Vitamin D is synthesized in the body in a reaction involving sunlight. Vitamin D can also be consumed in the diet by eating fatty fish, mushrooms and supplements. A meta-analysis of 23 studies showed that vitamin D deficiency was 35 percent greater in obese than normal weight people. The relationship was independent of age or geography. Several recent studies have linked low vitamin D levels to poor bone health, muscle weakness, deficiencies in reproductive hormones, low aerobic capacity and increased body mass index (the proportion of weight to height). A study led by Caitlin Mason from the Fred Hutchinson Cancer Research Center in Seattle found that supplementing the diet with 2,000 international units of vitamin D per day had no effects on weight loss during a 12-month study.
Source: Obesity Reviews, 16: 341-349, 2015; American Journal of Clinical Nutrition 99: 1015-1025, 2014