Food combining became well-known in the early 20th-century lead by Herbert Shelton (its founder) and Sylvester Graham (for whom the graham cracker is named). More recently, it has resurfaced in the best-selling book Fit for Life. The theory is that the full digestion of nutrients is enabled by "food combining", which will aid in the prevention of certain chronic metabolic diseases. The Hay diet is one type of food combining diet.
The basic principle of food combining is that the enzymes needed for food digestion function best when foods are eaten in the proper combination. Specifically, fruits should be eaten alone, fruits and vegetables should not be eaten together, nor should protein and starches. Many people attest to high energy levels and improved health by following food combining principles. Others dispute the theory that proteins and starches cannot be digested at the same time, citing foods like legumes and whole grains, each of which contains appreciable amounts of both protein and starch within an individual food and is diet staples around the world.