Should substance addiction be classified as a disability? If so, what threshold(s) should be used to substantiate an “addiction”?

1. Statistically, two in every three Americans are medically obese today. Should obesity be classified as a disability? What are the implications for businesses hiring obese employees under the ADA? Before you answer, though, consider a much more fundamental question: How do we (legally) define “obese”? What metric should be used? Some argue the Body Mass Index (BMI) is the medical gold standard, but consider that BMI is a far-from-perfect tool. What would you suggest as a solution to this apparent problem?
2. Should substance addiction be classified as a disability? If so, what threshold(s) should be used to substantiate an “addiction”? Does it depend on the kind of substance? For example, should addiction to controlled substances such as alcohol or tobacco be viewed differently under the law (for disability purposes) than addiction to illegal substances? What are the implications for for businesses hiring “addicts”?