Glycemic Index, Glycemic Load and What It Means

"Glycemic Index, Glycemic Load and What It Means" was written by Pip Taylor. Pip is an accredited sports nutritionist and accredited dietitian, professional triathlete, author and mother of two. Pip recently had her first book published, 'The Athlete’s Fix’. Check out Pip's favorite snacks for fueling workouts here. We all do it. Every day. Multiple times a day. Eat. And yet often little if any thought goes into what we actually eat or what happens after we have chewed it up. Just one effect though is on blood sugar. As athletes, we know that blood sugar levels can be important for fueling exercise. We even have a term for that feeling when glycogen stores are running low – the dreaded ‘bonk’. We also know that eating something can bring us back from that brink. But how and what does this mean? Whenever we eat, blood sugar levels rise. Carbohydrates, broken down to glucose, cause the most rapid increases in blood sugar levels with proteins and fats having a lesser effect. Food composition, serving size and timing are among other factors that influence how quickly blood sugar will rise. Insulin, released from the pancreas then works to lower blood sugar levels by pushing the glucose into cells where it can be used as energy (great) or the excess stored as fat (not so great). If we have consistently high blood sugar levels though, then either the pancreas cells can become worn out and insulin produced decreases (Type 2 Diabetes); or insulin continues to be produced at a high level but our cells start to ignore its incessant knocking trying to push glucose in the door – in other words we become insulin resistant. Of course those consistently high blood sugar levels also increase risk of obesity and this layer of fat in turn makes it harder again for glucose to push through and enter cells, in effect helping maintain high sugar levels in the blood. The issue with sugar remaining in the bloodstream is that it damages vessel walls increasing risk of stroke and heart disease. When insulin levels are raised they also signal the body to stop burning fat and instead to store excess energy as body fat. Insulin then is a key driver of body weight and body fat. So spiking of blood sugar and the ensuing insulin response, or consistently high blood sugar levels, is not really a great thing from a health perspective. But how do we know what foods will cause this spike? One way is to look at the carbohydrate content of foods – but this can be confusing and misleading since the fiber content, type of starch or sugar present in the food and how it can ben processed or cooked,  as well as other foods those carbs are consumed with affects how much they cause blood sugars to rise. Enter the Glycemic Index and Glycemic Load. Or at least the premise behind the index, because as more recent research suggests things are more complicated than first thought. The Glycemic Index (GI) ranks food from 0-100 based on the amount they supposedly raise blood sugar levels. Pure glucose has a ranking of 100. Low GI foods have a ranking of 55 or less (like yogurt, porridge, apples, legumes), medium 56-69 and a high GI food (like white rice, bread, many cereals and a baked potato) ranks over 70. But the Glycemic Index tells us little about the effect on blood sugar levels when we actually eat a food. That’s because we don’t eat the same amount (in weight) of rice as we do lettuce. The weight or portion size of the food matters and this is where the Glycemic Load comes in. The Glycemic Load then is calculated by multiplying the Glycemic Index of a particular food by the number of carbohydrates present in a 10g serve. Foods with a glycemic load (GL) under 10 are considered low-GL foods; between 10 and 20 moderate-GL foods and above 20 high-GL foods. Foods with a lower GL and GI typically are typically high in fiber and nutrients – most whole fruits and vegetables are low GL. They are generally more satiating too, breaking down more slowly and helping keep us feel fuller for longer. Which sounds nice and logical. But new research suggests that the Glycemic Load of a food is not a set value but varies from individual to individual on how it is metabolized[1]. In other words the same food, when eaten by different people causes very different responses and rises in blood sugar levels. Evidence suggests that these differences may stem from differences in gut bacteria. Researchers collected stool samples from the study participants and found correlations between what foods caused spikes in blood sugar levels and the types and numbers of bacteria present in the gut. These findings actually support the growing evidence suggesting that the micro-biome is closely linked with a range of health conditions including glucose tolerance, obesity and diabetes. This of course means it is hard to predict what foods will cause increased blood sugar levels and a spike in the insulin response. Aside from personalized nutrition the best advice remains to eat a diet that focuses on fresh whole foods. We also know that healthy gut bacteria thrive with lots of fibrous, whole fresh foods so it makes sense from that perspective. But is it ever beneficial to have a spike in insulin? Yes. For athletes, after strenuous exercise when glycogen levels are depleted, an insulin spike will help drive glucose into cells and aid in the recovery process. During exercise too where fuel is in demand, pushing glucose into the working muscles is a key factor in performance and staving of declines due to fatigue. In these instance, more refined foods such as sports drinks, bars or other foods can be a great supplement. But timing is critical and keeping these sports foods as sports foods and relying on fresh foods for the majority of your dietary intake will help manage blood glucose levels as well as supply maximum nutrients. It’s important to understand the effect of the foods you are eating have on your body and ultimately on health and performance. Diet goes a long way beyond that of calories – the take home message (yet again) is that rather than trying to decipher rankings or eat according to the Glycemic Load (which may be inherently flawed anyway) of certain foods and meals – instead focus on keeping your food real and fresh. Include lots of fresh fruits and vegetables and avoid or minimize processed and refined foods. Supplement with sports foods as warranted  - i.e. when energy needs are high and when it is beneficial to be providing your body with plenty of energy and quickly. Check out Pip's favorite snacks for fueling workouts HERE. [1] David Zeevi, Tal Korem, Niv Zmora, David Israeli, Daphna Rothschild, Adina Weinberger, Orly Ben-Yacov, Dar Lador, Tali Avnit-Sagi, Maya Lotan-Pompan, Jotham Suez, Jemal Ali Mahdi, Elad Matot, Gal Malka, Noa Kosower, Michal Rein, Gili Zilberman-Schapira, Lenka Dohnalová, Meirav Pevsner-Fischer, Rony Bikovsky, Zamir Halpern, Eran Elinav, Eran Segal. Personalized Nutrition by Prediction of Glycemic Responses. Cell, 2015; 163 (5): 1079 DOI: 10.1016/j.cell.2015.11.001