A Low Fat Diet Would Have The Most Impact On The Bioavailability Of Which Vitamin?

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Plant-based diets are becoming increasingly popular, but not all sources of nutrients are the same. Animal-based foods like milk or meat are rich sources of certain essential nutrients that are easily absorbed by the human body. These same nutrients are present in many plants but can sometimes be less available to the human body to digest and absorb. Therefore, nutrient bioavailability must be considered when consuming a plant-based diet. Many factors can affect nutrient bioavailability such as anti-nutrients<1> like oxalates, phytates, and tannins; cooking and processing methods; and factors in the human body

This science review looks at:

Nutrients commonly under-consumed from a plant-based diet, and how to increase intake of those nutrients from plantsWhich nutrients are less bioavailable from plant-based foodsHow bioavailability can be improved by cooking and processing

The nutrients of special concern in plant-based diets are (click to jump to that nutrient on the page):

What does bioavailability mean?

The European Food Information Council (EUFIC) defines bioavailability as “the proportion of a nutrient that is absorbed from the diet and used for normal body functions”<2>. Everything food that is eaten needs to be digested and absorbed in the intestine, and the presence of some compounds in plants can make that process more difficult for the body. For example, antinutrients can block digestive enzymes from reaching parts of a food to be digested. Oxalic acid is a molecule that plants produce to bind extra calcium within the plant. This molecule helps the plant function properly, but it also means that when we eat the plant, the calcium is harder for the human body to digest and absorb. In this example, the calcium would have a low bioavailability.

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Cooking can increase the bioavailability of many nutrients from plants.

Calcium

Plant-based calcium sources

Plant sources that are naturally rich in bioavailable calcium are limited<3>,<4>. Commonly recommended plant sources of calcium include kale, legumes, figs, bok choy, and broccoli. However, the quantity and bioavailability of calcium within these foods is far lower than dairy products or calcium fortified foods<5>,<6>. For example, the EPIC-Oxford cohort observed that vegans had inadequate intakes of calcium, approximately half the mean intake level of non-vegetarians<7>. The presence of oxalic acid, or oxalate, reduces calcium bioavailability<8>. Oxalic acid, which is present in many calcium rich plant foods, particularly leafy vegetables<9>, binds to calcium to form oxalate, which is not very well absorbed across the gut<10>.

Spinach is a renowned example of a food high in calcium, yet absorption is very low due to the oxalate content. Turnip greens have a similar calcium level but lower oxalate content, thus absorption is significantly higher than from spinach<11>. Grains and legumes, which in general make up a substantial part of a plant-based diet, are high in phytates, which bind calcium strongly and these complexes are insoluble in the small intestine, making them hard to digest and absorb. It is estimated that 32% of calcium from dairy-based foods is absorbed, but only 5% of calcium from spinach is absorbed.

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Turnip greens have a lower oxalate content than spinach, making the calcium from turnip greens more bioavailable.

Improving bioavailability of plant-based calcium

Studies have shown reducing phytates levels significantly increases calcium absorption from grains, pulses and legumes<12>,<13>. Tannins and fibre can also negatively affect calcium bioavailability. In vitro tests have shown that germinating and de-hulling cowpeas, lentils or chickpeas to reduce tannin and fibre levels can significantly increase calcium bioavailability<14>.

Factors in the human body can also influence calcium bioavailability. Calcium is absorbed across the gut by vitamin D dependent active transport and facilitated diffusion. Therefore, an individual’s vitamin D levels can affect calcium absorption. Factors such as sex, age, and individual calcium stores affect the rate of facilitated diffusion. The lower a person’s calcium stores, the more the gut will absorb this nutrient, but this ability decreases with age<15>.

To summarise, bioavailability of calcium in a plant-based diet is not optimum mainly due to the quantity and presence of innate inhibitors. Cooking or processing plants to remove antinutrients can improve bioavailability, and some plant-based sources of calcium are more bioavailable than others. However, it is commonly suggested that people who do not consume animal products, particularly dairy, should eat foods fortified with calcium or take a calcium and vitamin D supplement to meet the recommended daily allowance (RDA) for this mineral<16>,<17>.

