Association between dietary folate intake and severe headache among adults in the USA: a cross-sectional survey

Folate, also known as vitamin B9, is a water-soluble vitamin. Previous studies on dietary folate intake in severe headache patients were equivocal. Therefore, we conducted a cross-sectional study to elucidate the relationship between folate intake and severe headache. This cross-sectional study used data from participants over 20 years old who participated in the National Health and Nutrition Examination Survey (NHANES) from 1999 to 2004. The diagnosis of severe headache was made through participants’ self-report in the NHANES questionnaire section. We performed multivariate logistic regression and restricted cubic spline (RCS) regression to explore the relationship between folate intake and severe headache. A total of 9859 participants took part in the study, 1965 of whom were severe headache patients and the rest were non-severe headache. We found that dietary folate intake was significantly and inversely associated with severe headache. Compared with participants with lower folate intake Q1 (≤ 229·97 ug/d), the adjusted OR values for dietary folate intake and severe headache in Q2 (229·98–337 ug/d), Q3 (337·01–485 ug/d) and Q4 (≥ 485·01 ug/d) were 0·81 (95 % CI: 0·67, 0·98, P = 0·03), 0·93 (95 % CI: 0·77, 1·12, P = 0·41) and 0·63 (95 % CI: 0·49, 0·80, P < 0·001), respectively. For women aged 20–50 years, there was a non-linear association between folate intake and severe headache in the RCS. Women aged 20–50 years should have higher awareness of dietary folate and increase their dietary intake of folate, which may aid in preventing severe headache.

Severe headache or migraine is a common headache disorder with a high prevalence in people less than 50 years old, causing severe disability to the individual and a major burden to society (1) .It is characterised by paroxysmal attacks with headache, nausea and increased sensitivity to movement, light and sound (2) .The American Migraine Prevalence and Prevention Study indicated that the majority of people reported to have 'severe headaches' met the diagnostic criteria for migraine or possible migraine (3) .
There are two main subtypes of migraine, including migraine with aura and migraine without aura.These two entities can be distinguished by focal neurological symptoms that typically precede or accompany headache attacks in patients having migraine with aura (4) .Cortical spreading depression is a phenomenon of spreading depolarisation across the cerebral cortex, which is related to the presence of aura (5) .Among the genetic and environmental factors that may contribute to the development of migraine, a study has shown a possible link between 5,10-methylenetetrahydrofolate reductase gene polymorphisms associated with reduced enzyme activity and migraine risk (6) .Defective or insufficient production of methylenetetrahydrofolate reductase, an important enzyme involved in the metabolism of homocysteine (Hcy), can lead to hyperhomocysteinaemia (7) .Rainero I et al suggested that interventions targeting Hcy metabolism, primarily supplementation with Hcy-lowering vitamins (including folate), may have beneficial effects on severe headache or migraine patients (8) .
Folate is a water-soluble vitamin and its synthetic form is folic acid (9) .The metabolism of Hcy is reported to be dependent on the presence of the cofactor folate, and in the presence of folate deficiency, Hcy levels tend to be elevated (10) .Smith AD et al revealed that there was a correlation between the level of serum Hcy and the frequency and characteristics of headache attacks (11) .And dietary compounds, folate, was found to reduce Hcy levels in many pathologies (12) .A previous study showed that a significant relieving in headache severity, frequency and duration of headache attacks in ninety-five migraine patients with a diet supplementation with pyridoxine and folate for 3 months (13) .However, the relationship between dietary folate intake and severe headache has not been reported in the general population.
The relationship between dietary folate intake and severe headache in adults was examined using data from the National Health and Nutrition Examination Survey to refine this study in the general population.Based on the nutritional patterns found in this population, we hypothesised that dietary folate consumption would be inversely associated with severe headache.Furthermore, a dose-response relationship between dietary folate intake and severe headache was descripted.

