Complex sentence: McLagan's explanation for the belief that fat is unhealthy comes from Ansel Keys's studies in the 1940s, which linked animal fat to heart disease--studies which have since been challenged by other studies which do not link the two.
Split into simpler sentences: McLagan's explanation for the belief that fat is unhealthy comes from Ansel Keys studies in the 1940s, which linked animal fat to heart disease. The studies have since been challenged by other studies which do not link the two.