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Eating the Hour Before Exercise

By Monique Ryan

Triathletes are well aware that beginning training and competition with a well fueled body can delay fatigue. One significant fuel source is stored glycogen, which remains relatively limited even with an optimal diet. Depending on the intensity of your workout, glycogen stores may last only for 75 to 90 minutes of exercise. Because of the multiple training sessions required for the sport, triathletes face the constant cycle of training and refueling, often with only several hours of recovery time. It is not unusual to have the need to fit a small meal or snack in between training sessions.

Eating Before Exercise
Research and practical experience makes a good case for consuming carbohydrate 2-4 hours prior to exercise. A relatively nice amount of food can be consumed and replenish or top off your muscle glycogen stores. Liver glycogen stores, which maintain blood glucose levels during training, can become significantly depleted after a training session and require a good meal to return back to their full levels. Eating adequately before exercise can also prevent hunger during longer training sessions. However, scheduling and other real-life considerations may not always allow for optimal meal timing before exercise. Often, consuming foods and fluids in the 30-60 minutes prior to exercise may be a viable and necessary option.

The Hour Before Exercise
There are a variety of scenarios that could necessitate the need for food 30-60 minutes prior to exercise. Every triathlete is all too familiar with those early morning training sessions and early race start times. Rising in the extremely early hours of the morning simply to eat food and fuel up for an early start may not be feasible or desirable. Scheduling may also result in a long time gap between the last meal and the start of a training session, and hunger and limited fuel during training may become a significant issue. It may also be helpful to eat closer to longer training sessions in which the added fuel provides a performance benefit. Despite these practical considerations, consuming carbohydrate in the hour prior to exercise has been a subject of some controversy over the years.

Consider the Research
It all began in 1979 when a well respected lab reported the results of one preliminary study. When subjects were fed 75 gm glucose 30-45 minutes prior to exercise, the researchers measured a decrease in blood glucose levels, or hypoglycemia, during exercise. An exercise test also measured a decrease in performance at 80% VO2 max when compared to a placebo. However, what was not as widely reported from this study, was that the measured hypoglycemia was short-lived, lasting only about 10 minutes into exercise, and was not associated with fatigue. 

These results spurred on more research. Nine studies had subjects consume a variety of supplemental carbohydrate sources such as fructose, glucose, sucrose, and amylopectin in supplement form (rather than from real food), anywhere from 30-60 minutes prior to exercise. Five of these studies found that the carbohydrate feeding did not impair exercise, while four of these studies found an actual performance improvement. So overall, eating 30-60 minutes prior to exercise is very unlikely to impair performance, and may even provide a needed performance boost. But what was most interesting about the study results was the highly individual metabolic responses among subjects, with some subjects experiencing hypoglycemia.

Because some athletes do experience more symptomatic transient hypoglycemia than others, researchers incorporated the glycemic index (GI) into their study design. The glycemic index measures the blood glucose profile of carbohydrate foods. Lower glycemic index foods produce more stable blood glucose and lower insulin responses than moderate or high GI foods. It was thought that consuming a lower glycemic index food would be well tolerated 30-60 minutes prior to exercise and could improve performance. In other words, a bowl of home cooked oats may give you a more sustained boost than a big glob of carbohydrate gel.

When a high glycemic index food was compared to a low glycemic index food, results for nine studies were split down the middle. Utilizing a lower GI food did appear to lessen the observation of marked responses in insulin and lowering of blood glucose levels. However, the performance implications of this manipulated metabolic response are less clear. Some studies found an improvement in performance, while others did not. And it appears that the best designed studies found that lower glycemic index foods did not result in a performance improvement, despite the blood glucose response being favorably altered.

Practical Considerations
From the triathlete’s perspective, whether you consume carbohydrate 30-60 minutes prior to exercise needs to be individualized to your tolerances and training schedule. Deriving a performance benefit from ingesting fuel 30-60 minutes prior to exercise is most likely to occur when the carbohydrate you consume replenishes compromised fuel stores. So consider consuming carbohydrate within an hour before exercise if you have not eaten for four hours or more, or prior to early morning training when liver glycogen is low. Eating before long training sessions also can prevent hunger, and would provide extra calories for triathletes who have very high energy requirements.

If you are still concerned that you are an athlete who is carbohydrate sensitive during this time period, there are a few sensible strategies that you may find useful. Consuming a high enough amount of carbohydrate may simply offset any lowered blood glucose levels and hypoglycemic symptoms. Amounts of 70 gm or more seem to maintain blood glucose levels in individuals susceptible to exercise hypoglycemia. Individuals not susceptible to hypoglycemia can consume anywhere from 50 to 100 gm carbohydrate prior to exercise. You can also consider a carbohydrate source with a lower glycemic index, though this often requires the consumption of real foods very close to exercise rather than easily digested sports nutrition products. Common sense would indicate that consuming a carbohydrate beverage or gel might be more practical than chowing down on a big bowl of lentils 30 minutes before exercise. Many of these products such as gels, bars, and sports drinks provide anywhere from 30-50 gm of carbohydrate per serving. You may choose one of the items or any combination of them in portions that you tolerate.

Insulin sensitive athletes can also consider another very practical strategy for maintaining blood glucose levels while exercising. One simple technique is to simply consume adequate carbohydrate, about 30-60 gm per hour, during exercise. This may be more feasible during some training sessions than others, but effectively diminishes any negative effects of pre-exercise carbohydrate ingestion.

Monique Ryan, MS, RD is the author of Sports Nutrition for Endurance Athletes, 2nd edition (VeloPress 2007). Click here to view more about the book or purchase. She was a member of the Athens 2004 Performance Enhancement Teams for USA Triathlon and USA Cycling Women’s Road Team. She is owner of Personal Nutrition Designs and offers her sports nutrition “E Program” at