The theory behind the fit for life diet comes from food combining, as our diet plan review explains. Advocates of food combining diets claim that certain foods, when mixed, become very hard to digest and turn to fat in the intestinal system. The key to the fit for life diet is to understand that different metabolisms require different diet meal plans for optimal benefit. Through understanding your metabolism the fit for life diet offers extraordinary results without the need to feel hungry ever again. The program requires each individual to develop their own personalised diet meal plans and recipes in order to get the very best from their diet. It is recommended that the pursuer of the diet purchase their unique eating programme from the founder's website.

The fit for life diet recommends eating fruit from the time you wake up until noon as it purifies the system. Dairy and meat products are strictly limited and diet meal plans and recipes are replete with healthy fruits and vegetables. Although there is some debate about the "scientific" basis for the programme, followers claim massive improvements in energy and weight loss. The programme was very popular in the 1980s and has become the inspiration for many weight loss programmes and eating plans. Critics of food combining claim that it has no basis in scientific fact, and that eating different foods at different times of the day places stressful restrictions on the lives of its followers. Nevertheless, there are many benefits to following eating programmes that are high in fruit and vegetables.