The Bio-Accessibility of Food

Is a vegan diet nutritionally sustainable?

Most of us, if considering a vegan diet, will have done some research on what to eat to make sure you don’t die of a nutrition deficiency. This research is often met with surprise and relief, as fruits and vegetables contain a wide array of almost all the minerals and vitamins found in animal products. However, just because a food contains certain minerals or vitamins does not mean that you are necessarily able to digest them, and this can be even more so true for fruits and vegetables.

A food’s ‘bio-accessibility’ is the proportion of a nutrient that can be released from the physicochemical matrix it is contained within, and then be absorbed by the body. The best way to work around

the difficulties of bio-accessibility is to eat a wide range of foods, health professionals usually recommend people to eat more plant-foods to do this; but wouldn’t completely cutting out any other food group make gathering a full spectrum of essential nutrients more difficult?

Plant foods do not all have the same bio-accessibility purely because they are all unique, and it is dependent on several factors. These include the state of the physicochemical matrix that the nutrient is contained within, as if it is not easily ruptured then very little of the nutrient will be released, as well as, the age, life stage, and chemical state of the nutrient, which determines whether and how much is going to be used, stored or excreted.

Plant cell walls are made up of ‘dietary fiber’, a carbohydrate-based material that is non-digestible by the body’s enzymes. A 2016 review from the British Journal of Nutrition regarding the mechanisms of dietary fiber provided an analysis of how the encapsulation of nutrients it provides often led to digestion taking place at a slower rate and to a lesser extent. Most of the macronutrients are usually absorbed (≥90%), but the bio-accessibility of micro-nutrients differs depending on the permeability of the cell walls: some only requiring mastication, or chewing, to be released while others are excreted from the body still intact.

The bio-accessibility of plant foods can often be increased by cooking them, which breaks down the cell walls so that they can be easily ruptured.

However, some methods of cooking, such as boiling, causes the nutrients to leach out. For example, by boiling carrots and spinach you can increase the bio-accessibility of the beta-carotenes inside, but the carotenes along with the other nutrients are lost into the water.

Some food particles are so small that it is incredibly hard to increase the bio-accessibility of the nutrients they contain to a sufficient extent. In the British Journal of Nutrition review, it was explained that once particles in the stomach reach a size smaller than 2 millimeters, they are released into the small intestine to be absorbed. However, many common plant tissues are composed of cells with dimensions several times smaller than this, so a large majority of the plant cells will still be intact. For example, 2mm of almond tissue contain 327,000

lipid-rich cells, of which only 19,000 would be ruptured; this is about five percent.

A 2008 study from the Journal of agriculture and food chemistry which looked at the release of nutrients from almond seeds, found that even when they are finely ground, only 39, 45, and 44% of lipid, vitamin E, and protein are available.

Many nutrients exist in a different chemical state in plant foods, which tends to lower their bio-accessibility. For example, iron exists in two dietary forms: haem and non-haem iron. Haem iron only exists in meat and fish, and nonheam iron only exists in plant foods. This is because the haem iron stems from haemoglobin that animals need to transport oxygen in their blood.

The haem molecule separates the iron from other food molecules and keeps it in a soluble state. Non-haem iron is poorly soluble, and its bio-accessibility is decreased even further by the action of anti-nutrients, such as phytates, which inhibit its absorption. Plants create these anti-nutrients as a defense mechanism against predators and to prevent premature seed germination.

Professor Paul Sharp, the current Head of the KCL Department of Nutrition & Dietetics, explains that someone following a vegan diet will have a hard time supplying their body with vital minerals largely “due to the presence in plants of ‘anti-nutrients’, such as phytic acid, which can inhibit the absorption of minerals.”

“Selecting plant foods that are rich in minerals and low in anti-nutrient factors,” Sharp says, “would

be a good option for those following a vegan diet.”

Non-haem iron also poses a problem in cereals: a 2012 study found that ion mass spectrometry has found that the iron is stored in aleurone cells, which predominantly store the iron as phytate-complexes inside phytingloboids. Any cereal products containing white flour are worse, as the milling process removes the aleurone cells. Elemental iron powder is added back into the flour, but this also has a very poor bio-accessibility.

Although plant foods have their limitations, a nutritious diet that meets your micronutrient requirements can be met by knowledgeably eating a wide variety of plant types that encompasses the full spectrum of essential nutrients.

Vitamin C is able to increase the absorptionof non-haem iron in the intestines, and soaking,fermenting, and cooking plant foods reduces thereanti-nutrient levels. So, what should be the mainconcern or difficulty for those on a plant-based diet?

Professor Tom Sanders, the former Head ofdepartment of Nutrition & Dietetics at KCL,explained that “The issues around transition metalssuch as iron and zinc are concerned with speciationof the metals (i.e. number of electrons in the compoundseg Fe++ is more available than Fe +++) aswell as compounds such as phytate and polyphenolsthat reduce their absorption.”

Sanders concludes that “the onlyvitamin of concern is vitamin B12which is generally absent from plantfoods unless contaminated by faeces.Rabbits get their B12 by eating theirown faeces and ruminants get thevitamin from rumen fermentation.Humans are monogastrics and so theB12 produced in faeces is not availableunless you eat them.”

The problem with B12 is quite wellknown and most vegans take a supplementfor this. Vitamin B12 is also added to manyplant-based milks. However, some nutritionists donot completely agree that vitamin B12 is the onlynutrient to be concerned about, as the bio-availabilityof minerals has been seen to be quite low(between seven and 35 percent for non-haem iron).

Professor Sharp explained that “[Sanders] is correctthat B12 is the major concern for vegans asit is not produced by plants — it is made by yeastand bacteria.”

However, Sharp says that “there are othersnutrients to be aware of – particularly calcium (maindietary source in the UK population is dairy), zinc(main source in UK population is meat) and nonhaemiron.”

Sharp goes on to explain that “the problem isn’tthat you can’t get enough of these minerals from avegan diet – you certainly can – but the minerals in

plant foods are less absorbable than those in animal-basedfoods.”

Both Sanders and Sharp believe that it is possibleto gather all the nutrients a person requires on a plantbaseddiet, and express that the main concern is doingenough research into what the best sources are.

It is also worth keeping in mind that each bodyhas different nutritional requirements: not everyonecan expect to eat the same diet. It is also importantto consider that those don’t have what would generallybe considered normal health or normal livingconditions could find it nearly impossible to remain

vegan while maintaining a proper nutrient intake:frequent travellers might struggle, as a vegan diet isstill considered a niche market in many places. Moreimportantly, people suffering from anemia and specificfood allergies, for example, will find veganismespecially challenging.

In conclusion, research shows that a full spectrumof essential nutrients can be consumed on aplant based diet, and this will massively decreasecalorie intake while increasing the intake of fibre,which is known to be beneficial for your body.

This certainly makes things more difficult,though, purely because you are cutting out some ofthe most soluble forms of the essential nutrients.

by Richard Rodrigo