Most people who have kids with MSPI will already know this, but many don't get the difference. MSPI is related to the protein in dairy, beef and soy. Lactose is a sugar. Lactose intolerance arises when a person's body cannot produce lactase, the enzyme required to metabolize lactose. Lactose intolerance presents itself later in life, generally in adulthood. Lactose is found in large quantities in human breast milk. Infants are never (or almost never) lactose intolerant, as this would be fatal.