A common theme in the fitness industry is attempting to classify foods as good or bad and even more commonly this is targeted towards carbohydrates. One of the popular methods for doing this is based on the glycemic index.
The glycemic index (GI) is a ranking of carbohydrates on a scale from 0 to 100 according to the extent to which they raise blood sugar levels after eating. (1)
0 is the lowest and 100 is the highest (the same as pure glucose). 55 or less is considered low, 56 to 69 is considered medium and greater than 69 is considered high.
This article will explain why relying on GI to classify foods as good or bad is not as simple as it seems.
The GI of a food is determined in an overnight fasted stated using a single food. This is fine for testing but the real world applicability of this is minimal. The digestion and absorption of previous meals as well as the fact that it is rare to have a whole meal with a single carbohydrate source and no other food mean that relying on the GI number is inaccurate. (Fat, fibre and certain proteins will affect the release of glycemic response)
Allen et al found that the cooking methods of sweet potato can change the classification from low to medium GI. (2) Dehydrated and raw being considered low while microwaving, baking and steaming to be considered medium. I’d have to say that if you are considering eating raw sweet potatoes to keep the GI low, then there may be some other psychological issues to sort out 🙂 As well as this, studies have shown that the ripeness of fruit will affect it’s GI rating. The starch generally has a higher GI than does the sugar, so as ripeness progresses, the GI decreases, such as was reported for bananas (3).
Lower GI foods have been associated with high levels of satiety. The issue is that the majority of the studies that have come to this conclusion have been short term and single meal experimental designs. As previously mentioned, this is not applicable to most real life situations. Kiens & Richter (4) completed a study that lasted 30days and came to the conclusion that “when ingesting a diet with an energy composition common in Western countries, switching the carbohydrates from high to low GI sources decreases insulin action on whole-body glucose disposal at a high but not at a physiologic plasma insulin concentration”. Basically, a lower resistance to insulin was seen in the high-GI group at the end of the trial. There has been no causal relationship found to show that lower GI = higher satiety. Holt et al found that boiled potatoes had the highest satiety index of all the foods tested despite being classed as medium GI. (5)
It is common for people who are worried about their insulin response to choose a lower GI food. Unfortunately, the glycemic index of a food cannot be directly correlated with it’s insulin response. Some examples of this are milk and yoghurt that have a very low GI of 15-30 yet have a high insulin index of 90-98. Another example is baked beans which are low GI but very high insulin index of 120. The opposite can be seen with beef, fish and cheese that have insulin index similar to many carbohydrate foods. (They are not listed on the GI tables as they are only for carbohydrates)
Raben (6) conducted a systematic review on published human intervention studies. Follow is the abstract with important points in bold.
In diabetes research the glycaemic index (GI) of carbohydrates has long been recognized and a low GI is recommended. The same is now often the case in lipid research. Recently, a new debate has arisen around whether a low-GI diet should also be advocated for appetite- and long-term body weight control. A systematic review was performed of published human intervention studies comparing the effects of high- and low-GI foods or diets on appetite, food intake, energy expenditure and body weight. In a total of 31 short-term studies (< 1 d), low-GI foods were associated with greater satiety or reduced hunger in 15 studies, whereas reduced satiety or no differences were seen in 16 other studies. Low-GI foods reduced ad libitum food intake in seven studies, but not in eight other studies. In 20 longer-term studies (< 6 months), a weight loss on a low-GI diet was seen in four and on a high-GI diet in two, with no difference recorded in 14. The average weight loss was 1.5 kg on a low-GI diet and 1.6 kg on a high-GI diet. To conclude, there is no evidence at present that low-GI foods are superior to high-GI foods in regard to long-term body weight control.
Miyoshi et al. (7) produced a study that tested the nutrient absorption of brown rice (low GI) versus white rice (high GI) in men on a lower protein diet. Abstract is below with important points in bold.
The effect of brown rice with low protein intake was studied in five healthy young men. Feces were weighed, the digestibility of nutrients was determined, and blood tests were made. Each subject followed a diet consisting mainly of polished rice for 14 days and one consisting mainly of brown rice for 8 days. Both diets contained 0.5 g protein per kg of body weight. The brown rice diet had 3 times as much dietary fiber as the polished rice diet. On the brown rice diet, fecal weight increased, and apparent digestibility of energy, protein, and fat decreased, as did the absorption rates of Na, K, and P. The nitrogen balance was negative on both diets, but more negative on the brown rice diet. The phosphorus balance on the brown rice diet was significantly negative, but other minerals were not affected by the diet. The levels of cholesterol and minerals in the plasma were not significantly different on the polished rice diet and the brown rice diet. Comparing these results with data on standard protein intake (Miyoshi, H. et al (1986) J. Nutr. Sci. Vitaminol., 32, 581-589.), we concluded that brown rice reduced protein digestibility and nitrogen balance.
This study goes to show that context matters when examining different foods. Looking at the diet as a whole to see if it is the correct energy balance, provides adequate macro nutrients, micro nutrients and fibre is more beneficial than looking at single food items.
The science surrounding GI is a maze and there are minimal causal relationships established. Because of this, I would not be recommending any healthy individuals to base their carbohydrate choices on the GI of a specific food. I would prefer to choose foods based on their micronutrient density for the most part and level of processing. As usual, I like the 80/20 rule where you focus primarily on whole food sources and leave a smaller portion for processed foods.
Majoring in the minors will just lead to excessive cortisol anyway 🙂
4. Kiens B, Richter EA. Types of carbohydrate in an ordinary diet affect insulin action and muscle substrates in humans. Am J Clin Nutr 1996;63:47-53.
5. Holt SH, Miller JC. A satiety index of common foods. Eur J Clin Nutr 1995 Sep;49(9):675-90
6. Raben A. Should obese patients be counselled to follow a low-glycaemic index diet? No. Obes Rev. 2002 Nov;3(4):245-56.
7. Miyoshi H, Okuda T, Okuda K, Koishi H. J Nutr Sci Vitaminol Tokyo. 1987 Jun;33 (3):207-18