Elevated folic acid (FA) intakes create analogous conditions to those of metabolic folate deficiency⁽Reference Troen, Mitchell and Sorensen¹⁾. In a similar way, vitamin B₁₂ status influences the physiological response to different FA doses in the organism⁽Reference Morris, Jacques and Rosenberg²⁾. In previous studies, we have demonstrated that FA deficiency compromises methionine metabolism, whereas supplementation does not show additional positive effect compared to control diet in growing animals⁽Reference Partearroyo, Ubeda and Alonso-Aperte³⁾. However, there is no information on the FA supplementation effect on the methionine cycle in vitamin B₁₂ deficiency in two critical physiological states such as growth and aging.

The methionine/methylation metabolism during growth and aging was determined to evaluate the global effect of different levels of dietary FA and vitamin B₁₂.

Male Sprague–Dawley rats (6 weeks, n 50, and 20-month-old, n 35) were assigned into four groups at three levels of folic acid both in the absence of vitamin B₁₂ for 30 d: [C_B/C_F (50 μg vitamin B₁₂; 2 mg FA), D_B/D_F (0 μg vitamin B₁₂; 0 mg FA), D_B/C_F (0 μg vitamin B₁₂; 2 mg FA) and D_B/S_F (0 μg vitamin B₁₂; 8 mg FA)]. Manipulation of the animals was performed following European Union Normative (2003/65/CE). Student's t test was used to evaluate the differences in the same group between the two populations (*P<0.05, **P<0.01 and P<0.001).

Fig. 1. Serum folate.

Fig. 2. Serum vitamin B₁₂.

Fig. 3. Serum vitamin B₆.

Fig. 4. Serum homocysteine.

Fig. 5. DNA methylation in brain.

Fig. 6. DNA methylation in liver.

Serum homocysteine, vitamin B₆ and FA concentrations were lower in aging v. weanling rats (Figs. 1, 3 and 4, respectively), but serum vitamin B₁₂ values were higher (Fig. 2). However, aging seemed not to affect the folate liver stores (data not shown). Only during growth, both vitamins deficiency induced a hepatic DNA hypomethylation that was reverted when rats were FA supplemented (Fig. 5).

There were some parameters of methionine cycle in weanling rats and none in aging rats that recovered similar values to controls when rats deficient in vitamin B₁₂ were supplemented with FA. Therefore, FA supplementation should also be accompanied by the same nutritional action for vitamin B₁₂ in order to solve these alterations in this vitamin B₁₂ deficient group.