<B>A Carbohydrate Is Not A Carbohydrate</B>
In this section, I'd like to demonstrate that not all carbohydrates were created equal. Specifically, I'll briefly discuss:
1. The insulin index vs. the glycemic index
2. The superiority of low-GI and II diets
3. The difference between liquid carbohydrates
While older carbohydrate classification schemes were centered on the notion of simple vs. complex carbohydrates (a structural classification), newer schemes focus more appropriately on the absorption profiles (glycemic index) and physiological effects (insulin index) of these carbohydrates (a functional classification).
The Glycemic Index (GI) is a classification scheme based on the blood glucose rise after consuming a carbohydrate food. This measure is based on the absorption profile of the food and was originally considered an indirect, but adequate measure of the insulin response to food. The assumption was that the insulin rise would be proportional to the glucose rise. However, recent research has demonstrated a dissociation of the glycemic response and the insulin response to the food. Therefore the insulin index was created.
The Insulin Index (II) is an index of the magnitude of insulin secretion as a result of food ingestion. Of course, this is the direct measure that the glycemic index could only approximate. Since insulin is a tricky hormone to manage, it's best to know exactly what's happening with this guy, especially if you have poor insulin sensitivity or poor carbohydrate tolerance.
Studies by Holt et al (1996) and Ostman et al (2001) highlighted some of these differences between glycemia and insulinemia. Interestingly, while the glycemic and insulin indices of many foods were similar, some foods caused unpredicted responses. As shown in the following graph, foods like yogurt and milk had relatively low-glycemic indices, but very high insulin indices. White and brown rice, on the other hand, had high-glycemic indices, but low insulin indices. The point here is that if you want to effectively manage body composition, you should choose your carbohydrates based on both the glycemic and insulin indices.
Unfortunately, there are only limited insulin data out there, leading us to continue to rely in some cases only on the glycemic index.
More complete glycemic and insulin indices can be easily located by doing an Internet search on these two terms.
So the next appropriate question would be, "What does the literature say about low GI and II diets vs. higher GI diets?" Well, here's a summary:
Ludwig et al (2000) described the following list of benefits for eating a low GI diet:
- Better nutrition (better micronutrient profile and more fiber)
- Increased satiety
- Decreased hunger
- Lower subsequent energy intake (second meal effect)
- Fat loss
- Better fasted insulin and glucose
In a study by Agus et al (2000), it was demonstrated that during a short, 6 day, low-calorie diet, a low-GI carb intake preserved metabolism and enhanced fat loss vs. a high-GI diet. The low GI group saw a 5% decline in metabolic rate and a 7.7lb weight loss while the high-GI group saw an 11% decline in metabolic rate and a 6.6lb weight loss. In these subjects, fasted glucose and insulin values were lower in the low-GI group, indicating better glucose and insulin sensitivity.
Spieth et al (2000) and Ludwig et al (2000) showed that 4 months of low-GI eating was superior to 4 months of high-GI eating in overweight teens. The low-GI group lost 1.5 points on the BMI scale and 2.2 lbs while the high-GI group gained 2.88lbs and increased their BMI. In addition, these studies showed that a low GI meal reduced food intake during subsequent meals while the high GI meal lead to overeating.
Finally, Pawlak et al (2001) showed that in rats, a low-GI diet led to decreased fasting insulin and glucose values, decreased fat mass, and decreased insulin and glucose values during a glucose tolerance test. Therefore, body comp as well as glucose and insulin sensitivity improved.
The bottom line here is that when all else is equal, a diet containing mostly low-GI carbohydrates is superior to a high-GI diet for losing fat, preserving metabolic rate, and maintaining healthy insulin sensitivity and glucose tolerance.
Next, I'd like to illustrate the differences between popular liquid carbohydrates including maltodextrin, dextrose, fructose, and sucrose.
Maltodextrin is a glucose polymer (a string of glucose units put together, similar to the protein peptide). It is therefore, by definition, a complex carbohydrate. However it's more complex nature does NOT slow digestion. Therefore, the GI and II remain high. Maltodextrin is the absolute best carbohydrate to consume during exercise for rapidly delivering blood glucose and for muscle glycogen recovery. It's also best for fluid uptake.
Dextrose (glucose) is a simple carbohydrate unit (similar to the amino acid). While it's good for exercise situations (malto is better), you're probably better off adding some dextrose to your maltodextrin formula. A little bit of dextrose may enhance the already excellent fluid uptake that occurs with maltodextrin during exercise.
Fructose is a simple carbohydrate unit, but it's structurally different from glucose. Due to its structure, it can possibly cause GI problems and/or decrease fluid uptake with exercise. Fructose, unlike other simple carbs, has to be "treated" in the liver and it reaches the muscle slowly.
Finally, sucrose consists of glucose and fructose units bonded together. Therefore, upon digestion, you get glucose and fructose in the GI (and the benefits and consequences of each).
Based on the three studies I reviewed (Blom et al 1987, ven Den Burgh et al 1996, Piehl et al 2000), it appears that dextrose is 72% faster than fructose for muscle glycogen resynthesis . As a result, at the end of 8 hours, muscle glycogen was 30% higher with dextrose ingestion. However, in another study, at the end of 4 hours, muscle glycogen was 15% higher with maltodextrin ingestion vs. dextrose. So dextrose kicks fructose's butt although malto beats up on dextrose.