In this set of analyses of 11 different, large datasets collectively containing >200,000 US women, we found that increased BMI was generally associated with significantly increased risk of headache, but not diagnosed migraines. We note that our results across all datasets, with the exceptions of WHI (diagnosed migraines) and NHANES I (taking headache medication), suggested that, as compared to a BMI of 20, mild obesity (BMI of 30) was associated with an ~35% increase in the odds of reporting headache whereas severe obesity (BMI of 40) was associated with an ~80% increase in the odds of reporting headache. Across the databases, a BMI of ~20 was commonly associated with the lowest risk for headache. These results were not materially altered when socioeconomic variables, alcohol consumption, and hypertension were also included in the model.
With regard to migraine headache, the results from our primary model of the WHI data, the only dataset that explicitly assessed migraine headache diagnosis, suggested that BMI may not be associated with migraine, but our extended model revealed a slight negative association. It is noteworthy that many people with migraines go undiagnosed. Therefore, the relationship between BMI and diagnosed migraines is not likely to reflect the BMI relationship with all migraines (both diagnosed and undiagnosed). Our main conclusion that BMI was associated with an increased likelihood of headaches is based on the following logic: (i) the WHI analysis showed no positive correlation between BMI and diagnosed migraines, and this is a finding of consequence because of the large WHI sample; (ii) the NHIS analyses showed a positive association between BMI and “headaches or migraines”; (iii) the ACHS and TCHS showed a positive association between BMI and “headaches” which were probably interpreted by most participants to include migraines; (iv) finding (i) suggested that there was no association between BMI and diagnosed migraines in our NHIS, ACHS and TCHS analyses; and (v) so we concluded that the findings in NHIS, ACHS and TCHS suggest that BMI was associated with non-migraine headaches and possibly undiagnosed migraine.
Most of the databases we analyzed individually provided ample statistical power to detect the estimated effect sizes we observed. That, along with the consistency of the results, obviated the need to conduct a formal meta-analysis. Our findings clarify and accord with previous studies. Specifically, after adjusting for age, gender, race, and education, Scher and colleagues (6
) found that obesity was associated with prevalent chronic daily headache (OR = 1.34). Similarly, Ohayon (10
) and colleagues found that overweight/obese (BMI > 27) respondents were more likely to report morning headache than were adults with BMIs 20–25 and among a sample of ~15,000 Australian women, Brown (9
) and colleagues found that obese persons were more likely to report headache (OR = 1.47). Also, consistent with our primary model analysis of WHI data, Bigal and colleagues (8
) using data from over 30,000 participants, found that BMI was not associated with migraine prevalence.
Interestingly, we observed some evidence in four datasets (NHIS 1997, 1999, 2003; and ACHS) that unusually low BMI may be associated with increased risk for headache. These results suggest that increased BMI may be associated with decreased risk of headache among the category of women with a BMI <20 and increased risk of headache among those with a BMI >20. This finding should be interpreted with caution since the association was statistically significant at the 0.05 level only in the NHIS 1997 and 1999 datasets. It was noteworthy that, since only ~5% of all study participants had BMI values <20, we may have lacked sufficient power to reliably detect the elevated risk levels estimated to be associated with low BMI across studies. To our knowledge, low BMI, in the absence of major illness (e.g., cancer), has not been previously associated with reports of headache. Nonetheless, this finding merits further investigation before definitive conclusions can be drawn.
Nine out of the eleven datasets we examined had no study variables missing >3% data, so any resulting effects from missingness were likely to be minimal in these cases. In NHANES I and TCHS where we saw higher levels of missing data for some variables it was less clear what, if any, effects missing values might have had on our results. Interestingly, in TCHS, those reporting headache were ~80% more likely to be missing BMI data than those not reporting headache, but we cannot know how this would influence the significant linear relationship we detected between BMI and headache. Considering the results available from the more complete datasets (i.e., NHIS 1997–2003 and ACHS) and the similarity of those results to those from TCHS, missingness may not have significantly affected the TCHS results.
The mechanisms that might be responsible for the obesity–headache association are unclear. However, obesity is associated with the metabolic syndrome, a pro-inflammatory, pro-thrombotic state that may contribute to headache development and progression (25
). Headache is also related to sleep apnea, a condition highly associated with obesity (27
). Hypertension is also associated with headache (28
) and obesity is a major risk factor for hypertension (29
). Moreover, headache is one of the side effects of many medications, including sibutramine, a medication to treat obesity (30
). Each of these offers a hypothesis meriting further study.
This study has limitations. First, the headache-related questions in the datasets differed, in some cases substantially. For example, the WHI headache question focused on migraine headache and asked, “Has a doctor told you that you have “migraine”?” By contrast, the NHANES headache question did not ask whether the respondent suffered from headache but, rather, whether they used medication for headache (“During the past 6 months have you used any medicine, drugs or pills for headache?”). Although we coded the headache variables in the datasets to create uniformity in outcome variables (see ), these two datasets (WHI and NHANES I) which asked about headache in a way related to diagnosis or treatment are the only two that did not detect clear and statistically significant associations. The assessment of headache in the other datasets focused primarily on the presence/frequency/severity of headaches. Second, we only considered cross-sectional datasets as they were more widely available and different statistical methodology would be required to analyze longitudinal data. Hence, our analyses were restricted to headache or migraine status concurrent with BMI status. We did not look at data on subjects who were free of headache at baseline, that were followed prospectively to see if BMI or changes in BMI would predict headache or migraine occurrence over time. Follow-up data were available in only the two smallest studies (ACHS and TCHS). In the future, we recommend analyzing any available longitudinal data on headache and BMI by using nonlinear methodology, similar to that which we have applied to these cross-sectional databases.
In conclusion, the results of estimating the association between BMI and headache in large, nationally representative samples of women indicated that obese women have significantly higher risk for headache. Further research is warranted to study the direction and mechanisms of causation as well as to investigate the possible BMI–headache relationship among men. The possibility that weight loss may alleviate severe or chronic headache problems among obese people also warrants investigation.