The best source of Vitamin-D is exposure to sunshine. It is best known for promoting bone health but it is also very important for activating and deactivating enzymes in the brain that are involved in the development of nerves and neurotransmitters. The Vitamin-D hormone will also uplift a bad mood and increases problem solving ability. It is synthesized when sunlight touches the skin so if you spend most o your day in doors then a supplement is definitely in order.
