to worry about. In certain cases, it may be advantageous to take supplements spread out through the day, however.
Believe it or not, lifestyle has an impact on how well our body is able to use nutrients. Granted, the lifestyle itself can promote or deny nutrient intake in its own right, but assuming nutrient intake was equated for, certain individuals would see worse nutrient absorption than others. Specifically, heavy alcohol consumption leads to degradation of nutrient activity; although, interestingly, moderate alcohol consumption has no, and if deficient in certain vitamins (B6, for example), to some positive impact on vitamin bioavailability (2). Similarly, smoking decreases the bioavailability of folate, vitamin C, B12, among other micronutrients in the process of metabolism (remember, bioavailability is A) Absorption and B) Use), so although absorption may not be impaired, the use of these vitamins may be (2).
This is a rather broad subject, so I will address a few examples, but the condition of your body is a huge factor for its intake of nutrients. While in most cases, the body is running at a rough average once young adulthood begins and continues to do so for life, situations like pregnancy, lactation, growth all play a role in increasing the demand in certain nutritional areas (15). The body suddenly requires substantially more of certain vitamins – this is often seen in calcium during pregnancy and childhood (1)(15), or iron in women once maturation occurs (16), among a series of other conditions.
When we discuss illness, we are not talking about the common cold. Instead, what we are interested in is chronic illnesses that typically affect the gut or metabolism. Diseases such as atrophic gastritis (inflammation of the stomach mucosa) show a decrease in nutrient absorption due to a variety of factors (7)(17). Without covering every gut disease in existence, disease can make a noticeable impact on bioavailability. Obviously, nutrient specific diseases will also hamper absorbability.
Probiotics & Need
Finally, there are two more areas that I investigated that I will mention briefly, but I can make no assertions, because I found little research on the subjects. While probiotics have been known to help with gut bacteria, and in theory one would imagine they would help with absorption, I have found no research on the subject so I am simply speculating.
On the other hand, as the body fills up its need of micronutrients throughout the day, the bioavailability of said vitamins diminishes as the body requires less and less to reach its optimal intake. This statement is again unsubstantiated, because again I could not find research in this area other than animal studies, which, in this area are rather questionable. However, I am mentioning it, because I briefly spoke to Lyle McDonald ( a well-respected nutritionist) on the subject and he seems convinced this to be the case, although he also admitted that the research is likely scarce. If I run into any research, I will edit this article and add it in the “citations” section.
So, in short, bioavailability is the amount of nutrient the body absorbs, as well as uses. Micronutrients especially are impacted by many factors that can either disrupt or enhance their availability. Disease, the physiological state of the body, and lifestyle probably play the biggest roles in influencing nutrient absorption and use, while other facets of bioavailability certainly play a role, they are not as readily impactful.
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