The glycemic index, or G.I., is a method of predicting how soon the sugar in food will enter the bloodstream. Glucose, the only type of sugar that circulates in the blood, is assigned a value of 100. The higher the glycemic-index number, the faster the food enters the blood.
The system was initially designed for diabetics, since the rapidity of carbohydrate entrance into the blood is critical for insulin use or any other medication designed to control elevated blood glucose. Eventually, the G.I. was used to determine the different metabolic effects of various carb foods.
The idea is that foods with higher G.I. numbers enter the blood faster and, by doing so, provoke a greater release of insulin. In someone who’s not diabetic, excess insulin can be a problem because it tends to promote and perpetuate bodyfat stores. Insulin blunts the body’s fat-mobilization and oxidation processes.
Expert consensus is that carbohydrate foods promote the greatest release of insulin, particularly so-called high-glycemic-index foods, or simple sugars. Protein is supposed to incur a minimal insulin release, since it doesn’t have much effect in increasing blood glucose levels. That’s the cornerstone of most high-protein, low-carb diets: the idea that by limiting carb intake you can control insulin and burn fat more efficiently. The high-protein intake inhibits appetite and conserves lean body mass—mainly muscle—that might be catabolized during low-calorie or low-carb diets.
Many energy bars and drinks have a high-protein, low-carb formula. The idea is that carbs promote an insulin release that rapidly lowers blood glucose, and prior to training that may result in hypoglycemia, or low blood sugar, or lead to premature exhaustion of limited muscle glycogen reserves, which are needed to power bodybuilding workouts.
A recent study that compared the glycemic effects of sugars and proteins came up with some paradoxical findings.1 Twelve healthy men, none of whom lifted weights or did any other type of exercise, got a 50-gram glucose drink, a white-flour bagel (high-G.I. food), peanuts (low-G.I. food), a protein bar containing 29 grams of protein and three grams of carbs or a protein drink containing 30 grams of protein and eight grams of carbs. The men then lay down, and the scientists drew their blood every 10 minutes for two hours to measure their glycemic responses to the foods.
The glucose drink, the protein bar and the protein drink elevated plasma insulin levels above baseline, or resting, levels starting at 10 minutes and lasting through 60 minutes. The bagel, a high-G.I. food, elevated insulin levels above baseline from 30 to 75 minutes. No change in insulin levels occurred in those who ate the peanuts. All the participants’ insulin levels returned to baseline by the 90-minute mark.
When insulin levels rise at the start of exercise, there’s a huge increase in muscle glucose uptake, a result of the interaction between insulin and the increase in muscle glucose transporter, or GLUT-4, that’s caused by the muscle contraction. The net effect is a drop in blood glucose levels so dramatic in some people that they nearly become hypoglycemic when they start exercise. For many the start of exercise releases catecholamines, such as epinephrine, which lower blood glucose levels and break down the liver glycogen that releases glucose into the blood. In low-carb dieters, liver glycogen may already be low. Result: premature fatigue during training.
The study highlights a few overlooked facts. The high-protein, low-carb diets that are supposed to work by limiting insulin release don’t work as advertised. That doesn’t mean low-carb plans don’t work, just that their insulin effect is overplayed. Certain amino acids are nearly as potent as sugar in promoting insulin release. That makes sense, since insulin is known to promote the entry of amino acids into muscle, an anabolic effect. In fact, insulin is only anabolic in the presence of a high blood amino acid level. The difference between carbs and protein is that carbs, especially the simple, or high-G.I., carbs, will promote an increase in blood glucose, and a protein meal doesn’t. Simple carbs and protein, however, have similar effects on insulin.
In practical terms, that means drinking a high-protein, low-carb shake prior to training may not be a good idea for many people. As the study shows, the high protein content will promote a considerable insulin release, which may cause low blood glucose at the start of exercise—not good for high-energy training.
Is there a solution? Eat protein with carbs, particularly low-G.I. carbs. Adding some form of soluble fiber, such as psyllium or guargum, to the drink, will slow the entry of nutrients into the blood. Of course, you may also increase your production of intestinal gas, which can be either an advantage or a disadvantage, depending on how crowded the gym is that day. The most practical option is simply to add an essential fat source, such as flaxseed oil, to the drink. That, too, will considerably lower the G.I. and insulin-releasing effect while providing some important essential fats.
One note here: Don’t add fish oil. By far the best source of the important omega-3 fatty acids, fish oil also blocks the synthesis of an important substance called prostaglandin F2A, which has anabolic effects in muscle. Aspirin and other over-the-counter pain relievers do the same thing and can inhibit muscle gains.
1 Parcell, A.C., et al. (2004). Glycemic and insulinemic responses to protein supplements. J Am Diet Assoc. 104:1800-1804.
©,2013 Jerry Brainum. Any reprinting in any type of media, including electronic and foreign is expressly prohibited.
