There are several camps of opinion when it comes to the virtues of high- or low-glycemic index (GI) carbs and performance. There are seemingly conflicting results from dozens of studies trying to figure out if high or low GI carbs are better for exercise performance. Fortunately, I’m not particularly interested in the performance angle for this article. That being the case I’m going to borrow the results of a recent carb study to shed light on the virtues of high or low GI carbs for fat loss.
The title of the study is “Ingestion of a high glycemic index meal increases muscle glycogen storage at rest but augments its utilization during subsequent exercise.” (1) The study was set up as follows, on two occasions, 14 days apart, seven trained men ran at 71%VO2 max for 30 min on a treadmill. Three hours before exercise, in a randomized order, subjects consumed either a high GI or low GI carb breakfast. The low GI breakfast consisted of 80 g bran flakes, 200 ml skimmed milk, 360 g canned peaches in own juice, 300 g apples and 500 ml unsweetened apple juice. The high GI breakfast consisted of 60 g corn flakes, 240 ml skimmed milk, 100 g white bread, 30 g raspberry jam, 265 ml Lucozade original energy drink, 587 ml water. Both meals provided about 175g carbs and 21 grams of protein. Both meals provided the same number of calories.
The two meals had significantly different effects on substrate utilization during exercise. Once again, I’m not going to explore the performance angle of this study. Instead I want to illustrate the impact that GI can have on fat burning. Here is a graph (1) from this study demonstrating what proportion of glycogen and fat were used during the subsequent treadmill session.
Although there was a trend for less disruption of fat burning during rest, the real effects were seen during the workout. During exercise, total carbohydrate oxidation was 12% lower during the low GI trial with a compensatory increase in fat oxidation compared with the high GI trial. Although the total number of calories burned were similar, the proportion of fat burned was significantly higher. This after eating the same amount of carbs (175g) before exercise!
Too often we get so caught up in avoiding carbs that we forget our real goal is to burn fat. If it were possible to burn just as much fat eating carbs as not, I think the overwhelming majority of us would choose to eat carbs.
Beyond the relief from walking around in a low-carb haze all the time, there are physiological benefits to having more exogenous or dietary carbs coming in. Research has demonstrated that not only is thyroid activity sensitive to overall calorie intake, it is also sensitive to carb intake. (2,3,4,5) It appears that the level of circulating T3 remains higher on fewer calories if more of those calories are from carbs. We’re not talking anything magic here, just a trend in the right metabolic direction.
So, we see that if we diet right we can eat our carbs, and burn our fat too, as long as we stick to the low-GI variety. Here are a few rules of thumb that you can use when choosing carbs to diet with.
- Use high bran cereals instead of low sugar cereals.
- Choose whole fresh fruit instead of processed fruit products
- Choose foods and meals with a low Glycemic Load (GL) (Glycemic Load = GI/100 x amount carbs in food portion or meal)
- Avoid high GI and high GL foods and meals
That’s really about it. You will find by following these rules of thumb you will end up eating whole fresh fruits and vegetables and lean meats and very little if any refined and processed foods. Sound familiar?
1. Wee SL, Williams C, Tsintzas K, Boobis L. Ingestion of a high glycemic index meal increases muscle glycogen storage at rest but augments its utilisation during subsequent exercise. J Appl Physiol. 2005 Apr 14;
2. O’Brian JT, Bybee DE, Burman KD, Osburne RC, Ksiazek MR, Wartofsky L, Georges LP. Thyroid hormone homeostasis in states of relative caloric deprivation. Metabolism. 1980 Aug;29(8):721-7.
3. Serog P, Apfelbaum M, Autissier N, Baigts F, Brigant L, Ktorza A. Effects of slimming and composition of diets on VO2 and thyroid hormones in healthy subjects. Am J Clin Nutr. 1982 Jan;35(1):24-35.
4. Mathieson RA, Walberg JL, Gwazdauskas FC, Hinkle DE, Gregg JM. The effect of varying carbohydrate content of a very-low-caloric diet on resting metabolic rate and thyroid hormones. Metabolism. 1986 May;35(5):394-8.
5. Reinhardt W, Holtermann D, Benker G, Olbricht T, Jaspers C, Reinwein D. Effect of small doses of iodine on thyroid function during caloric restriction in normal subjects. Horm Res. 1993;39(3-4):132-7.
Photo credit: Muscletime