IRS Publication 502, Medical and Dental Expenses, explains what you can and cannot deduct. You can get the publication at the IRS web site ? IRS.gov. Just beware that most IRS publications tend to be written in a dry format and the explanations aren't always user friendly. So, here's a synopsis of the rules, in layman's terms.

The IRS allows you to deduct the cost of joining and maintaining membership in a diet program, but, except under specific circumstances, not the cost of the food. In order to qualify for the deduction ? and this a very important detail ? a doctor must diagnose you as having a medical condition that requires weight loss. Simply carrying a few extra pounds is not enough; you must have a medical diagnosis such as obesity, high blood pressure or heart disease. If the doctor instructs you to lose weight for medical reasons, make sure you get a written copy with a diagnosis of the disease or ailment that requires the need for weight loss. Records are your best defense against an audit.

The IRS is very specific about what you can and cannot deduct and the rules a bit tricky, so pay attention. The costs for joining a weight loss program, such as Weight Watchers, are deductible. If you join a gym and the membership includes nutritional counseling, you cannot deduct the membership. However, the gym offers a separate weight loss program that includes a separate charge from the cost of the gym membership, you can only delete the cost of the weight loss program. For example, if the gym membership costs $25 a month and you pay an additional $15 a month to belong in the gym's weight loss program, you can only deduct $15 a month for the weight loss program ? not the $25 for the gym membership. If there is a combination fee and you cannot separate the cost of the gym membership from the weight loss program, you lose the deduction.

If the program requires you to eat special food, in general you cannot deduct the cost of the food. The rationale is that you have to eat anyway and you are simply switching out one meal choice for another meal choice. There is an exception, however. The IRS will allow you to deduct the cost of the food if the diet food does not satisfy normal nutritional needs, the food alleviates or treats an illness and the requirement for the food is substantiated by a doctor.

The cost of exercise equipment and clothing is not deductible. Nor are materials related to the diet such as journals or software. And you cannot deduct the cost of a diet book.

Tax laws are tricky, so it's prudent to consult a tax professional. And remember, a healthy lifestyle will help to lower the medical costs you incur over the course of your life, so even if you can't deduct that new treadmill, you still will save money by getting on it every day.