Common Questions about Sugars and the Glycemic Index

by Jenny Hegmann

Jenny Hegmann, MS, RD, is co-author of The Cyclist’s Food Guide: Fueling for Distance (© 2005 Sports Nutrition Publishers) with Nancy Clark, MS, RD. Hegmann is a sports nutritionist and long-distance cyclist. She lives and works in the greater Boston area.

What type of carbohydrate is best for health and for riding?

The quality of a carbohydrate used to be judged by whether it was complex or simple. Complex carbohydrates (potato, bread) were considered “slow,” providing a steady, longer-lasting supply of glucose to the body. Simple carbohydrates (sweets, sugar) were “fast,” causing a quick blood sugar “high” followed by a sugar “low.”

Today we know that how you respond to a carbohydrate is not reliably determined by whether it is simple or complex: a potato (complex) is quickly converted into glucose by the body; fructose (simple) much more slowly. How a carbohydrate will affect blood sugar is better predicted by the glycemic index (GI). The GI ranks foods by their ability to raise blood sugar compared to a reference food, usually glucose, which has a GI of 100. The GI is based on a portion of food that provides 50 grams of available carbohydrate. High GI foods are quickly digested and absorbed and provide a fast, steep increase in blood sugar. Low GI foods are slowly digested and absorbed and provide a flatter, sustained release of glucose into the blood stream.

The problem with high GI foods is that when blood sugars rise, insulin levels also rise. Insulin signals the cells to use glucose for energy and the body to store fat (rather than burn it). A spike in blood sugar and resulting spike in insulin levels can cause low blood sugar in sensitive individuals, although this effect is probably lower in exercising athletes (the insulin response is blunted during exercise). Research shows that high GI diets are associated with obesity, diabetes, and cardiovascular disease — high insulin levels are a possible factor.

Following a low GI diet is prudent. Studies show that it helps to curb the appetite, is helpful for weight loss, and lowers risk of cardiovascular disease. Low glycemic foods eaten before exercise may or may not give you an advantage; they increase the utilization of fat for fuel, but this does not always translate into using less glycogen, faster times, or longer time to fatigue.

Blood sugar is affected by not only the quality (GI) of a carbohydrate but also the quantity. GI is based on a portion of food that contains 50 grams of carbohydrate (such as one potato or four tablespoons of honey). These may not be the amounts you would actually eat. The glycemic load (GL) describes a food’s effect on blood sugar based on variable portion sizes. To calculate: GL = GI x grams carbohydrate in your serving divided by 100. While a potato and honey are both high GI, eating one potato (50 grams carb, GL = 31) raises blood sugars faster/higher than only one tablespoon (12 grams carb, GL = 10) of honey.

There are limitations to the GI:

  • GI measurement is based on people who fasted and then ate the selected food by itself. They ate nothing else while their blood sugars were tested over 2-3 hours. Eating mixed foods or eating frequently will change the glycemic response.
  • Foods vary in GI based on origin and processing. An Idaho potato has a different GI than one from Maine; mashed a different GI than baked.
  • Large variations exist in response to the same food. What quickly raises your blood sugar today may not have the same effect on you tomorrow, or on your buddy.
  • Most foods in the US have not been evaluated for GI. New Zealand has standardized testing of products available in that part of the world. The GI of food in New Zealand may not be the same for food in the US.