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Protein

Differences between plant-based protein and animal-based protein

The most obvious concern regarding protein in plant-based diets is that sources are generally limited in one or more essential amino acids that cannot be made by the human body. Therefore, plant-sourced proteins are often referred to as ‘incomplete’. This is contrary to animal derived protein sources, which contain complete combinations of essential amino acids. The most common limited essential amino acids in plant-based diets are lysine (mainly limited in cereals), methionine (legumes, nuts and seeds), tryptophan (cereals) and cysteine (legumes)<18>.

Protein complementation

Protein complementation, the combination of vegetable proteins to get all of the amino acids that are essential for the body, is the most effective way to meet protein needs when consuming a plant-based diet<19>. Individuals who eat a variety of plant protein sources such as legumes, nuts, grains, and seeds in enough quantities can meet optimum protein needs through plant sources alone. Interestingly, protein complementation is not required for each meal, as the body has the capability of storing amino acids<20>,<21>.

Table 1. Examples of Protein Complementation<22>,<23>

Food Limited Amino Acid Complement
Grains

(Oat, Brown Rice, Wheat)

Lysine, Threonine Legumes

(e.g. Soy, Pea, Lentils, Beans)

Nuts and Seeds Lysine Legumes

(e.g. Soy, Pea, Lentils, Beans)

Legumes

(e.g. Soy, Pea, Lentils, Beans)

Methionine Brown Rice, Wheat, Potato
Corn Tryptophan Legumes

(e.g. Soy, Pea, Lentils, Beans)

However, the amino acid content is not the only limitation to plant protein bioavailability. The presence of other components such as fibre, tannins, and phytates can reduce protein digestibility, thus making it more difficult for the body to utilise the amino acids.

Vitamin D

Sources of vitamin D

The human body acquires vitamin D by two methods: (1) vitamin D is produced in the skin via UV rays from sunlight and (2) intake from the diet. There are two forms of vitamin D: vitamin D3 (active form) and vitamin D2. Vitamin D3 is considerably more bioavailable than the plant source vitamin D2, which means vitamin D3 is more effective than vitamin D2 at raising serum 25(OH)D concentrations, which is an important molecule for the body to actively absorb calcium<24>. Vitamin D3 is produced by human skin in the presence of ultraviolet light from the sun, or sourced from animal products are rich in vitamin D3, whereas plant sources contain vitamin D2 only<25>,<26>.

Vitamin D levels of vegans and non-vegetarians

The EPIC-Oxford cohort reported the average vitamin D intakes of vegans were approximately 73% lower than non-vegetarians<27>. Vitamin D deficiency is evident within the European population at concerning rates of prevalence<28>. Recent national UK surveys identified 1 in 5 people with low vitamin D levels (serum levels below 25 nmol/L)<29>. Individuals that derive vitamin D from sunlight and a plant-based diet alone will unlikely meet the RDA for vitamin D, especially during winter. The Scientific Advisory Committee on Nutrition (SACN) advises to consume fortified foods and supplements to meet adequate vitamin D requirements<30>. More recently, England’s national health service (NHS) extended their recommendation of taking a daily supplement containing 10 micrograms vitamin D to the entire UK population. This is to counteract the risk of getting less sun exposure due to current measures enforced by UK government to keep people in their homes to control the spread of Covid-19<31>.

Iron

The World Health Organisation (WHO) describes iron deficiency as the most common and widespread nutritional disorder in the world<32>. It is prevalent in developing countries where diets are predominantly plant-based. Deficiency is a major issue due to a significant amount of the population having high iron needs such as women of childbearing age, combined with the low bioavailability of iron in available foods.

Haem iron and non-haem iron

Iron is present in two forms: haem and non-haem iron. Haem iron is more readily absorbed across the gut compared to non-haem iron<33>. Red meat and other animal derived foods are rich sources of haem iron<34>. Plant sources contain non-haem iron only<35> and include foods such as green leafy vegetables, legumes, nuts, seeds, and grains.

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Iron bioavailability can vary significantly due to inhibitors within the same or other foods in a meal<36>. Phytates, which are complexes found in legumes, grains, oil seeds and nuts, are arguably the most potent inhibitors to non-haem iron absorption<37>. Phytates form insoluble complexes in the gut, reducing iron bioavailability considerably<38>.

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