Is a diet recommended by a physician deductible medical expense?

[MaryK4]

It depends- the IRS rules: You can include in medical expenses amounts you pay to lose weight if it is a treatment for a specific disease diagnosed by a physician (such as obesity, hypertension, or heart disease). This includes fees you pay for membership in a weight reduction group as well as fees for attendance at periodic meetings. You can't include membership dues in a gym, health club, or spa as medical expenses, but you can include separate fees charged there for weight loss activities.

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:

1. The food doesn't satisfy normal nutritional needs,

2. The food alleviates or treats an illness, and

3. 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.

(Weight-Loss Program)

[Opus 17]

That's tricky.  The diet must be prescribed to treat a specific disease, not merely recommended for general health. And you can only deduct the excess cost compared to "normal" food.  If you can't prove an excess cost, then it is not deductible, even if prescribed.