Study population
Data analysed in this cross-sectional study was obtained from NHANES, administered by the Centers for Disease Control and Prevention.The NHANES data is a series of cross-sectional, stratified, multi-stage probability surveys for the Americans, non-institutional population of the USA (14) .The NHANES collects information on demographics, laboratory tests, physical examinations, diet surveys and other health-related questions via home visits and mobile examination center.All NHANES protocols were authorised by the National Center for Health Statistics Ethics Review Committee and the NHANES obtained written informed consent signed by all participants.No additional institutional review board approval was required for the secondary analysis (15) .Relevant data from NHANES are publicly available, and its methodological details and survey design can be found at https://www.cdc.gov/nchs/nhanes/index.htm.
The data were not subjected to statistical weight calculations prior to the study, and the sample size was based on all available data in NHANES.We conducted a cross-sectional study of American adults from the 1999-2004 NHANES survey, as this was the only cycle to include headache questionnaires for adults.The data were combined for our analysis, resulting in 31 126 participants, and our study was limited to adults 20 years old or older.We excluded participants without information such as severe headache, pregnant women and missing data.Finally, 9859 subjects remained in our study (Fig. 1).

Severe headache classification
We evaluated severe headache by self-report in the NHANES questionnaire section.We divided participants who answered 'yes' into those with severe headache: 'In the past 3 months, have you had severe headaches or migraines?'We may consider that the majority of participants with severe headaches have migraine.Our findings are consistent with those of the American Migraine Prevalence and Prevention study.The study revealed that 17•4 % of participants reported 'severe headache', of which 11•8 % met the International Headache Disorder Type II criteria for migraine, 4•6 % met the criteria for 'probable migraine' and only 1 % were identified as 'other severe headache' (3) .

Dietary folate assessment
Dietary folate intake was identified through a 24-h recall survey.The survey is a retrospective dietary evaluation method that provides detailed information on all meals and beverages consumed over a 24-h period (16) .The dietary assessments were conducted in person by trained dietary investigators.Participants were shown a standard set of measurement guidelines that help them to be able to accurately report the quantity and size of food.NHANES codes the acquired data by using the Food Intake Analysis System and the USA Department of Agriculture survey nutrition database, which in turn converts them into total nutrient intakes (17) .In the present study, all participants performed their first 24-h dietary recall from 1999 to 2004 (18) .Furthermore, we grouped each participant by their dietary folate intake.

Potential covariates
Various potential covariates were assessed based on previous literature (18)(19)(20) .Demographic covariates included age, gender, ethnicity, education level, marital status and family income, all of which were obtained through self-expression during the interview process.Lifestyle-related covariates included smoking status, alcohol consumption and BMI.Dietary-related covariates included energy consumption, protein consumption, carbohydrate consumption and fat consumption.Clinically relevant covariates included hypertension, diabetes, stroke and coronary heart disease and C-reactive protein.

Statistical analyses
We performed statistical analysis by using R software (version 4.2.1).All analyses in this study were based on appropriate sampling weight to account for the complex survey design (21) .The statistical descriptions of continuous variables were presented as sample-weighted means (standard errors), whereas categorical variables are reported as sample-weighted percentages and frequencies.To compare the differences between different groups, one-way analyses of variance was performed for continuous variables, but χ 2 test was performed for categorical variables.Also, we divided the participants into quartile by dietary folate consumption.Multifactorial logistic regression was used to examine the association between dietary folate consumption and severe headache, and relevant data were expressed as OR (95 % CI).To assess the robustness of the results, a sensitivity analysis was also conducted on participants without extreme energy intake (consuming < 500 or > 5000 kcal per day) for sensitivity analyses.In addition, restricted cubic spline regression was used to explore the non-linear relationship between dietary folate and severe headache.And dietary folate consumption was involved in the model as a continuous variable.Two-sided P values < 0•05 were considered statistically significant difference.

Baseline characteristic
The basic characteristics of the included and excluded individuals are exhibited in the Supplementary Materials (Table S1).Table 1 shows a total of 1965 individuals with severe headache.The average age of the individuals in this study was 46•10(0•30) years, and 4852(50•65) individuals were Association between folate intake and severe headache female.Participants who consumed more folate often tended to be younger, male, married, non-Hispanic White, had a higher educational level, had a high family income, never smoking never drinking, had a lower incidence of hypertension, diabetes and stroke, had higher consumption of energy, proteins, fat and carbohydrates and lower serum C-reactive protein levels and BMI.As shown in Table 2, the results of the univariate analysis indicated that age, sex, race, education, family income, smoking status, drinking status, coronary heart disease, BMI, protein intake and C-reactive protein were associated with severe headache.

