Weight loss programs or drugs are eligible for reimbursement with flexible spending accounts (FSA), health savings accounts (HSA), and health reimbursement accounts (HRA) with a Letter of Medical Necessity.
While medical treatments for weight loss are deductible if there is a medical reason, special food is not deductible. More specifically, special food is only deductible if it does not meet normal nutritional needs, and is medically necessary. And in that case, you can only deduct the extra cost of the special food over normal food (assuming you can prove what that is). You could deduct membership fees and counseling, but you will have a very difficult time deducting the cost of special food.
The IRS says:
You can't include the cost of diet food or beverages in medical expenses because the diet food and beverages substitute for what is normally consumed to satisfy nutritional needs. You can include the cost of special food in medical expenses only if:
The food doesn't satisfy normal nutritional needs,
The food alleviates or treats an illness, and
The need for the food is substantiated by a physician.
The amount you can include in medical expenses is limited to the amount by which the cost of the special food exceeds the cost of a normal diet.