Candy died from a double whammy. He was obese for almost all of his adult life and his father also died young from a heart attack. So he had both the lifestyle and genetic factors stacked against him.

Hollywood actors get top of the line (and very well paid) personal trainers/chefs/nutritionists to lose the weight quickly.

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I don't understand why they don't just use actors/actresses who better fit the size requirements, unless a person's size/weight has to change in the movie like Tom Hank's "Castaway" and "Philadelphia". As RK is fond of saying (usually regarding women but also applies to men), we come in all different shapes and sizes. I take that to mean the differences in frame/skeletal, fat/muscle amount and distribution (assuming everyone having a reasonably healthy lifestyle). There are natural outliers at both extremes of normal, so why not use them for these roles?