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The metabolic response of carbohydrate ingestion within the body

By Ronan Costelloe MSc. BA (Hons)

Never before have there been as many low fat or fat free products available to consumers, and yet Western society is more obese than ever. Much of this has to do with a lack of understanding of our bodies metabolic response to carbohydrates. Frequently, very fit, healthy and active people struggle to achieve their desired body composition due to misinformation about carbohydrates.

There is an ever-increasing amount of research being produced that if correctly applied will improve body composition and bring about healthier lifestyles. Consequently, the general population seem to spend an ever-increasing amount of time sieving through information in an attempt to identify a healthier lifestyle. Nutritional generalisations also seem to have passed through generations and it is perhaps not surprising that there is a problem. In a sporting or athletic context, all too often information has become generalised to “all athletes” and for “all sports” and herein lies the problem.

The metabolic response of carbohydrates:

After eating, the gut breaks down carbohydrates into glucose, the glucose is absorbed from the gut and it is burned for energy or stored as fat. If the carbohydrates you eat require little digestion (fast burning) glucose becomes rapidly available in the gut and is absorbed quickly. Energy is now rapidly available for high intensity exercise. If the carbohydrates you eat are more fibrous (slow burning) the digestion occurs slowly and takes longer to break down into glucose.

Insulin, the hormone that can both help and hinder us:

The more rapid the rise in blood sugar the more insulin is required. Insulin, as the hormone of storage, takes glucose from the blood and stores it away. The help that this offers exercise is that insulin will assist the replacement of muscle or liver glycogen consumed during high intensity training or competition, but once the body’s glucose stores are replenished any excess would be converted and stored as fat. This can be called the “spillover effect”.

Carbohydrates that have a high glycaemic index (fast burning) and glycemic load produce a very large insulin response, particularly when consumed away from exercise and can be more readily converted to fat. A reduction of healthy fat consumption in the diet and moving away from an intake of complex to simple carbohydrate sources seen in Western societies has been paralleled by an increase in obesity. Hence knowing the glycemic index (GI) in food is essential to good diet for athletes and generally active people.

GI is a number that is given to carbohydrate foods to show how fast blood glucose will rise when ingested. The level of glucose in our blood is controlled by insulin. Insulin acts to ensure that blood glucose levels do not rise above the normal ranges. It moves glucose from the blood into cells. The faster blood glucose rises the more insulin produced. Effectively, this is where high and low glycemic values (GI) originate. Higher GI (56 -100) raises blood sugar quickly and produces an insulin response and can contribute to fat storage.

To keep things simple, sugar we put in our tea (sucrose) has the top GI value of 100. All other foods are rated against this, and lower numbers are foods that tend to be better for stabilizing hormones. The higher the GI the more disruption to blood glucose is caused. The lower the GI the slower the food is broken down in the body.

Much of the early sports research focussed on the nutritional requirements for endurance events. What has been clearly established is that when competing in endurance events, especially those lasting 90 minute or more, 30-60g/hr of carbohydrates is essential for performance, and recently we have seen ultra-endurance athletes consume up to 90g/hr of carbohydrates. Endurance athletes should consume higher glycemic carbohydrates (fast burning) before and during an event and adequately replenish the body’s muscle and liver glycogen after an event. Quantity, quality, and timing of carbohydrate is essential in maximize performance, recovery and body composition.

The performance window:

Foods higher in carbohydrate should be eaten earlier in the day when exercise is greater and energy requirements are high. Lower GI carbohydrates such as Oatmeal, wholegrains and fibrous fruit provide long lasting energy throughout the day (slow burning carbohydrates). When we exercise and intensity is high (fast burning carbohydrates) sources such as white refined bread with some jam or honey, bananas, energy bars and fruit smoothies are easily digestible at this during the performance window.

Lower glycemic (GI of 55 or less): Most fruits, vegetables, beans, wholegrains, nuts and seeds.
Higher glycemic (GI of 56 or higher): White refined bread, rice cakes, crackers, bagels, energy bars, refined

breakfast cereals and jellies.

Has this led us to being overly ‘obsessive’ about consuming just any carbohydrate? A lack of knowledge may be evident, we see this through poor performance and inferior body composition. High glycemic carbohydrates (fast burning) have a place in our diet essentially within the “performance window”, before, during and after high intensity (anaerobic) exercise, as both protein and essential fat intake during this period have lesser importance. However, we are seeing an increased proportion of hidden sugars consumed outside the “performance window” from high glycemic carbohydrates (fast burning) such as certain low fat yogurts, refined breakfast cereals, and condiments.

Summary and Top Tips

• Outside the “performance window” choose foods which break down slowly (low GI) so the body can burn it rather than have to store calories as fat (high GI). Foods that are absorbed slowly also have the added advantage of making you feel full and decrease appetite.
• Eat carbohydrates in expectation of exercise. It is not advisable to eat large volumes of carbohydrates before bed when requirements are going to be low.
• Ingest fast release carbohydrates (high GI) pre, during and post exercise to replenish lost energy. Always eat breakfast (unless advised). Skipping breakfast results in low blood sugar for an extended period of time, your symptoms will worsen, and compensatory eating will often be excessive later in the day.
• Watch for ‘hidden’ sugars and carefully read labels! Avoid those foods with modified corn starches or syrups. These are the hidden refined sugars!
• High Fibre Foods such as vegetables and whole grains tend to have a good fibre content. Fibre slows the absorption of sugar from the gut, by decreasing gastric emptying and glucose uptake.
• Increase the intake of quality protein foods (i.e. fish, poultry, and lean meats. Vegetarian proteins like tofu and plant based protein is effective at controlling appetite while slowing down the absorption of carbohydrates.Ronan Costelloe MSc. BA (Hons)

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