As an athlete, you know that sugar is needed for energy. Yet this substance could cause your performance to deteriorate, hinder repair of tissue and could potentially lead to numerous diseases. So when does this friend turn into foe?

All carbohydrates are made up of molecules of sugar or saccharides. The two most familiar single sugars (monosaccharides) are glucose and fructose. Sucrose is a disaccharide formed from a glucose molecule bonded to a fructose molecule and is also known as table sugar. High fructose corn syrup, found in most drinks and many processed foods, is composed of a mixture of glucose and fructose.

The speed at which sugar is absorbed into the bloodstream is rated on the Glycaemic Index (GI). Although high GI foods are important for athletes during and directly after exercise, what are the consequences of ingesting high GI and sugary foods (note that fructose is not high on the GI) at other times of the day?

Cancer

Research suggests that large intakes of high GI foods, fructose and sucrose are related to an elevated risk of colon and rectal cancer. It is thought that the quantity of refined sugar in the diet can significantly influence gut function and the composition of bowel content. An increase in the ratio of sucrose to dietary fibre was observed to elevate the risk of cancer as stool bulk is reduced and transit time increased, adversely affecting intestinal flora. Vegetables, whether cooked or raw, had an inverse association with colon cancer as did fruits. Bread, pasta, cakes, desserts and refined sugar all showed a positive association with colon and rectum cancer risk.

Serum Lipids

Contrary to what was previously believed, a high fat diet is not the main cause of a change in serum lipids. In rodents, the direct association of sugar intake and elevated plasma triglycerides and hypertension has been reported in numerous studies. Similarly, in humans, a high carbohydrate diet has a marked negative impact on fasting triglycerides, with carbohydrates and not calories effecting the major changes in blood lipids. Fructose is thought to be the main culprit as its hepatic metabolism favours lipogenesis, inducing hyperlipidemia.

Vasdev et al reports of increased risk of cardiovascular disease on diets high in sucrose or fructose, with increased incidence of atherosclerosis, ischaemic heart disease, diabetes and high blood pressure.

Oxidative Stress

Heavy exercise is known to enhance free radical production. Similarly, a diet high in fat and/or refined sugar has been shown to induce oxidative stress7. It has been hypothesised that oxidative stress plays a pertinent role in the development of degenerative diseases. A strong positive association between oxidative DNA damage and sugar intake, mostly derived from fruit, sugar added to beverages, milk, soft drinks and desserts was found in 71 healthy volunteers residing in Florence.

Additionally, it is thought that fructose metabolism has the ability to induce generation of reactive oxygen species due to increased activity of xanthine oxidase and generation of glyceraldehydes, an inducer of free radicals. As such, the liver, where fructose is metabolised, may be exposed to oxidative stress after the consumption of fructose.

Fructose intake is also thought to decrease the activity of the copper enzyme, superoxide dismutase (SOD), an antioxidant which would offer some protection against oxidative stress. Ceriello et al reported that the ingestion of oral glucose was found to deplete plasma antioxidants and to increase oxidative stress both in normal and in Type 2 diabetic subjects.

Mineral Status

The consumption of fructose has been reported to affect homeostasis of numerous trace elements. It has been shown to increase iron absorption and to reduce the concentration of serum and hepatic copper. Iron overload is associated with abnormalities of glucose homeostasis. Furthermore, iron is a strong oxidant which has the ability to generate reactive oxygen species hence causing further oxidation.

A study into the intake of high-fructose corn sweetener, found in many drinks and snacks, showed an increased magnesium balance but a depressed calcium and phosphorous balance. These results are worrying and warrant further research as this adverse effect on macromineral homeostasis could lead to bone loss.

Mental health

In a study of six countries correlating per capita consumption of sugar with the prevalence of major depression, it was found that there was a highly significant correlation between sugar consumption and the annual rate of depression. Elsewhere, refined sugar consumption was a dietary predictor of the prevalence of depression whilst a high consumption of fish, and therefore omega-3, related to a decreased rate of depression. How sugar consumption may have an impact on depression is unclear, although possible relationships include the stimulation of β-endorphins by sucrose and the speculative link between sugar consumption, oxidative stress and major depression.

Conclusion

Table sugar as well as fructose are found in processed and ready-made foods, baked goods, confectionary, carbonated and sports drinks, sports bars, sauces and condiments. Athletes, because of their lifestyle, are at risk of over-consuming foods high in sugar for the following reasons:

- Available foods during events or competition may be processed or lacking in variety
- A need for quick energy resulting in the purchase of a sports bar or something similar
- Not making the time to prepare food for the day ahead
- The misconception that all sugar ingested will be used for energy with the worst side-effect being weight gain

Although sugar is used for energy, especially during physical activity and directly afterwards, the intake of sugar at other times should be avoided. With an increased risk of cancer, cardiovascular disease and possible risk of oxidative stress and weaker bones, dietary habits of athletes and non-athletes alike, need to be revised.

Meals should include a variety of foods, freshly cooked or raw, to include colour from different legumes and vegetables. Preparation and organisation is required when travelling away from home. It may seem like a nuisance, but taking the time to better prepare daily food intake will improve performance, mental focus and most importantly, short and long-term health.

Watch out for sugars under another name

- White sugar
- Barley malt
- Glucose
- Glucose polymers
- Sucrose
- Invert sugar
- Honey
- Maple syrup
- Brown sugar
- Rice syrup
- Corn syrup
- High fructose corn syrup
- Concentrated fruit sugar
- Dextrose
- Molasses
- Maltodextrins

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