I’m not sure exactly when “Low GI” really caught on in the world of fad diets. The theory itself was developed 30 years ago, and has been the basis of many very popular diets such as Atkins, South Beach, and the Zone. But is a high GI rating really a high dietary risk?
The glycemic index (GI) is a measure of how quickly and significantly a food can raise blood sugar. The index dates back to 1981, when it was created as a proposed management tool for diabetics. Though the relationship between GI levels and health problems has been widely researched, there is still much debate in the academic world as to how closely linked a high GI diet is to obesity and lifestyle diseases.
Though the scientific evidence is inconclusive, we do know a few things for sure. High GI foods raise blood sugar quickly, leading to a large spike in insulin. This in turn prompts glucose uptake by the muscle, liver, and adipose tissue (body fat deposits), and decreases levels of the hormone glucagon. Increased insulin leads to increased glycogen synthesis and lipogenesis (fat storage), and decreased glucagon concentrations slow lipolysis (fat breakdown). Conversely, low GI foods result in a lower, more sustained glucose and insulin response, leading to less fat storage and greater use of fat as an energy source, among other things. If this is the case, shouldn’t we leave the high GI foods alone completely?
Unfortunately, it’s never so straightforward. There are several factors that affect your blood glucose and insulin response other than the GI rating of a food, some of which can render the measurement absolutely useless.
One of the biggest problems with the GI is that it rates foods based on their individual glucose response (for reference, low GI foods have a rating less than 55, medium GI foods rate between 55 and 70, and high GI foods are rated above 70). This would be all well and good if we ate meals that consisted of one food only, but personally I’m not a big fan of a plate full of only carrots. I would imagine I’m not alone in enjoying some variety in each meal, and so we run into a problem – how do you rate the GI of a combination of foods? Theoretically, this can be achieved by averaging the GI ratings of each food in the meal: high GI foods would be balanced out by any fats, proteins, and fibre in the meal, as these take longer to digest and slow the whole digestive and absorptive process.
However, as often happens, what sounds good in theory doesn’t work so well in practice. Even if you had a plate full of only carrots, you could still get different GI ratings from plate to plate. Cooking method influences the GI rating: carrots sautéed in olive oil would have a lower GI than steamed carrots, since the added fats would increase digestion time. The time of day of the meal, and the time relative to your last exercise session also affect the rating. In fact, immediately post-exercise is a great time to have high GI foods. Your recently-depleted muscle will snap up the excess glucose and put it to work, enhancing your recovery rather than allowing said glucose to be stored as fat.
GI ratings are also based on a standard amount of carbohydrate in a food, rather than a standard amount of food itself. Specifically, the rating is based on the blood sugar response to 50g of carbohydrate in a given food, whether that be in an 85g milk chocolate bar, or 500g of raw carrots (the actual amounts needed to get to 50g carbohydrate). Enter the glycemic load index. The GL was designed to provide a more accurate measure of blood sugar response; it’s the product of a food’s GI and the amount of that food you eat. Then we go back to the mixed food content of a meal, and prep methods, etc – you can see this is not a easy fix.
Lastly, the GI has no real connection with nutritional quality. Some naturally high GI foods are highly nutritious (watermelon is rated 72) and some low GI foods are not so great nutritionally: a Snickers bar has a GI rating of 40. I know which one I would be suggesting to my clients!
The bottom line is that the GI tends to classify foods and “good” or “bad” based on a scale that you don’t want to use. Do yourself a favor: stick to lean meats, fruit and veggies, and healthy fats, and make low GI and low priority.