Buttermilk is high in lactic acid, which makes for a great natural exfoliant. Lactic acid can brighten your skin, help to even out skin texture and can help fade brown spots. Lactic acid helps to cleanse and nourish the skin.
The moisturizing power of buttermilk is so strong, it can help alleviate the discomfort of sunburns. Buttermilk's astringent properties can also firm and tighten skin.
The presence of milk protein and lactic acid are very good for those who have dry or acne prone skin. These compounds can exfoliate and rejuvenate skin.
Many vitamins are only soluble or bioavailable with a fat attachment. Which makes some nutrients completely useless without sufficient fats. Fats are what protect, and nourish the skin. Buttermilk is higher in lactic acid than any other milk.
Milk used in soap or other topical products has no direct relation to lactose-intolerance.
However, if a milk allergy or general milk intolerance has been diagnosed by licensed health practitioners, these people should NOT use any milk products anywhere near their bodies.