Bioavailability of nutrients from animal vs plant foods
When we talk about ‘most nutritious foods for humans’, we need to consider something called bioavailability.
If we do not, we run the risk of developing nutrient deficiencies.
Let’s see why bioavailability is such an important concept to keep in mind.
Defining bioavailability
Bioavailability refers to the amount of a nutrient that an organism is able to absorb and utilise immediately—without modification—in normal physiological functions.
We’ll use Vitamin A as a case study to illustrate how bioavailability works, and why it’s relevant to all humans.
Case study: Vitamin A
Vitamin A is a fat-soluble vitamin, involved in a myriad of essential physiological functions:
Vision
Bone growth
Reproduction
Immune function
Maintenance of skin barrier integrity
Transportation and assimilation of nutrients
Regulation of several hundred genes
Again, it’s a critically-important nutrient, and many of us modern folks seem to exhibit signs of deficiency.
Deficiencies in Vitamin A are linked to many diseases, but most notably, those that affect eyesight.
Each day, the human body uses up ~5% of Vitamin A stores.
Athletes and labourers (highly-active populations engaging in strenuous, loaded movement patterns) deplete stores even more rapidly.
If we do not replenish our stores (by eating foods rich in Vitamin A), we may quickly become deficient, leading to a host of (preventable) problems.
Recall that Vitamin A is crucial for optimal reproductive health, immune function, vision, bone, skin health etc.
Vitamin A is not a single nutrient, but actually a group of compounds called retinoids.
Some types of Vitamin A that the human body uses are called retinal, retinoic acid, and retinol—which are referred to as ‘physiologically-active’ or ‘pre-formed’ Vitamin A.
Remember this for later.
Wait, aren’t carrots ‘known’ to improve eyesight?
I’m glad you asked.
This seems to be a popular myth, perpetuated during World War 2.
Carrots have not been shown to improve eyesight, however, there is a shred of truth to the matter.
Carrots contain building blocks of Vitamin A—called carotenoids, such as alpha- and beta-carotene.
Plant compounds, such as alpha- and beta-carotene, can be converted by humans (in the liver) into a physiologically-active (usable) form of Vitamin A, however, it takes time and energetic resources.
Studies show that multiple units (at least 3, possibly up to 30) of carotenoids are required to produce a single unit of physiologically-active Vitamin A.
Studies also indicate that this ability (to convert carotenoids into Vitamin A) is highly-variable amongst humans—and largely dependent on your ancestry.
Gut health also plays a key role in enabling the conversion and absorption of Vitamin A. This is more relevant than ever, given that the majority of humans alive today who live in ‘developed’ nations are suffering from some degree of gut dysfunction—see mounting prevalence of metabolic disorders, allergies, food intolerances, autoimmune disease, bacterial/fungal overgrowths and so on.
Herein lies the importance of the bioavailability discussion.
Carotenoids are not useless, but eating carrots is not a great strategy for meeting Vitamin A requirements.
Especially when we have better options.
Animal foods are abundant in pre-formed Vitamin A (not ß-carotene, but the real deal).
Meat, butter and eggs, all contain pre-formed Vitamin A—physiologically-active forms that the human body can absorb, and use straight away, sans conversion.
How?
The animal (say a herbivorous cow) has already done the hard work of taking the building blocks from their foods (like ß-carotene in grass) and transforming them into a biologically-active form of Vitamin A (like retinol).
Many nutrients that come from animal foods are considered ‘more bioavailable’ for humans because of such qualities.
A similar story is true for Vitamin D, Vitamin K, iron, omega-3 fats and many other nutrients.
We ‘can’ get some of these nutrients from plants and fungi, but they are much less valuable / lower-quality options for humans.
We need to make up the difference by eating animal foods—that are rich in pre-formed, bioavailable nutrients—otherwise we run the risk of developing nutrient deficiencies.
An actual cure for night blindness (and pernicious anaemia)
Ox liver has been known to cure some types of night blindness since ancient times.
This was first documented in the Eber’s Papyrus (~1500 BCE), and also noted by the father of Western medicine, Hippocrates (~400 BCE).
Modern scientific analysis has helped to determine some reasons why…
Liver is extremely rich in pre-formed Vitamin A.
The liver is where animals, like cows, store much of their surplus nutrients, like Vitamin A.
Organ meats, liver in particular, are veritable storehouses of hard-to-find nutrients, like bioavailable Vitamin A, that are essential for optimal physiological function in humans.
Liver, as it happens, is also known to quickly reverse pernicious anaemia—the 1934 Nobel Prize in Medicine was awarded to three doctors who (re)discovered ‘liver therapy’.
How?
Along with Vitamin A, and many other nutrients, liver is rich in bioavailable B vitamins, copper and iron (heme-iron)—which are all essential for blood oxygenation and optimal cardiovascular function.
These are just a couple of examples of bioavailability at play, but the same story repeats elsewhere—Vitamin D, Vitamin K, iron, zinc, omega-3 fats…
Humans need animal foods
If humans do not consume sufficient quantities of animal foods, we will become deficient in dozens of essential micronutrients;
Vitamin A
Vitamin K2
Vitamin B2 (riboflavin)
Vitamin B9 (folate)
Vitamin B12 (methylcobalamin)
Minerals, like heme-iron
The bioavailable forms of these nutrients are only found in animal foods.
This is to say little of the importance of macronutrients—animal proteins and fats—which many people seem also to be deficient in.
As it happens, the micronutrients are stored within the macronutrients (muscle meat + fat) and humans seem to need all of them.
So, eat well-sourced and well-prepared plants and fungi for pleasure and variety, but do not be fooled—the most nutritious part of a Sunday roast is actually the meat.
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References
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