Table 5. Genetic variations associated with responses to nutrient intake.

Gene (rs number)	Variants	Function	Dietary factor	Phenotypic traits
FTO gene (rs1558902/rs9939609)	T;A	Plays a key role in metabolism and is linked to weight and BMI	Protein/SFA:PUFA	Individuals with an A allele of the FTO gene (gene variant rs1558902) who consumed a high-protein diet were more likely to have a higher BMI and an increased risk of obesity compared to TT allele carriers [32,33,34]. Additionally, people carrying an A allele of the FTO gene had a higher risk of obesity compared to TT homozygotes when SFA intake was high and PUFA intake was low [35].
BCMO1 gene (rs11645428)	G;A	Encodes a gene that is a key enzyme in the conversion of beta-carotene to vitamin A	Vitamin A	Individuals carrying the GG genotype do not efficiently convert dietary provitamin A carotenoids into the active forms of vitamin A and may have a higher risk of vitamin A deficiency [38].
CYP2R1 gene (rs10741657)	A;G	Encodes the enzyme 25-hydroxylase related with vitamin D activation and is associated with vitamin D binding and transport to tissues	Vitamin D	Participants with the GG or GA genotype of CYP2R1 (rs10741657) have an increased risk of low levels of 25(OH)D3 [39,40,41].
GSTT1 gene	Ins/Del	Involved in vitamin C utilization through glutathione S-transferase enzymes	Vitamin C	Individuals with the GSTT1 Del/Del genotype are at higher risk of serum ascorbic acid deficiency when consuming less than the RDA of vitamin C compared to those with the Ins allele [37].
FUT2 gene (rs602662)	G;A	Involved in vitamin B12 cell transport and absorption	Vitamin B12	Carriers of the G alleles possess a higher risk of low vitamin B12 serum levels when they consumed diets with low bioavailable sources of vitamin B12 compared to AA genotype carriers [42].
GC gene (rs7041 and rs4588)	A;C and G;T	Encodes a protein that binds to vitamin D and transports it to target tissues	Calcium	Individuals homozygous for the G allele of rs7041 and the C allele of rs4588 have an increased fracture risk compared to other genotypes when they consumed a low-calcium diet (< 1.09 g/day) [43].
MTHFR gene (rs1801133)	C677T	Produces the enzyme methylenetetrahydrofolate reductase (MTHFR), which is involved in the conversion of 5, 10-methylenetetrahydrofolate to 5-methyltetrahydrofolate	Folate	Individuals carrying the T allele have a higher risk of low serum folate levels due to a lower MTHFR enzymatic activity [44]. MTHFR gene mutations are associated with neural tube defects, vascular disease, and hyperhomocysteinemia [45,46].
HFE gene (rs1800562)	G;A	Encodes a membrane protein that is similar to MHC class-I protein and is associated with beta2-microglobulin, which in turn are associated with iron absorption regulation by modulating the interaction between transferrin receptor and transferrin	Iron	Individuals with the AA genotype of the HFE gene rs1800562 are associated with a higher risk of hemochromatosis compared to those with the G allele [47].
TMPRSS6 gene (rs4820268), TF gene (rs7385804), and TFR2 gene (rs3811647)	G;A, C;A, and G;C	Involved in the regulation of the expression of hepcidin, a peptide hormone that modulates iron absorption	Iron	Individuals carrying the GG genotype in the TMPRSS6 gene have an increased risk of low hemoglobin and transferrin saturation compared to those with the A allele [48,49,50]. Individuals with the AA genotype of the TF gene tended to have a higher risk of elevated transferrin and low ferritin compared to those carrying the C allele [50,51]. Polymorphisms in the TFR2 gene can affect red blood cell count, hematocrit, and mean corpuscular volume, and individuals homozygous for the CC genotype have a higher risk of low serum levels [50].

BMI, body mass index; SFA, saturated fatty acid; PUFA, polyunsaturated fatty acid; RDA, recommended dietary allowance.