Vitamin D deficiencies in dogs can cause health problems over time, but so can an oversupply. Because vitamin D is fat soluble, it accumulates in body fat. Overdoses can be toxic and even fatal, but some vitamin D is necessary to a dog's health. Vitamin D will aid in the absorption of calcium and phosphate, increases bone cell activity, influences the formation and growth of long bones, and speeds the healing of fractures. Adequate D levels may help prevent heart disease, joint inflammation, skin and coat problems, cancer, vision problems, depression, mental illness, infections, inflammatory bowel disease, dental problems, hyperparathyroidism, and kidney disease.