DNA damage develops rapidly during micronutrient deficiencies (12
), but the consequences of this damage, cancer and degenerative diseases, are delayed in onset (3
). This temporal dissociation can be explained by redistribution of limited nutritional resources through evolutionary adaptation of biochemical pathways (9
). As the scarcity of a micronutrient increases, a triage mechanism for allocating scarce micronutrients is activated that favours short-term survival at the expense of long-term health, in part through an adjustment of the binding affinity of each coenzyme for its required micronutrient (9
). An important corollary of the ‘triage theory’ (9
) is that the degenerative diseases accompanying ageing might be delayed by an inexpensive micronutrient intervention (14
). There is little societal concern because no overt pathologies have been associated with marginal to moderate levels of deficiency. But the pathology is insidious in the short term, and the chronic degenerative disease of the future is presaged by the DNA damage observed with micronutrient deficiencies.
Substantial experimental evidence supports the critical role played by micronutrients in the preservation of genomic integrity (15
). We have shown the deficiency of several micronutrients (iron, magnesium, zinc and vitamins B6
, C, folic acid and biotin) results in increased DNA damage in primary human cells in culture or in rodents (3
). Other studies have documented DNA damage in humans who are moderately deficient in iron, zinc, folate and B12
or choline and in rodents or human cell cultures for mostly severe deficiencies in selenium, copper, calcium, niacin and choline (9
). The depletion of zinc, which is a cofactor for several DNA repair pathways, increases single-strand breaks that return to normal upon zinc repletion (18
). The vitamin K-dependent transforming growth factor beta-inducible protein (Tgfbi) is critical to integrity of the mitotic spindle, and mouse-knockout model demonstrates increased chromosomal aberrations and spontaneous cancer (14
). Human intervention trials with micronutrients report a decrease in DNA damage (20
), though more studies are needed to reach a definitive conclusion.
Micronutrient status is associated with cancer in epidemiological studies. High serum copper, high serum iron, low serum magnesium and concomitance of low serum zinc with high serum copper or low serum magnesium contribute to an increased mortality from cancer in middle-aged men (21
). Low vitamin B6
intake correlates with the risk for colorectal cancer, and the risk is further increased by alcohol intake (23
). Meta-analysis of nine prospective studies estimated that risk of colorectal cancer decreased by 49% for every 100 pmol/ml increase in blood pyridoxal 5′-phosphate (24
). Low magnesium consumption also increases the risk of colorectal cancer (25
Assay methodologies used for evaluating the link between MN frequencies and micronutrient status are the lymphocyte cytokinesis-block assay, the erythrocyte MN assay (and the related reticulocyte MN assay) and MN formation in exfoliated cells (13
). Studies using these techniques show that dietary adequacy of micronutrients affects the frequency of spontaneous MN in humans. Owing to the complexity of human diet, the relationship between individual micronutrients and MN frequency can be difficult to interpret, unlike in vitro
experiments or animal models. The association of folate (and B12
) status with MN formation is clearly an exception to this observation ().
Studies on micronutrient status and MN frequency in RBCs
Folate status and MN frequency
The folate coenzymes are required for de novo
synthesis of nucleotides used for DNA replication and for DNA methylation. Hence, folate deficiency has a genome destabilising effect (33
). The profound effects of folate deficiency on DNA integrity and MN formation arise from decreased N
10-methylenetetrahydrofolate availability, which reduces synthesis of thymidylate (dTMP) (33
). The result is an increase in the cellular dUMP/dTMP ratio and DNA polymerase-mediated deoxyuridine triphosphate misincorporation into DNA (34
). Excision of uracil from DNA generates single-strand breaks that could result in a double-strand break if two opposing nicks are formed. These mechanistic studies established that the deleterious effects of folate deficiency on DNA integrity develop prior to the appearance of overt haematological manifestations.
The initial visible manifestation of severe folate deficiency affects the haematopoietic system (35
) in the form of abnormal nuclear maturation of precursors of blood cells in the bone marrow. Thus, the erythrocyte MN assay is particularly suited to the study of folate inadequacy. This was demonstrated in a splenectomised adult with mild folate deficiency, who had marked increased in MN erythrocytes compared to folate-replete individuals, and rapidly corrected to the normal value after treatment with folate (12
). Subsequently, analysis of blood samples from >150 splenectomised subjects showed that individuals with high observed MN frequencies (>0.1 per 100 erythrocytes) were all deficient in either folate or B12
). With the development of the flow cytometry-based reticulocyte MN assay, effect of folate status on MN frequency was also examined in non-splenectomised individuals (26
). This study showed significant negative correlation between serum folate level and reticulocyte MN frequency in 99 subjects who all had serum folate levels within the normal range. Folate repletion reduces MN formation in deficient individuals, but supplementation has no effect on MN frequency when folate status is normal (26
Despite the uniformity among various studies in identifying the strong link between folate status and MN frequency, there is wide individual variability of MN frequency with normal folate status (26
). Consequently, apart from a state of extreme deficiency, MN frequency cannot be predicted from folate status with certainty. Whether this variability in MN is from inherited traits, influence of other micronutrients or the environment has not been determined.