Buttermilk is much
lower in fat than regular milk.
(Only half a percent more fat than skim milk),

What's in buttermilk?
You might be surprised to learn there is no butter, per se, in buttermilk, and it is lower in fat than sweet milk. Old-fashioned homemade buttermilk is the slightly sour, residual liquid which remains after butter is churned, ie. milk from the butter or buttermilk. It was usually flecked with tiny spots of sweet, creamy butter that didn't quite make it to the top to be skimmed. The flavor of buttermilk is reminiscent of yogurt and most people prefer it well-chilled. You'll find it is slightly thicker in texture than regular milk but not as heavy as cream. It takes one gallon of milk to yield one-half pint of true buttermilk.

Buttermilk Health
It is also high in potassium, vitamin B12, calcium, and riboflavin as well as a good source of phosphorus. Those with digestive problems are often advised to drink buttermilk rather than milk, as it is more quickly digested. Buttermilk has more lactic acid than skim milk. One cup of buttermilk has 99 calories and 2.2 grams of fat, whereas whole milk has 157 calories and 8.9 grams of fat.

Buttermilk history
In days gone by, nothing went to waste in the standard homestead, and this included the liquid leftover after churning butter. Combined with natural airborne bacteria, this liquid thickened and soured, taking on a pleasingly tangy flavor. The resulting buttermilk made an excellent addition to biscuits, pancakes, and baked goods.

Irish folklore claims a glass of buttermilk will cure a hangover. When heated with a clove of garlic, it was sure to cure any variety of ailments. In American folklore, drinking buttermilk will immunize one against poison oak and ivy. Many pioneer women used buttermilk as a facial wash, believing the flecks of butter brought a smooth and creamy complexion.