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 . 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

glycemic index

Illustration by Elaine Liu

Foods with values below 55 are considered low-GI, while choices 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 . 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 Mayer, J. USDA Human Nutrition Research Center on Aging, Tufts University, Boston, MA, USA. Nutrition Reviews, 2000 Jun; 58(6):163-9. . Ludwig, D.S., Mazjoub, J.A., Al-Zahrani, A., et al. Division of Endocrinology, Department of Medicine, Children's Hospital Boston, Boston, MA. Pediatrics,1999 Mar; 103(3):E26.. 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 . Turner-McGrievy, G.M., Jenkins, D.J., Barnard, N.D., et al. Department of Nutrition, Gillings School of Global Public Health, University of North Carolina, Chapel Hill, NC. The Journal of Nutrition, 2011 Aug;141(8):1469-74. . Marsh, K., Barclay, A., Colagiuri, S., et al. Northside Nutrition & Dietetics, Chatswood, NSW, Australia. Current Diabetes Reports, 2011 Apr;11(2):120-7. Niwano, Y., Adachi, T., Kashimura, J., et al. Carbohydrate Task Force, International Life Sciences Institute Japan, Tokyo. Journal of Nutritional Science and Vitaminology, 2009 Jun; 55(3):201-7..

On the flip side, some studies have shown no difference in hunger, satiety, or energy level after eating high- or low-GI foods . Makris, A.P., Borradaile, K.E., Oliver, T.L., et al. Center for Obesity Research and Education, Temple University School of Medicine, Philadelphia, PA. Obesity, 2011 Jun 30.. 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 . Dodd, H., William, S., Brown, R., et al. Departments of Human Nutrition and Preventive and Social Medicine, University of Otago, Dunedin, New Zealand. American Journal of Clinical Nutrition, 2011; 94(4): 992-996.. Plus, effects on blood glucose differ from person to person— everybody’s body is a unique and beautiful snowflake, no matter what Niwano, Y., Adachi, T., Kashimura, J., et al. Carbohydrate Task Force, International Life Sciences Institute Japan, Tokyo. Journal of Nutritional Science and Vitaminology, 2009 Jun; 55(3):201-7..

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 . 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 ().