martes, 23 de octubre de 2018

Genetic Factors Impacting Nutritional Requirement

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By Lois Zoppi, BAReviewed by Liji Thomas, MD

While nutrition has a very recognizable role in overall health management, the role of genetics in nutritional impact is yet to be fully understood. Genetic variability is accountable for many differences among people, including eye, hair, and skin colour, weight, and many others.

Image Credit: ARTFULLY PHOTOGRAPHER / Shutterstock

When considered in relation to nutrition, genetic variability may be at least partly responsible for differences between individuals in:

Absorption of food
Metabolism
Enzyme digestion
Biosynthesis
Catabolism
Transport across cell membranes
Uptake by cell receptors
Storage
Excretion

Genetic variations may also be linked to individual food likes and dislikes. For instance, phenylthiocarbamide (PTC), found in foods like cabbage, broccoli, cauliflower, kale, and Brussels sprouts, can either taste very bitter or be almost tasteless, depending on the genetic makeup of an individual.

A single gene, TAS2R38, was found to be responsible for a person’s ability to taste PTC. Consequently, this can have an effect on a person’s individual nutrition as people are less likely to eat foods they don’t like, consequently leading to lower intake of certain food groups and their nutritional benefits.

Genetic Factors Affecting Vitamin Absorption

Genetic variations in the central nervous system can also affect the degree of satiation (how full a person feels after eating) and taste perception.

The absorption of certain vitamins can also be affected by genetics, such as increased iron absorption in hemochromatosis, a condition where the body’s iron levels build up and overload. people with hemochromatosis possess a gene variant on the short arm of chromosome 6, which is linked to HLA Locus A, and which allows increased iron absorption.

The absorption of vitamin B12 can also be adversely affected by the genetically determined absence of gastric intrinsic factor, leading to the development of pernicious anemia.

The ability to absorb vitamin D, among other fat-soluble vitamins like A and E, according to some studies, varies by a factor of 34 between individuals.

The body cannot produce these vitamins itself and the bioavailability of vitamin E through the diet is modulated by single nucleotide polymorphisms (SNPs).

The particular SNP linked with the bioavailability of vitamin E is controlled by a combination of 28 SNPs across 11 genes that are involved with the intestinal uptake and transport of these vitamins.

Genetic Factors Affecting Lactose Malabsorption

Humans produce an intestinal enzyme called lactase to digest lactose, a type of sugar found in the milk of most mammals, into glucose and galactose.

In some people, the ability to absorb lactose disappears after they are weaned, and such people are called lactose malabsorbers.

Some populations, particularly those of European origin, do not lose this ability to digest milk and retain the intestinal lactase activity required, which is controlled by a gene particular to this process. Those who don’t carry this gene are homozygotes at the locus for lactase deficiency.

Race and Ethnicity

As the amount of gene sharing by family members can be affected by their degree of relatedness, racial and ethnic groups are treated somewhat like an ‘extended family’ when considering links between genetics and nutrition, due to the inheritance of similar genes through common ancestors.

Identifying specific gene markers to explain the existence of certain traits or diseases in different ethnic or racial groups can often differentiate between environmental factors that could account for differing disease incidence rates. For instance, divergent obesity rates between different populations are thought to be due to genetic factors, but no gene marker has been found to definitively prove this.

Genetics and Obesity

Genetic factors in obesity among family members have been identified in studies involving monozygotic and dizygotic twins.

Nutritional Policies and Guidelines

Differences in enzyme activity contribute to differences in nutritional requirements. Currently, nutritional guidelines like recommended dietary allowances (RDA) are based on metabolic outcomes but are not adjusted for genetic subgroups. Therefore, these nutritional policies, when formulated on the basis of only widely experienced variations, can pose potentially life-threatening risks for small groups of people with rare variations.

Again, differences between racial and ethnic groups are important to consider in nutritional policy-making, as a nutrition policy may be well suited to one population, but be unsuitable for another.

Nutrigenetics and Nutrigenomics

Two areas of study regarding nutrition and genetics are nutrigenetics and nutrigenomics, which investigate the effect of genetic variations on dietary responses, and the role nutrients play in bioactive food compounds and gene expression.

Discoveries in these areas will accelerate the development of personalized nutrition plans and strategies for better health and disease prevention. There are three key factors in nutrigenetics and nutrigenomics: ethnic diversity, food availability and malnutrition.

Regarding food availability, depending on cultural, environmental, and economic differences, groups of people will make very different food choices due to availability, affordability, taste perception differences and habits.

In the presence of chronic malnutrition, gene expression and genome stability can be affected, with genome stability sometimes leading to gene sequence mutations or mutations at chromosomal levels.

Summary

While there is evidence suggestive of genetic influences on various areas of nutrition, there is a paucity of reliable studies of genetic factors impacting nutrition, partly due to flaws in their methodology.

Many studies rely on self-report questionnaires that allow for bias and under- and overestimation of nutrient intake.

Although nutrigenetics and nutrigenomics show great promise in understanding nutrition on a genetic level, we are not close to a time when dietary advice can be based on genetic testing.