A Sports Nutrition Perspective on Glucose Intolerance

By Ellen K. Chow, MS, RD, CSSD, CD

Endurance sports rely heavily on glucose to provide energy. But what if this mechanism goes wrong? Is it healthy to use protein and fat in place of carbohydrates? How would you know if you consumed too many of them for a specific type of exercise? Should a family history of diabetes be a concern? Carbohydrate metabolism is a complex biochemical process. This article aims to address some basics regarding glucose control for athletic performance and long-term health goals.

Glucose Metabolism

The human body maintains tight control of blood glucose level, often referred to as blood sugar. Normal pre-meal level or fasting blood glucose (FBG) values for healthy individuals vary between 70-99 mg/dl. In the U.S., individuals with FBG between 100 and 125mg/dl may be considered as glucose intolerant or prediabetic, while values above 126mg/dl are often a diagnostic criterion for diabetes.

The following environmental factors can significantly influence glucose levels:

• Carbohydrate intake

• Physical activities

• Stress

• Hydration status

During endurance exercise, the body disproportionately uses glucose as fuel. However, at rest, normal metabolism utilizes balanced fuel sources among carbohydrates (glucose), protein (amino acids), fat (fatty acids), and alcohol when it is present. While the transition between regular activities and exercise is not exclusive of each other, increased glucose metabolism may begin as soon as minutes into an exercise session and continues for a few hours following its end, as trained muscles replenish energy store.

The timing and amount of carbohydrate consumption among recreational athletes had been an interesting trend due to the popularity of low-carb diets at one end of the spectrum, and low-fat, high-carb diets at the other end. The truth is, carbohydrates are not an enemy. They simply need to be the right kind, consumed with the right amount and at the right time.

Too Much, Too Little, Just Right

Multiple hormones tightly regulate the fluctuations of glucose levels. In the simplest terms, when blood glucose is elevated for a prolonged period of time, the body converts the excess glucose into triglycerides (fat) and stores it as a reserve for later use, creating a drop in glucose. This can be a result of too much carbohydrate intake relative to expenditure. When the change between such glucose “high” and “low” is significant, one may feel sluggish or even describe a crush-like sensation in energy level.

Conversely, low blood glucose triggers the breakdown of protein and fat for energy, which is commonly seen in very low-carb diets or when meal-skipping. However, this back up mechanism using alternative fuel has its drawbacks — seen as a stressor and a potential threat to our survival, the body’s protective nature can unintentionally cause an overproduction of glucose, resulting in an elevation, thus simulating a similar biological response as in high blood sugar. Such glucose rebound is more pronounce, and sometimes accompanied by physical symptoms. This phenomenon is well-known among diabetics but much less discussed for those with glucose sensitivities, hypoglycemia, and insulin resistance, albeit it’s an important message.

The best way to start correcting unstable glucose is establishing a baseline. This can be easily done at your primary care physician’s office through a fasting glucose and insulin check. Note that fasting glucose does not necessarily reflect fluctuations throughout the day and following meals, but it would provide useful information.

Next, set appropriate carbohydrate goals based on exercise time of less than or more than one hour. This should be done with a health/fitness provider familiar with sports nutrition. Remember that stabilizing glucose does not necessarily mean a low-carb diet. In fact, it is paramount to consume the right kind, right amount, and at the right time. Always use non-exercise or low-activity days as a reference point before adding the extra nutrients to meet your training needs; you may refer to previous articles on athletic weight management [June 2012 Bicycle Paper] and sports drinks for carbohydrate needs [July 2012 Bicycle Paper].

Typically, recreational female athletes can start with 150-180 grams of carbohydrates per day and male athletes somewhere between 180-210 grams per day. Once you have selected your carbohydrate level, divide that amount into three meals and one or two snacks. Eating times are flexible depending on your schedule, but preferably avoid going beyond four to six hours without any intake. When carbohydrate amounts are controlled, it may be necessary to adjust fat and protein intakes.

It may take a few days to several weeks to learn the carbohydrate content of your eating choices. While many foods have a nutrition label, just as many will require a little investigation. Most of this information can be found through online search engines, health and fitness websites, or the USDA Nutrient Database. Reference books are also available in addition to manufacturer and some restaurant websites.

Finally, add in the carbohydrate need for training and time this intake at one to two hours before each session. When implemented properly, one can expect to experience improved energy within a couple of weeks and improved blood work, if monitored, within three months.

Health and Performance

Not only does stable blood sugar improve overall well being and cycling performance, it provides a foundation for healthy metabolism, lowering risks for diabetes, heart disease, and other inflammatory conditions. Unfortunately, no one would be able to guarantee absolute prevention of metabolic diseases especially for those with a strong family history. The key is not to be discouraged, even though it is easier said than done. Imagine delaying diabetes and its complications by decades, avoiding medications and insulin injections.


U.S. Department of Agriculture, Agricultural Research Service. 2011. USDA National Nutrient Database for Standard Reference, Release 24.

Ellen is a Seattle-based sports and wellness nutritionist. She works with athletes of all ages. She also works extensively with athletes who have stable cardiovascular and metabolic diseases. Ellen can be contacted at www.endgamenutrition.com.

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