Low-carb diets suggest that we need to cut the amount of carbohydrate in the diet, whereas the low-GI diet claims that we need to change the type of carbohydrate. To understand this, we need a little background information.
Very briefly, the theory behind the GI diet is as follows. When a food containing carbohydrate is eaten it is digested in the small intestine and various sugars – mainly glucose - are released from the carbohydrate into the bloodstream. The body responds to the increase in blood glucose by releasing insulin—a hormone that causes the glucose to be stored in the liver, muscle and other body tissues for later use. Glycaemic Index (GI) is a measure of how long it takes for the glucose from a food to be absorbed into the blood. Foods with a low GI value release glucose slowly, thus providing a steady supply of energy and leaving you feeling satisfied longer - so that you're less likely to snack. In contrast, high GI foods produce a rapid - but short-lived - rise in blood sugar. This leaves you lacking in energy and feeling hungry within a short time, so that snacking is much more likely. If this pattern is repeated, you're likely to gain weight.
Do low GI diets result in greater weight loss?
If the theory behind the low-GI diet is true, we might expect that weight loss would be substantially better with low-GI carbohydrates. But is that actually what the scientific evidence tells us?
In a systematic review of 20 studies published in 2002, Raben found that average weight loss in the low- and the high-GI diet groups was identical1. The author concluded that “there is no evidence at present that low-GI foods are superior to high-GI foods in regard to long-term body weight control”. This general conclusion is supported by a number of other well conducted trials.
Ebbeling et al compared low-fat and low-GI diets in obese young adults and found no difference in weight loss2
In another randomized trial involving 45 women, Sloth et al investigated the effects of high- and low-GI diets on weight loss over a 10-week period. No significant difference in weight loss or in appetite was found between the two groups3
Raatz et al conducted a 12-week randomized trial in 29 obese subjects to compare weight loss between 3 diets; high-GI (HGI), low-GI (LGI) or high-fat (HF), all of which delivered approximately 500 kcal less than each subject’s estimated energy needs. After 12-weeks the investigators found that all three groups lost weight and that there was no significant difference between them4. The authors conclude that utilizing low-GI foods does nothing to improve weight loss when compared with other diets.
Two longer term studies have delivered similar verdicts on the low-GI diet. Das et al conducted a 1-year trial in which they compared the effects of a high-GI and a low-GI diet in 34 healthy adults. There found no effect of dietary composition on changes in hunger, satiety, satisfaction or weight loss between the two groups5. Moreover, the authors noted a tendency among those on the low-GI diet to regain weight and body fat which suggests “that reduced energy intake may be somewhat harder to sustain with low GI regimens long term”.
Finally, Sechieri et al carried out an 18-month study of 203 healthy Brazilian women, who were randomly assigned to either a low-GI (LGI) or a high-GI (HGI) diet. The main outcome measure for the study was weight change after 18 months. After 2 months the LGI group had slightly better weight loss than the HGI group, but after 12 months both groups began to regain weight. At 18 months there was no difference in weight loss between the HGI and LGI diet groups6.
Taken together, these findings show that low-GI diets provide no benefit in terms of long-term weight loss when compared with conventional reduced calorie diets.
Limitations of the GI concept
It’s important to remember that the GI of a food taken in isolation is not necessarily the best indicator of healthy food choices. Some soft drinks, milk chocolate and ice creams have low to moderate GIs, whilst wholemeal bread, brown rice and brown spaghetti all have the same GI values as their ‘refined’ white versions.
The GI concept has other limitations. Firstly, the GI of a food can be misleading because certain fruits (such as watermelon) and vegetables (such as carrots) may have high GI values but relatively small amounts of carbohydrates and calories. Secondly, individual responses to carbohydrates vary, so a food that significantly raises one person's blood sugar, may not have a similar affect on anothers. Thirdly, and most importantly, the GI value of a food may vary depending on the company it keeps. For example, if you put peanut butter on a slice of bread, the GI value for both foods will change. This is a problem because – as a rule – we don’t eat a single food and the moment foods are mixed, the GI becomes difficult to determine.
So whilst the general advice behind GI – to avoid highly processed foods and to eat more vegetables, fruit, whole grains, nuts and legumes – is sound, in practice it is much more complicated to apply.
Summary
There is no evidence to suggest that the low-GI diet is any more effective for long-term weight control than more conventional calorie-controlled alternatives. What matters in any successful diet is calorie restriction – and all carbohydrates have the same energy value. Whilst the general nutritional advice behind the low-GI diet is sound, attempting to adhere to low-GI foods in particular may be difficult and unnecessary for successful weight loss.
Scientific References
1. Raben A. Should obese patients be counseled to follow a low-glycaemic index diet? No.
Obes Rev 2002; 3:245-56
2. Ebbeling CB, Leidig MM, Feldman HA. Effects of a low-glycaemic load vs low-fat diet in obese young adults: A randomized trial. JAMA 2007; 297: 2092-2102
3. Sloth B, Krog-Mikkelsen I, Flint A, et al. No difference in body weight decrease between a low-glycemic-index and a high-glycemic-index diet but reduced LDL cholesterol after 10-wk ad libitum intake of the low-glycemic-index diet. Am J Clin Nutr. 2004;80:337-47
4. RaatzSK, Torkelson CJ et al. Reduced glycaemic index load diets do not increase the effects of energy restriction on weight loss and insulin sensitivity in obese men and women.
J Nutr 2005; 135: 2387-91
5. Das SK, Gilhooly CH, Golden JK, et al. Long-term effects of 2 energy-restricted diets differing in glycemic load on dietary adherence, body composition, and metabolism in CALERIE: a 1-y randomized controlled trial. Am J Clin Nutr. 2007; 85:1023-30
6. Sechieri R, MouraAS et al. An 18-month randomized trial of a low-glycaemic index diet and weight change in Brazilian women. Am J Clin Nutr 2007; 86: 707-13
