The idea of separating foods into nutrient groups for optimal digestion and, by extension, weight loss, dates to the early 1900s. Dr. William Howard Hay first initiated the theory and wrote several books about it. Some modern diets, such as the Beverly Hills diet, use Hay's premise as a springboard for other weight loss guidelines. Modern science has debunked the theory to some extent, but separating protein and carbohydrates into separate meals might have other benefits beyond weight loss.
Digestion
Protein foods require hours to digest, while carbohydrates pass through your stomach relatively quickly. Theoretically, if you eat both together, the carbs will have to wait around in your stomach for the protein to fully process and break down, so everything can move on to your small intestine together. According to an article in "Women Fitness," the carbs may begin to ferment, resulting in a feeling of indigestion. However, the American Dietetic Association says that digestion doesn't happen this way. Macronutrients do break down at different rates, but they also absorb independently of each other. However, it's possible that if you don't consume both protein and carbs in the same meal, your digestive system will have an easier time processing your food and you'll feel better after eating.
Mood
Your brain uses carbohydrate foods to produce serotonin, the neurotransmitter that gives you a sense of calm and peace. Carbs contain tryptophan, the amino acid that your brain converts to serotonin. Only one amino acid can cross the blood-brain barrier at a time, and your brain will convert whichever one gets there first. In theory, if you combine protein and carbs in the same meal, you add another amino acid to the field, so the tryptophan contained in the carbohydrates might not win the race. However, Middle Tennessee State University indicates that this theory doesn't entirely hold water. Your body reacts to carbs by producing insulin, and insulin inhibits all amino acids except tryptophan. Therefore, even if you consume carbs with protein, the tryptophan will probably get to your brain first. However, there's also no contest if you don't combine the macronutrients. A meal consisting solely of carbs will ultimately produce serotonin, making you feel calmer and happier.
Alertness
Combining carbohydrates and protein in the same meal will affect your concentration and alertness. Protein foods contain tyrosine, which the brain converts to dopamine. Dopamine perks you up, sharpens your mental processes and gives you energy. If you combine protein with carbs, the resulting production of insulin will prevent the tyrosine from getting to your brain first. However, if you consume protein by itself, you'll feel sharper afterward.
Tips
Because tryptophan can also suppress your appetite, you might not feel hungry for a while after an all-carb meal. If you eat less, you will eventually lose weight. "U.S. News & World Report" indicates that Mariah Carey lost 20 lbs. in 2008 by separating her macronutrients into different meals, but in the process, she was also consuming fewer than 1,500 calories a day. There is no scientific basis for the idea that weight loss will result from separating your protein and carbs. However, if you need "brain food" for an upcoming exam or challenge, you can benefit from an all-protein meal.
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