The weakest of Marshall Mandell's guides relating food allergies to health problems (Dr. Mandell's Five-Day Allergy System, Dr. Mandell's Lifetime Arthritis Relief System)--but harmless as a diet. The unproven premise, in this case, is that certain persons react allergically to particular foods (most commonly, wheat and eggs) in ways that cause them to gain weight directly (by causing water retention, for example), or indirectly (by causing behaviors, such as anxiety reactions, that lead to compulsive eating). And not only may the sufferer not recognize the allergy, he or she may actually crave the implicated food (witness many people's need for coffee). To eliminate the problem, the Mandells recommend a rotation diet which removes common allergy-causing foods; after several days of feeling worse (because of a kind of withdrawal), sufferers will feel much better--and lose weight. They can then pinpoint the problem-causers (since rotation diets gradually add foods) and avoid them permanently. The diet itself is OK, if spartan (for breakfast, papaya with blueberry sauce; for lunch, tuna-carrot-tomato casserole); in any case, it's not meant to be long-term. Anyone who has real reason to suspect a food allergy and who can apply the time and effort to work out this complicated regimen should heed the Mandells' caveats (be sure your physician knows what you're doing, for one) and be aware that it's unsupported, as yet, in recognized scientific terms.