His secret is CLEAR to all of us.
I met Mike a few times back in the early 80's and when a bunch of us were skeptical about him being "natural" he passed around his daily diet log, the guy is a real nutriton expert he even kept track of the vitamins and mineral content along with pro, carbs and fat of all food he ate.
The guy was NOT even 200 lbs on stage but ripped with awesome muscle bellys.
He won a natural show where he had to pass a BLOOD analysis for drugs,THAT, he passed.
He passed more drug tests than you can imagine, plus he was real matter of fact about it, not defensive.
If you remember BALCO and Victor Conte got their start doing DIET and supplemnt routines for elite athletes and was 100 % legit, UNTIL they branched out into the vitamin S deal

At the end of his career hard to say if he was 100% natural, no matter, the guy was a phenom and remeber he passed the drug test and was given the title over Shawn Ray at ASC in 91. (was it 91?)
Larry Scott summed up the real deal on 'roids pretty well . I will paraphrase him here; ' drugs can make a really good bodybuilder become a great one but never transform an avg one into a champ.'