Tips to make the best use of the GI:

  • Experiment. If you quickly run out of energy after having a bagel and jelly for breakfast, try meulsi or oatmeal, yogurt, and a banana instead.
  • Consuming fat or protein with a carbohydrate food lowers the GI because these slow digestion and absorption. But don’t overdo it; remember, your diet should be predominately carbohydrate. Choose healthful unsaturated fats such as vegetable, olive, and canola oils, avocado, and nuts; lean proteins; and low-fat or fat-free dairy products.
  • To lower the GI of your overall diet, consume one low glycemic food with each meal or snack. Enjoy nuts or dried apricots with cereal, milk, and lentils or beans as a side (or main) dish.
  • Include lower GI foods before your ride, especially if you will not consume much or any fuel on a shorter ride. This provides a sustained source of glucose to working muscles.
  • Choose higher GI carbohydrates during your ride, but always choose what makes you feel the best.
  • Choose high GI foods immediately after your ride to begin to recover your glycogen stores.
  • High GI foods aren’t “bad.” Whole wheat bread and potatoes have nutrients important for good health. Eat a healthful diet overall with smaller quantities of high GI foods and more lower GI foods.
  • Many foods are so low in carbohydrates (blueberries, leafy vegetables), that their GI cannot be tested. Meats, poultry, fish, and fats have no GI because they contain no carbohydrate.

Is sugar bad?

No, but too much sugar is not good. Sugar provides a concentrated source of carbohydrates to fuel muscles and brain. Increasingly overweight Americans consume excessive amounts of added sugar, nearly 20 teaspoons a day, twice that recommended by the USDA. Diets high in added sugar tend to be low in nutrients and high GI. Endurance cyclists easily burn up extra sugar calories but risk displacing nutrients from their diet if they overeat sugary foods. Sweets and energy bars/drinks lack the fiber, vitamins, and other nutrients needed for good health. Rides and recovery present a good opportunity to substitute some sugary foods with more wholesome, natural foods. Instead of cookies, try a peanut-butter-and-banana sandwich; instead of a sports bar, have an apple and chocolate milk; swap the recovery shake with a fruit-and-yogurt smoothie. If don’t tolerate such foods, then consume what works during your ride and make a point to eat more nutritiously off the bike.

Should high fructose corn syrup (HFCS) be avoided?

HFCS is no better or worse for you than regular table sugar. HFCS is sugar syrup made from corn starch. Its fructose content has been criticized by people who say it is a culprit in obesity, among other diseases. The HFCS used to sweeten soft drinks and other sweetened beverages is 55% fructose and 45% glucose, similar to table sugar (sucrose), which is a 50-50 mix of fructose and glucose. HFCS has sweetness almost identical to sucrose, extends shelf life, and can improve texture of foods, all at a lower cost to manufacturers. The digestion, absorption, and metabolism of HFCS are similar to that of sucrose. Most nutritionists agree that excessive sugar consumption, not HFCS per se, is a threat to health.

Is “natural” sugar like honey or maple syrup better than refined table sugar?

No. All sugar — honey, maple syrup, brown sugar, turbinado, cane juice, table sugar, HFCS — contain fructose and glucose in varying amounts. They are all digested into glucose by the body and have four calories per gram or roughly 50 calories per tablespoon. None provides significant vitamins or minerals. Choosing sugar is a matter of taste; all will fuel your muscles equally well.

Note that fructose in high concentrations can lead to gastrointestinal upset because it is absorbed slowly in the intestine (it has a GI of 12). Fruit, fruit juice, and the sugars mentioned above contain some fructose. If these bother your stomach, experiment with eating less of them, switching to foods sweetened with dextrose (glucose) or maltodextrin, or eating carbohydrate foods that taste less sweet (like breads, milk, crackers).

Is maltodextrin preferable to regular sugar?

Maltodextrin is preferable if you’re looking for carbohydrates that taste less sweet or if you have trouble tolerating sugary foods. It is a glucose polymer (complex carbohydrate) made from corn starch. It is water soluble, quickly absorbed (has a GI the same as glucose, 100), and has low sweetness, making it perfect for sports drinks and gels.

Glycemic Index for common foods

Jenny Hegmann, MS, RD, is co-author of The Cyclist’s Food Guide: Fueling for Distance ((c) 2005 Sports Nutrition Publishers) with Nancy Clark, MS, RD. Hegmann is a sports nutritionist and long-distance cyclist. She lives and works in the greater Boston area.