Association between dietary folate consumption and severe headache
The results of the multi-factor logistic regression models are descripted in Table 3.Crude was the unadjusted model, and model 1 was adjusted for age and gender.In addition, model 2 was adjusted for age, gender, race, marital status, education level, family income, smoking status, drinking status, hypertension, diabetes, stroke, coronary heart disease, BMI, energy consumption, protein consumption, carbohydrate consumption, fat consumption and C-reactive protein.
There was a significant negative association between dietary folate intake and severe headache, when analysed by quartiles of dietary folate intake, after adjusting for potential confounders.3).
After excluding the participants with extreme energy intake, 9605 participants remained, and the relationship between dietary folate intake and severe headache kept   4), respectively (Table 4).In the restricted cubic spline analyses (online Supplementary Fig. S1), we found a non-linear association between folate intake and severe headache (P = 0•006).The OR values for the relationship between folate intake and severe headache were decreased with increasing folate intake.
In the restricted cubic spline (Fig. 3), for women aged 20-50 years, we found a nonlinear relationship between dietary folate consumption (continuous variable) and severe headache (P = 0•04), using a reference point of 425 ug/d of dietary folate consumption.Moreover, we observed that the risk of developing severe headache declined with increasing dietary folate consumption until it reached 716 ug/d, after which the risk of severe headache reached a plateau.

Discussion
In this large cross-sectional study, we provided the first nationally representative evidence of the relationship between dietary folate consumption and severe headache in American adults.Dietary folate intake was significantly and inversely associated with severe headache.The sensitivity analyses revealed a robust association between dietary folate intake and severe headache in adults.After stratified analyses by sex and age, we observed that dietary folate consumption was negatively associated with severe headache in women aged 20-50 years.For women over 50 years of age and for adult men, there was no significant association between dietary folate consumption and severe headache.And we further observed by restricted cubic spline analysis that the risk of developing severe headache decreases with increasing dietary folate consumption.A great deal of interest has been accumulated in dietary interventions in the prevention of migraine and headache (22) .In 2015, Menon et al had already explored dietary folate intake and clarified its negative association with headache frequency (23) .There were also other nutrients that have been extensively investigated, among them vitamin B 2 (riboflavin) and co-enzyme Q10 supplementation which have been shown to be beneficial for migraine patients (19,24) .Headache may be associated with hyperhomocysteinaemia (25) .In this context, homocysteine-lowering (Hcy) folate has been shown to be beneficial in the management and prevention of migraine (26) .It is noteworthy that most of the current studies are case reports or case series, and no specific studies have been conducted to explore the relationship between dietary folate and severe headache in the general population.The NHANES provides USA a unique opportunity to assess whether there is a negative relationship between dietary folate consumption and severe headache in the general population.
Although the underlying mechanism of the negative relationship between folate intake and severe headache is still to be investigated, our results are biologically plausible based on the available evidence.Currently, correlative applications of phosphorus nuclear magnetic resonance spectroscopy elucidate changes in energy metabolism in the brain of migraine patients (27) .It suggests that an imbalance between brain  energy demand and ATP production plays an important role in severe headache.And since glycolysis is the main process of energy production in the brain, the energy-producing functions of mitochondria are closely related to the pathogenesis of migraine (28) .A study on family hemiplegic migraine revealed that the energy metabolism of brain and muscle in migraine patients was defective (29) .Additionally, a previous study showed that elevated levels of Hcy were related to mitochondrial dysfunction and energy production in the central nervous system (30) .However, the dietary compounds pyridoxine (vitamin B 6 ) and folate have been identified to reduce Hcy levels (31) .All in all, folate deficiency may be related to headache attacks.These showed that maintaining normal mitochondrial function and reducing serum homocysteine properties of folate to may contribute to its beneficial effects on severe headache.
Severe headache or migraines are highly prevalent in adults under the age of 50 years and women, on average, are at roughly three times the risk of developing migraine than man (32) .In stratified analysis by gender and age, dietary folate consumption was only associated with severe headache in women aged 20-50 years old.Therefore, the results of this study further explain that dietary folate deficiency may be one of the reasons for the high prevalence of severe headache in women aged 20-50 years.As the body cannot synthesise folate, dietary intake is an important source of folate for the body, which is absorbed by the body in the proximal jejunum.Folate was found in many natural foods such as pulses, yeast, fruit and green leafy vegetables, especially dark green vegetables and in some animal foods such as liver and kidney (33) .Women, especially those of childbearing age, are becoming more aware of folate supplementation and are actively taking appropriate amounts of folate before and during pregnancy, and dietary folate intake is the preferred method of supplementation (34)(35)(36) .It is now mostly believed that the gastrointestinal tract is less functional in the elderly than in younger people (37) .As a result, older adults may have a reduced intake of dietary folate.Therefore, women aged 20-50 years may have higher folate absorption than men and women over 50 years, which may be the reason why dietary folate was associated with severe headache only in women aged 20-50 years.In the restricted cubic spline, when 425 ug/d of dietary folate consumption was used as a reference point, we observed that the OR was significantly lower than 1•00.Therefore, we recommend that women between the ages of 20 and 50 should have a daily dietary intake of the appropriate amount of folate.In sum, the negative association between folate intake and severe headache in a population of women aged 20-50 years is important for proposing strategies to prevent severe headache in adults in a specific population.
There are some limitations that must be taken into account in the study.At first, the diagnosis of severe headache or migraine was based on the self-report questionnaire 'In the past 3 months, have you had severe headaches or migraines?'In addition, no data was available on other characteristics of participants' severe headaches or migraines, such as severity or other symptoms and which subtype of migraine.Therefore, we used the term severe headache uniformly.This may not be the best terminology, but at least it eliminates the confusion as if the headache was migraine or not.Second, dietary data were obtained through 24-h recall, an approach that had inherent limitations in terms of reliability and validity of nutritional assessment.However, Prentice RL et al. suggested that 24-h recall may provide more detail about food types and quantities than food frequency surveys (38) .Third, since the study was conducted on USA adults and did not include special groups such as minors, we cannot analyse special populations or other ethnicities which resulted in insufficient extrapolation power for this study.Therefore, further researches are necessary to verify the generalisability of these results.And, we cannot eliminate the interference of nonrandom missing data on the results because of baseline differences between included and excluded participants.Finally, our study was a crosssectional study, which meant that causal inferences cannot be made.Thus, further prospective longitudinal investigations are needed to clarify the causal relationship between dietary folate intake and severe headache.Our study also has several advantages.This study provided epidemiological evidence of the significant relationship between dietary folate intake and severe headache in a representative general population across the USA.In addition, we provided reliable correlations by controlling for multiple potential confounders in our statistical analyses.Furthermore, we performed stratified analyses for both sex and age to detect differences between sex and age groups.

