Vitamin D is an important
micronutrient involved in several processes. Evidence has shown a strong association between hypovitaminosis D and cardio-
metabolic diseases, including
obesity. A
ketogenic diet has proven to be very effective for
weight loss, especially in reducing fat mass while preserving fat-free mass. The aim of this study was to investigate the effect of a
ketogenic diet-induced
weight loss on
vitamin D status in a population of obese adults. We enrolled 56 obese outpatients, prescribed with either traditional standard hypocaloric
Mediterranean diet (SHMD) or very low-calorie
ketogenic diet (VLCKD). Serum 25(
OH)D concentrations were measured by chemiluminescence. The mean value of serum
25-hydroxyvitamin D (25(
OH)D) concentrations in the whole population at baseline was 17.8 ± 5.6 ng/mL, without differences between groups. After 12 months of dietetic treatment, in VLCKD patients serum 25(
OH)D concentrations increased from 18.4 ± 5.9 to 29.3 ± 6.8 ng/mL (p < 0.0001), vs 17.5 ± 6.1 to 21.3 ± 7.6 ng/mL (p = 0.067) in the SHMD group (for each kilogram of
weight loss, 25(
OH)D concentration increased 0.39 and 0.13 ng/mL in the VLCKD and in the SHMD groups, respectively). In the VLCKD group, the increase in serum 25(
OH)D concentrations was strongly associated with body mass index, waist circumference, and fatty mass variation. In a multiple regression analysis, fatty mass was the strongest independent predictor of serum 25(
OH)D concentration, explaining 15.6%, 3.3%, and 9.4% of its variation in the whole population, in SHMD, and VLCKD groups, respectively. We also observed a greater reduction of
inflammation (evaluated by
high-sensitivity C reactive protein (
hsCRP) values) and a greater improvement in
glucose homeostasis, confirmed by a reduction of HOMA values, in the VLCKD versus the SHMD group. Taken together, all these data suggest that a dietetic regimen, which implies a great reduction of fat mass, can improve
vitamin D status in the obese.