A myth that never dies
As if people didn't have enough worries, there are always books on the market purveying notions about food combining. Some say that it's vital to eat foods in the right combinations - never combining, for instance, carbohydrates and protein at the same meal. They usually also recommend that fruits always be eaten raw and alone, because otherwise they will ferment and turn toxic in the stomach.
There's no evidence to support such contentions, according to Dr. Sheldon Margen, Professor Emeritus of Public Health Nutrition. Nearly all foods are themselves combinations. If you eat beans, for example, you're getting carbohydrates (sugars and starches), protein and fibre, among other things. Bread combines protein, carbohydrates, a little fat and many other things. A simple dish like macaroni and cheese, a peanut butter sandwich, or oatmeal with milk contains sugars, starches, protein and fat. Our digestive system handles food combinations very efficiently. The process begins in the mouth as we chew food and saliva acts upon it, beginning the breakdown of starches into sugars. Other enzymes come into play along the line, resulting in almost complete digestion and absorption of nutrients, no matter how they are combined.
As for that supposedly fermenting fruit, anyone who has studied human physiology can tell you that fermentation does not occur in the stomach. Fruit is nutritious, raw or cooked, and is readily digested in combination with other foods, including vegetables, grains and dairy products. Fruit is not a hard-and-fast category anyway: many things we call vegetables, such as tomatoes, are really fruits.
Nearly all foods are themselves combinations
The overwhelming weight of evidence is on the side of a varied, balanced diet with foods eaten in nutritious, appetizing combinations. Most vitamins and minerals are best utilized when consumed as part of a complex mixture of foods. For instance, foods high in vitamin C (such as fruits) boost the body's absorption of nonheme iron from grains. That's one reason fruit and whole grains make such a good breakfast combination.
Variety aids digestion, rather than making it more difficult.
University of California at Berkeley Wellness Letter, April 1996
[Editor: Of course, if certain combinations of foods disagree with you, then eat them separately.]