In this study, we showed that low serum folate concentrations and low folate intakes are associated with greater BMI, higher body fat accumulation, and greater waist circumference, independently of the overall diet quality. Altogether, our findings support the hypothesis that higher folate intake could be a protective factor against obesity. An important strength of the study is that it is the first to consider folate intake, MTHFR and DHFR genotypes, serum folate levels, and plasma Hcy levels, controls for total energy intake, physical activity, and diet quality. The associations between BMI, as well as fat mass and folate concentration, have been presented in several studies before, in different age categories and populations (e.g. [14, 15, 22,23,24,25,26]). However none of these studies included as many confounding factors as here, especially intake of diet quality, which may have had a significant impact on the results. When the diet quality is not assessed, an effect assigned to folate intake may turn out to be an effect of the overall higher nutritional diet quality. It has previously been reported that starting with dietary pattern, then including nutrients, provides more reliable research outcomes, as it makes it possible to control for the effect of diet [27]. Furthermore, only a few studies have shown that folate serum concentration is lower in people with obesity and overweight, independently of folate intake (e. g. [15, 25, 26]).
The associations observed in this study could be explained in two ways: Firstly, low serum folate level may be an outcome of obesity, which could alter the pharmacokinetics of folate and lead to increased dietary folate requirements. Secondly, low serum folate level may be a cause of obesity through altered patterns of epigenetic modifications involved, for instance, in lipid metabolism [24]. Our finding that subjects with BMI > 25 kg/ m2 had lower serum folate independently of intake supports the hypothesis of da Silva et al. [18] that obesity may be an independent factor that alters folate redistribution by increasing the cellular uptake of dietary folate. Tinker et al. [23] have previously shown higher levels of RBC folate and lower serum folate in women with obesity, suggesting that people with obesity may store more folate in RBCs. However, the mechanism of such altered cellular uptake of folate remains unknown, and we did not measure RBC folate to analyze associations with anthropometrics. Another hypothesis was proposed by Bird et al. [28]—namely, that adiposity may affect the absorption of folate by the intestinal epithelium, which would be a second explanation of our findings. On the other hand, the study of da Silva et al. [18] denied this explanation, by showing that there were no differences in the area under the curve of serum folate concentration after an oral dose of 400 μg folic acid in women with obesity and in nonobese women aged 18–35; this suggested that absorption of folic acid is equal in all women. It should also be considered that obesity is often related to specific dietary patterns, which could be another factor affecting body weight. However, as mentioned above, the statistical model in our study included differences in diet quality, as well as differences in micronutrient and macronutrient intake models, to avoid confounding.
We also showed there to be associations between serum folate level and MTHFR genotype. Subjects with CT or TT genotype had lower serum folate levels than those with CC genotype. Despite the effect of genotype on folate levels, we did not find any associations between MTHFR polymorphism and anthropometric parameters. The lack of a direct association may be a result of a very small effect of the MTHFR genotype on body weight status and composition; for this reason, although the study used 400 subjects, this sample size may not have been large enough to detect such an effect. Previous results for the associations between MTHFR C677T genotype and obesity are contradictory, and raise further questions. For instance, the study of Lewis et al. [29] showed that MTHFR C677T TT genotype was not associated with BMI, waist circumference, or WHR in two adult cohorts, but was associated with the prevalence of obesity. The study was performed on three large cohorts: The British Women’s Heart and Health Study (BWHHS), n = 3416; the Avon Longitudinal Study of Parents and Children (ALSPAC), n = 5130; and the Copenhagen City Heart Study (CCHS), n = 9173. In the ALSPAC cohort, the CT and TT genotypes were associated with lower BMI values in boys aged around nine. Contradicting these results, the same researchers found in the BWHHS cohort a positive association between TT genotype and the prevalence of obesity [29].
Another result of ours that deserves attention is the demonstration of an association between serum folate level, folate intake, and waist circumference, with a simultaneous lack of this association with hip circumference. This suggests that low folate status may cause only abdominal fat accumulation. These results are in line with the findings of Bird et al. [28] and of Piyathilak et al. [30]. The mechanisms linking folate deficiency and abdominal visceral adipose tissue accumulation could include systemic oxidative stress [31], glucose metabolism [26], and DNA methylation [32]. For instance, the study of Piyathilak et al. [30] showed that women with low plasma folate concentrations had lower PBMC (peripheral blood mononuclear cells) L1 methylation, which was related to higher waist circumference, BMI, and body fat percentage. This finding suggests that folate affects body fat distribution through epigenetic mechanisms.
According to our results and those of others [15], lower serum folate concentrations in subjects with overweight and obesity are unrelated to Hcy concentration. We have observed a lack of association between folate and Hcy level, in accordance with Fonseca et al. [33], as well as between Hcy level and anthropometric measurements. Our results are in line with those of Fonseca et al. and Terruzzi et al. [33, 34], where no relationships were found between Hcy levels and BMI. Further, we did not find an association between MTHFR genotype and Hcy level, but Hcy metabolism depends on many factors, including metabolic capacity, diet, gender and, age. Some previous studies have reported different findings. In contrast to our study, Pereira et al. [16] reported that serum folate levels are positively associated with dietary folate and negatively associated with serum Hcy. Yet a few studies have also indicated Hcy to be positively correlated with BMI, fat mass, and waist circumference [35, 36], which suggests that Hcy may not be directly involved in mechanisms leading to obesity, but may in some cases act as an indicator of metabolic abnormalities.
This study has several limitations. First, we focused only on short-term folate status and did not measure RBC folate; we thus did not have a comprehensive view of folate status. Second, daily dietary folate intake was calculated as a mean value of 3 days’ intake using three-day food records, which may be too short a period to mirror daily folate intake. We only studied people aged 20–40, so the conclusions cannot be generalized to other age groups.
Taken together, our study showed folate is one of dietary micronutrients associated with increased body weight development, and so it may be reasonable to increase the daily recommendation of folate intake for people with obesity, who may have greater requirements for folate intake.