I’m sure we’ve all ran into these terms at some point and possibly seek out low-GI foods at the supermarket without really knowing why. I mean, what possible reason could I have for buying 2% yogurt over fat-free yogurt? Turns out, GI is key.
GI (refers to glycemic index here, not gastrointestinal) is a quantitative measure of how much one’s blood-sugar is elevated by consumption of food relative to consumption of pure glucose. Eating pure glucose is assigned a value of 100. The GI is mathematically the ratio of the integration of blood sugar with respect to time after eating the food of interest compared to the integration curve of blood sugar and time after consuming glucose. Reasonably it approximates how quickly or to what degree certain foods elevate blood sugar. Notably though, the GI calculation doesn’t account for the actual volume of food consumed. The calculated glycemic load gives the number of equivalents of glucose based on blood sugar response to consumption of food. For example, consuming one unit of glycemic load corresponds to one gram of glucose.
So why does GI matter? A number of studies have suggested benefits of low-GI diets. A 2012 publication found that low-GI diets significantly increase energy expenditure (ie calories burned). The study concluded:
Among overweight and obese young adults compared with pre–weight-loss energy expenditure, isocaloric feeding following 10% to 15% weight loss resulted in decreases in REE and TEE that were greatest with the low-fat diet, intermediate with the low–glycemic index diet, and least with the very low-carbohydrate diet.
It is hypothesized that low-GI foods do not as effectively activate fat storage via Lipoprotein mediated mechanisms. Thus consuming low-GI foods may encourage direct glucose metabolism by somatic cells rather than store glucose as fat for later use. There is even some evidence that eating foods with high-GI values can disrupt normal cell structure in the Islets of Langerhans, the pancreatic cells that produce insulin, glucagon (approximately antagonistic effect to insulin), and somatostatin (slow digestion for more efficient nutrient absorption). It is worth noting that destruction of beta cells in the Islets of Langerhans by autoimmune response characterizes Type I diabetes.
The takeaway? There is mounting evidence that low-GI diets contribute to weight loss and increased calorie expenditure when compared to high-GI foods. Though dietary mechanisms are notoriously difficult to pinpoint specifically, the fact that a number of high-GI foods such as white bread and processed carbs are already thought to be poor to health is testament to the merits of low-GI dieting.