From www.marmite.com:
"Marmite is not only renowned for its errr, distinctive flavour, but it's also a good source of B vitamins, completely vegetarian, is a low fat food and is very low in sugar.
B vitamins play a crucial role in helping your body to function properly and Marmite, along with other foods, is a good source of these important nutrients. B vitamins are water soluble, which means that they are harder for your body to store so you need to get them from your food regularly."