By Sarah Colquitt

Slap an Australian $100 bill down on supermarket counter in Stockholm and you’ll struggle to leave with any groceries. Sure, it’s money but not the right sort. A similar process occurs in the body: you can use proteins and fats as fuel but first they have to be converted to glucose: it’s the
only currency the cells understand. Which is why sports drinks are full of glucose. Well, actually they’re full of sucrose, which the digestive system easily breaks down into glucose. It goes like this: Glucose is a monosaccharide along with galactose (a sugar in milk), and fructose, a sugar found in honey. Put two of these “monos” together and you get the disaccharides: sucrose (common table sugar: glucose + fructose), lactose (major sugar in milk = glucose + galactose) and maltose (a product of starch digestion = glucose + glucose). Start pasting disaccharides together and you get polysaccharides: the starches. Long chains of glucose, the starches serve as storage depots of glucose in plants that convert any excess glucose into starch. Together all these types of saccharides are known as the carbohydrates and while plants such as wheat, rice and corn are busy
putting the glucose molecules together, it’s the role of your digestive system to break them down and make the glucose available. The simpler molecules, the mono and disaccharides taste sweet while the more complex carbohydrates require further digestion to release the glucose molecules. But the need to supply our bodies with a constant source of glucose has been an evolutionary challenge long before we started craving Mars bars and trying to get the best performance from our legs for fun. Because our glucose intake rises with each meal our bodies developed a system to store as much of the glucose away as possible and then release it slowly as the cells require it. The principal hormone in this is insulin. After a meal, blood sugar levels (BSL) rise so the pancreas secretes insulin. The insulin prompts the cells to take up as much glucose as they can, with the rest stored in the liver until it’s needed when you miss lunch. It was generally accepted that the simpler carbohydrates would be absorbed faster and get to the cells quicker than the more complex carbohydrates, but 15 years ago scientists discovered not all carbohydrates
are created equal and their rate of digestion and absorption can vary unexpectedly. A ranking of how fast a carbohydrate is absorbed is now called its Glycaemic Index (GI): a 1 to 100 scale with pure glucose equal to 100. For general health GI is important because limiting the amount of high GI
foods in your diet decreases the fluctuations in BSL and reduces your chances of obesity, heart disease and diabetes. But for athletes such as cyclists, the GI index can help you plan out your eating to ensure maximum performance both before and during your ride. It usually surprises
athletes that some simple carbohydrate foods do not always produce the high and short-lived blood sugar rises: for example, fruit and sweetened dairy products produce a relatively flat blood glucose curve, sugar (sucrose: a disaccharide) has a medium blood sugar profile, and some foods high in complex carbohydrates such as bread and potatoes actually produce a relatively rapid blood glucose response. Even dietary fibre does not always delay absorption and flatten the blood sugar curve – blood sugar levels after eating whole-grain breads are similar to those after white bread. What is very clear is that there is no way to predict the GI of a specific food without actually measuring the response and tables of specific GIs for different carbohydrates are available at www. glycemicindex.com.A number of factors appear to determine a food’s glycaemic index and one of the most important is how highly processed the food is. In highly processed carbohydrates, the outer bran and inner germ layer are removed from the original kernel of grain, which causes bigger spikes in blood sugar levels than would occur with less-processed grains. Whole-grain foods tend to have a lower glycemic index than their more highly processed counterparts. For example, white rice, which is highly processed, has a higher glycemic index than brown rice, which is less highly processed. Some athletes and coaches have speculated that altering the GI of their training diet or a meal before a race meal might influence their performance with a low GI pre-race meal giving them an advantage (less insulin surge and blood
sugars remaining elevated over a longer period after the meal) but controlled studies have failed to demonstrate any advantages of a low compared
to a high GI pre-race meal if the athlete uses carbohydrate supplements during the ride. But for non-elite cyclists GI is important. Those studies looked at maximal performance and the athletes involved were not worried about weight gain, or the size, cost and time of eating. For us mere mortals knowing you have time to eat well before a ride (ie your commute home) allows you to choose foods with a lower GI that will fill you up for a couple of hours but they’ll give a longer rise in BSL. High GI foods tend to leave you hungry only an hour after you eat and tend to increase your kilojoule intake by encouraging you to eat more. On the other hand, when you’re actually struggling on the road, high GI foods can dig you out of the hole and foods like sports drinks have been shown to increase performance on longer rides.

Factors that influence GI:?

FIBRE CONTENT: Fibre helps shield the
carbohydrates in food from immediate
digestion, so the sugars in fibre-rich foods
tend to be absorbed into the bloodstream
more slowly.
RIPENESS: A ripe fruit or vegetable has a
higher sugar content than one that is still green
and, therefore, has a higher glycaemic index.
TYPES OF STARCH: The type of starch
granules in a food influences how fast the
carbohydrates are digested and absorbed into
the bloodstream. The starch in potatoes, for
example, is digested and absorbed into the
bloodstream relatively quickly.
FAT AND ACID CONTENT: The higher a food’s
fat content or acid content, the slower its
carbohydrates are converted to sugar and
absorbed into the bloodstream.
Physical form. Finely ground flour has a
higher glycemic index than more coarsely
ground flour.