Vitamin D insufficiency is very common in older people,1 and many cross-sectional studies of older populations have reported higher levels of 25-hydroxyvitamin D (25-OHD) in men than in women.2-4 These differences have not been adequately explained but have been attributed to sex differences in behavioural and cultural factors associated with sun exposure.4 Male-pattern hair loss (baldness) occurs almost universally with ageing in men, and more than 70% of men older than 70 years have significant hair loss.5 As vitamin D production in humans occurs in exposed skin, particularly in the hands, arms and face,6 we hypothesised that men with more baldness would have more exposed skin suitable for vitamin D production and, consequently, higher serum 25-OHD levels. If so, baldness in men might explain sex differences in 25-OHD levels. Also, preservation of vitamin D status might provide an evolutionary explanation for the high prevalence of baldness in older men. We therefore assessed whether 25-OHD levels are associated with degree of baldness in middle-aged and older men.

Methods We recruited 323 healthy, independent-living, middle-aged or older men to take part in a study on calcium supplementation. Full details of the study protocol have been published elsewhere;4 briefly, men aged 40 years and older, who did not have significant medical disease and were not receiving cholecalciferol supplements in doses exceeding 1000 IU/day, were eligible to take part. This substudy was approved by the Northern X Regional Ethics Committee. A total of 296 men consented to have photographs of the head taken. Degree of baldness was independently assessed by two of us (M J B and R W A); the photographs were compared with standardised pictures from the validated Hamilton–Norwood scale for assessing male-pattern hair loss.5 Results were then grouped into three categories: no hair loss or mild frontotemporal recession (Group 1), predominant vertex loss (Group 2), and significant scalp and vertex loss (Group 3). Lifestyle information, including the frequency of outdoor hat wearing and whether sunscreen was applied to the scalp, was gathered using a questionnaire. Skin type was self-rated by participants using the Fitzpatrick scale7 (Box). Serum 25-OHD was measured by radioimmunoassay (DiaSorin, Stillwater, Minn, USA) in the first 192 men who were recruited, then using a chemiluminescence assay (Nichols Institute, San Juan Capistrano, Calif, USA) in the other 104 men. All data obtained using the Nichols assay were converted to predicted DiaSorin results, as previously described.4 Analysis of variance (ANOVA) was used to test for differences between groups for continuous variables, and the χ2 test was used to test for differences between groups for categorical variables. After adjusting for potential confounding factors, analysis of covariance (ANCOVA) was used to test for differences in 25-OHD levels between the three baldness groups. Statistical analyses were two-tailed and performed using SAS, version 9.1 (SAS Institute, Cary, NC, USA), and P values less than 0.05 were considered statistically significant. The study had 80% power, at the 5% significance level, to detect a between-groups difference in 25-OHD of 13 nmol/L.

Results Initial assessment of baldness showed a high level of agreement between the two researchers (κ = 0.93). In 13 men for whom there was disagreement, degree of baldness was reclassified by consensus. In the final classification, 141 men had no hair loss or mild frontotemporal recession (Group 1), 45 men had predominant vertex loss (Group 2), and 110 men had significant scalp and vertex loss (Group 3). The descriptive and biochemical characteristics of the three groups are shown in the Box. There were significant differences between the groups for age, use of sunscreen on the scalp, and frequency of outdoor hat wearing. After adjusting for potential confounding factors — including age, month of 25-OHD measurement, exercise levels, use of sunscreen on the scalp, skin type and frequency of hat wearing — there were no significant differences in 25-OHD levels between the groups (F2,242 = 0.48; P = 0.60). Analyses that controlled for each of these factors individually produced similar results. Analysis restricted to men in Groups 1 and 3 who did not use sunscreen on their scalp and did not often or almost always wear a hat outdoors revealed that the mean 25-OHD level was higher in Group 3 (91 nmol/L [SD, 35 nmol/L]) than Group 1 (85 nmol/L [SD, 31 nmol/L]), but this difference was not statistically significant (F1,92 = 0.54; P = 0.50). However, the study did not have sufficient power to detect a difference of this magnitude. Finally, when the above analyses were repeated using raw 25-OHD values (ie, before Nichols assay values were converted to predicted DiaSorin results) and controlling for the 25-OHD assay, the results were similar.