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 f iber 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 t he b io-accessibility o f m icro-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 non-heam 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 absorption of non-haem iron in the intestines, and soaking, fermenting, and cooking plant foods reduces there anti-nutrient levels. So, what should be the main concern or difficulty for those on a plant-based diet?
Professor Tom Sanders, the former Head of department of Nutrition & Dietetics at KCL, explained that “The issues around transition metals such as iron and zinc are concerned with speciation of the metals (i.e. number of electrons in the compounds eg Fe++ is more available than Fe +++) as well as compounds such as phytate and polyphenols that reduce their absorption.” Sanders concludes that “the only vitamin of concern is vitamin B12 which is generally absent from plant foods unless contaminated by faeces. Rabbits get their B12 by eating their own faeces and ruminants get the vitamin from rumen fermentation. Humans are monogastrics and so the B12 produced in faeces is not available unless you eat them.”
The problem with B12 is quite well known and most vegans take a supplement for this. Vitamin B12 is also added to many plant-based milks. However, some nutritionists do not completely agree that vitamin B12 is the only nutrient to be concerned about, as the bio-availability of minerals has been seen to be quite low (between seven and 35 percent for non-haem iron). Professor Sharp explained that “[Sanders] is correct
that B12 is the major concern for vegans as it is not produced by plants — it is made by yeast and bacteria.”
However, Sharp says that “there are others nutrients to be aware of – particularly calcium (main dietary source in the UK population is dairy), zinc (main source in UK population is meat) and nonhaem iron.”
Sharp goes on to explain that “the problem isn’t that you can’t get enough of these minerals from a vegan diet – you certainly can – but the minerals in plant foods are less absorbable than those in animal-based foods.”
Both Sanders and Sharp believe that it is possible to gather all the nutrients a person requires on a plant- based diet, and express that the main concern is doing enough research into what the best sources are.
It is also worth keeping in mind that each body has different nutritional requirements: not everyone can expect to eat the same diet. It is also important to consider that those don’t have what would generally be considered normal health or normal living conditions could find it nearly impossible to remain vegan while maintaining a proper nutrient intake: frequent travellers might struggle, as a vegan diet is still considered a niche market in many places. More importantly, people suffering from anemia and specific food allergies, for example, will find veganism especially challenging.
In conclusion, research shows that a full spectrum of essential nutrients can be consumed on a plant based diet, and this will massively decrease calorie 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 of the most soluble forms of the essential nutrients.