The term vitamin describes a small group of organic compounds that are essential in the human diet. Although for the most part, dependency criteria are met in developed countries through balanced diets, this is not the case for the five billion people in developing countries who depend predominantly on a single staple crop for survival.
Therefore, providing a more balanced vitamin intake from high-quality food remains one of the grandest challenges for global human nutrition in the coming decade(s).
Over the next few posts I will describe the known importance of vitamins in human health and current knowledge on their metabolism in plants.
Deficits in developing countries are a combined consequence of a scarcity of specific vitamins in major food staple crops, losses during crop processing, and/or over-reliance on a single species as a primary food source. Plant science can play a role in addressing these problems and vitamin pathways have been successfully engineered.
While considerable advances have been made in understanding vitamin metabolic pathways in plants, more cross-disciplinary approaches must be adopted to provide adequate levels of all vitamins in the major staple crops to eliminate vitamin deficiencies from the global population.
As plants are autotrophic – self-sustaining or self-nourishing organisms that have the ability to synthesise their own food from inorganic materials – they have the ability to acquire the basic elements (minerals) and synthesise the full spectrum of organic molecules required to support their growth and propagation.
While humans require the same basic elements as plants, they lack the ability to synthesise many organic molecules – i.e., so-called essential micronutrients, including certain amino acids and vitamins – for which plants are the main dietary source. Therefore, human nutritional health is dependent on plant food either directly or indirectly – feeding on animals that feed on plants.
In contrast with the three major nutrients (carbohydrates, proteins, and lipids), micronutrients by definition do not provide energy and are needed in relatively small amounts by organisms. It has been known for well over a century that micronutrient deficiency is directly linked to human disease. Indeed, such observations instigated the very discovery and categorisation of various micronutrients, most notably the vitamins.
The term 'vitamine' was coined by the Polish biochemist Casimir Funk in 1912 in his paper ‘The etiology of the deficiency diseases’, published in The Journal of State Medicine. He isolated a substance – beri-beri vitamine – that was present in rice bran, but not in polished rice (Oryza sativa) and could alleviate the deficiency disease beriberi, prevalent in many Asian countries. At the time, he assumed – wrongly, as it turned out – that all such essential compounds in the diet contain an amine group, hence, the term vitamine (vital-amine); the final ‘e’ was later dropped to deemphasise the amine connection.
Micronutrients are essential for all life; however, the term ‘vitamin’ is a medical definition relating to humans, highlighting the fact that we have lost the ability to synthesise these compounds de novo. Therefore, the building blocks of vitamins must be obtained from the diet.
To date, 13 compounds are classified as vitamins, they can be broadly classified into fat-soluble (A, D, E, and K) and water-soluble (vitamin B complex: B1, B2, B3, B5, B6, B8, B9, and B12, and vitamin C). Bacteria, fungi, and plants synthesise these compounds and their main function – both as micronutrients in these organisms and as vitamins in humans – is as cofactors or coenzymes in various enzymatic reactions.
Additionally, some of them play distinct roles, for example as antioxidants (vitamins C and E), in vision (β-carotene), or as a (pre-) hormone involved in calcium and phosphorus homeostasis in the blood stream (vitamin D).
Being micronutrients in plants as well as animals, it follows that vitamin compounds are synthesised in tiny amounts. On the one hand, this makes it challenging for researchers to study the corresponding pathways and enzymes involved, but on the other hand, it means that even small alterations in the levels of these compounds can have a disproportionately positive impact on aspects of human health.
It is only relatively recently, with the arrival of genomic sequence information and the interest in manipulating the levels of these compounds in plants, that the metabolic pathways of these substances have begun to be understood.
The concentration of many vitamins in the edible portions of the most abundantly grown plants used globally for human food is below minimal requirements, for example, wheat [Triticum aestivum], rice, maize [Zea mays], potato [Solanum tuberosum], and cassava [Manihot esculenta]. This has profound implications for global human health. This deficit is compounded by the limited variety of foods that make up the bulk of the average diet and a severe depletion of specific micronutrients in the five major crops previously listed as a result of postharvest processing.
For example, whole-grain rice is a good source of vitamin B1, but polished rice has been depleted of this vitamin. Fruits usually provide several vitamins and carbohydrates but are poor sources of minerals or protein. Therefore, a diversified, balanced diet with the right concentration and combination of nutrients is required to support human health.
Although the human requirements for the 13 vitamins are reasonably well known and well defined, at least at a population level, the vitamin status is far from being adequate in major sections of the global population. This is especially true in developing countries where billions of people still suffer from hunger and protein-energy malnutrition and, at the same time, deficient in numerous micronutrients, i.e., vitamins and minerals. In these countries, many people do not have the means to consume a diverse diet and rely on a single staple crop, which is almost invariably a poor source of several essential micronutrients.
For example, in ‘Behavioral dimensions of food security’, published in the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS), Timmer outlines that nearly one-half of the world’s population consumes rice as a staple food (typically produced by small farmers using highly labour-intensive techniques); while cassava is consumed by millions of people, mostly in tropical countries.
In Western countries, most people have access to a broad variety of foods that provide all the required vitamins or are fortified during processing to achieve this; in turn, vitamin deficiencies are reduced. Nevertheless, a significant portion of all populations do not have optimal intake in several vitamins for various reasons.
Current technology presents us with the opportunity to develop strategies to counteract these deficits and improve the nutritional quality of unprocessed foodstuffs. The understanding of vitamin biosynthesis, transport, storage, and recycling in plants has progressed considerably in recent years. In addition, the accessibility of genomic tools, such as high marker density genetic maps, genome sequences, and genetic resources, is enabling the identification of vitamin-improved alleles and their introduction into elite varieties for many crop species.
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