Nutrient Bioavailability

What is nutrient bioavailability?

The most common and easiest understood definition is the proportion of a nutrient [vitamin, mineral, or protein] that actually makes it through our digestive pathways being absorbed and metabolized into good use in our bodies. This is almost always quite different from the nutritional components found in the foods or supplements we are putting into our bodies. Meaning ‘nutrients contained in foods’ are NOT equal to ‘nutrients absorbed by the body.’

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For example, in a vegan diet it can be quite difficult to obtain the ‘essential amino acids,’ defined as those that your body cannot synthesize on its own and therefore must obtain through outside sources, i.e. diet, ideally. This is made difficult as the most quantifiable and qualitative sources of EAA are found in whole foods such as red meat, fish, poultry, eggs, and dairy products. In these sources you will find the most bioavailability [read as absorbable, readily metabolized, and therefore, viable nutrients].

It is not hopeless for vegans to obtain these EAA, however, it is much more difficult. Because while plant foods may contain EAA—think nuts, seeds, legumes, and beans— you have to be eating a wide variety, daily in order to meet your quota, in often obscene quantities, and even then the bioavailability of these plant sourced EAA is much lower than that from animal sources.

As a general rule, plant based EAA are much more fragile and easily hindered or damaged throughout our digestive pathways than those from animal sources. A good example of this phenomenon is iron.

Iron deficiency is currently the most commonly prevalent condition in the US. The World Health Organization estimates that half of anemia cases worldwide are due to iron deficiency. Anemia is when your blood is lacking an adequate supply of healthy red blood cells or hemoglobin, resulting in weakness from oxygen starved organs and muscles.

Some of you may already know that there are two main sources of dietary iron—heme and non-heme iron. The main difference being heme iron is from animal sources and non-heme iron is from plant sources. It gets a little confusing because meat sources [meat, poultry, & seafood] contain both heme & non-heme iron, while plant sources, dairy products, and eggs contain only non-heme. So there’s no viable way for plant based people to obtain heme iron.

The next difference between these two types is bioavailability of the iron. Heme iron has a very good rate of absorption [high bioavailability; 15-35%] and comes from hemoglobin and myoglobin molecules, which are responsible for oxygen transport & storage in our blood. The heme molecule binds to the central iron atom acting as a protective barrier as it moves through our systems. This acts as a shield from interacting poorly with other nutrients, and keeps the iron content soluble and absorbable in the gut. Also, eating heme iron [meat] in addition to non-heme iron [plants] increases absorption of non-heme iron.

One big reason why plant nutrients are less bioavailable to us is due to the interaction of nutrients inside our bodies at absorption sites. Nutrients can enhance or inhibit each other and sometimes cancel each other out before being utilized. This is what happens with most non-heme iron.

One example of this phenomenon is that non-heme iron is easily blocked by the presence of calcium. Calcium essentially acts as a guard, blocking passage through the gut lining. That is, if any of the delicate non-heme iron actually survives the trek through our intestines, which is a harsh environment for such a delicate iron source.

Road trip snack; pan-fried, grass fed & finished steak in butter and garlic with a side of spinach sautéed in butter and pink salt.

Road trip snack; pan-fried, grass fed & finished steak in butter and garlic with a side of spinach sautéed in butter and pink salt.

Spinach is a great example of this. Spinach has been touted as a ‘high iron food’ even though its total iron content is much too low for that descriptor [between 2.1 & 2.7mg/100g, while high iron is considered 4.2mg/100g, by definition] and its bioavailable iron is even lower. The iron absorption rate from meat is 15-35%, while for spinach it’s as little as 2%!

Why is that rate so poor?

Because as we just read, non-heme iron is not very resilient, especially as it travels unprotected through our harsh gut environment, unlike heme iron which maintains its own protection. Spinach also contains high levels of oxalic acid, known to bind to iron and inhibit absorption in the gut.

Can oxalic acid be reduced? Yes, but not removed. Soaking spinach for 80 minutes at 172 degrees F can significantly reduces oxalic acid, as well as boiling spinach for 12-15 minutes, which can reduce oxalic acid from 30-87%, and also makes folate more bioavailable. Raw spinach is higher in Vitamin C, and Vitamin C is known to enhance the absorption of both types of iron. However, calcium is known to inhibit or completely block the absorption of non-heme iron. Other inhibitors include grains, legumes, tannins, teas, and coffee, as each of these contain various anti-nutrients I’ll be talking about soon.

For now, suffice it to say that while there are plant based foods that do contain the proper, life sustaining EAA their bioavailability is much lower than that of animal sourced EAA and therefore it takes an extremely conscious and aware eater to be truly nourished on a vegan diet.

And then you still have to delve into proper preparation of these foods for optimal bioavailability, as all plants are protected by natural defenses labeled ‘anti-nutrients,’ some of these include: phytochemicals, phytic acid, saponins, polyphenols, lathyrogens, alpha amylase inhibitors, and lectins.

The long and the short of it is that nutrition facts on a label do not translate directly to nutritional value gained. Just because a food is “fortified with vitamins A-X!” doesn’t mean that any of those are useable to your body, or ‘bioavailable.’ This is a crucial concept and a foundational understanding in true nourishment, and real nutrition. It’s going to come up a lot as we talk about different foods, food preparations, and supplementation, a complicated topic all it’s own. I hope this has given you a good, basic understanding of bioavailability.