Conclusions
Our study first explored the relationship between dietary folate consumption and severe headache in American adults.This study suggested that dietary folate may play an important role in the prevention of severe headache.We recommend that women aged 20-50 years should have higher awareness of folate and increase their dietary intake of folate if necessary.

Fig. 1 .
Fig. 1.Flow chart of participants inclusion and exclusion for analysis.

Table 4 .-
Association between dietary folate intake and severe headache in participants with extreme energy intake was not included Crude was adjusted for nothing.Model 1 was adjusted for age and sex.Model 2 was adjusted for model 1 þ marital status, race, education level, family income, smoking status, drinking status, hypertension, diabetes, stroke, coronary heart disease, BMI, energy consumption, protein consumption, carbohydrate consumption, fat consumption and C-reactive protein.

Fig. 2 .
Fig.2.Association between dietary folate intake and severe headache in different sex and age groups.OR values were adjusted for marital status, race, education level, family income, smoking status, drinking status, hypertension, diabetes, stroke, coronary heart disease, BMI, energy consumption, protein consumption, carbohydrate consumption, fat consumption and C-reactive protein.

Fig. 3 .
Fig. 3. Association between dietary folate intake and severe headache in female of 20-50 years in RCS.The model was adjusted for marital status, race, education level, family income, smoking status, drinking status, hypertension, diabetes, stroke, coronary heart disease, BMI, energy consumption, protein consumption, carbohydrate consumption, fat consumption and C-reactive protein.Solid line, OR; shade, 95 % CI.RCS, restricted cubic spline

Table 1 .
Population characteristics by categories of dietary folate intake

Table 2 .
Relationship of covariates and severe headache risk

Table 3 .
Association between dietary folate intake and severe headache

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Crude was adjusted for nothing; model 1 was adjusted for age, sex; model 2 was adjusted for age, sex, marital status, race, education level, family income, smoking status, drinking status, hypertension, diabetes, stroke, coronary heart disease, BMI, energy consumption, protein consumption, carbohydrate consumption, fat consumption and C-reactive protein.