Vitamin D replacement improves muscle efficiency New research shows for the first time a link between vitamin D levels and muscle efficiency. Vitamin D supplementation may also be effective in improving skeletal muscle function. This study is presented today at the Society for Endocrinology annual conference in Harrogate, UK. The findings may explain the physical fatigue commonly experienced by patients with vitamin D deficiency, with broad implications for a large section of society. Vitamin D is a hormone produced in the skin using energy from sunlight, and to a lesser extent derived from dietary sources. Vitamin D deficiency is a significant public health problem as diagnosed cases are on the rise and the hormone is essential for good bone health. Alongside poor bone health, muscle fatigue is a common symptom in vitamin D deficient patients. This fatigue could be due to a problem in the mitochondria: the 'power stations' within each cell of the body. Mitochondria u...