Ever munch on a candy bar, get a sudden burst of energy, and then crash and burn? What about snacking instead on a whole-grain bagel, and having a healthy dose of energy for hours? While some foods cause intense spikes in blood sugar levels, others keep them more constant. The glycemic index (GI) indicates how carbohydrates in foods affect those levels. When carbohydrates turn to sugar, or glucose (where the gly- in “glycemic” comes from), it turns to one of the body’s main sources of energy.
The Glucose Rollercoaster — Why It Matters
Illustration by Elaine Liu
Foods with values below 55 are considered low-GI, while choices above 70 are considered high-GI. Most fruits and veggies are on the lower end, with values in the 30s and 40s, as are most whole grains. On the other end of the scale, potatoes rank way up in the 90s, and white bread falls in the 70s. But don’t go looking for GI values for proteins like chicken or steak: if a food doesn’t contain carbs, it won’t have a GI value. And these protein- and fat-rich items have another fun trick up their sleeves— they can actually lower the GI value of high-GI foods when eaten together when them!
Since lower-GI foods keep blood sugar levels steady and higher-GI foods cause dramatic blood sugar spikes (read: sugar rush and quick crash), low-GI foods are sometimes considered preferable. The body digests high-GI foods quickly, and that means a crash can soon follow, along with feelings of hunger and tiredness. Looks like that donut is now public enemy number one!
The Highs and Lows — The Answer/Debate
It’s unclear whether GI values have any effect on our health in general. Some studies suggest a low-GI diet can aid in weight loss by eliminating the sudden spikes and crashes associated with high-GI foods  . Low-GI diets may also help reduce type-2 diabetes by keeping blood sugar levels steady, but the studies have often involved small samples, so their conclusions may not be representative of larger populations   .
On the flip side, some studies have shown no difference in hunger, satiety, or energy level after eating high- or low-GI foods . One possible hiccup in the research is the way researchers tend to isolate the foods their subjects consume. In life outside laboratory walls, people eat foods in combinations of all different nutritional values, so the results of studies can’t necessarily be replicated by regular folks . Plus, effects on blood glucose differ from person to person— everybody’s body is a unique and beautiful snowflake, no matter what Brad Pitt says .
The slam-dunk on the GI debate: The GI scale itself is a little wonky and there is only one team consistently working on creating an official database. Searching for some foods can bring up many different values (a search for “banana” turns up values ranging from 30 to 62) so it’s hard to tell what’s accurate. And just because the number is low doesn’t mean it’s time to chow down. For example, a pear and a Snickers candy bar have the same low-GI value (41), but it’s clear which one is more nutritious (just kidding).