Australasian Science: Australia's authority on science since 1938

The Truth About Carbs

carbs

Most Australians do not eat a high carbohydrate diet, but many eat too much refined carbohydrate with a high GI.

By Alan Barclay

Hardly a week goes by without some new study proclaiming the negative health effects of carbohydrates in our food, but are carbs really that bad for us?

Carbohydrate is an essential nutrient like protein, fat, vitamins and minerals. It is the only fuel used by our brains, kidneys and red blood cells, and the preferred fuel of our muscles.

All of the carbohydrate we consume in food is digested in the stomach and small intestine, where it is broken down into glucose, fructose and/or galactose. These sugars are then absorbed through the walls of our intestine and end up in our bloodstream.

Most glucose circulates throughout the body, providing fuel for our red blood cells, brains and muscles. Glucose can be stored in our muscles and liver as glycogen, and is converted back to glucose for energy in times of need (such as when you are exercising).

Fructose and galactose are extracted from the blood as it passes through the liver, where they are also converted to glucose that can either be released into the blood, stored as glycogen or converted to pyruvate, which is released into the bloodstream as an alternate fuel source. If very large amounts of pure fructose (>50 grams) are consumed in a single “dose” it can also be converted to fat, although this is thought to make up a relatively small fraction (<1%) of the amount consumed in humans.

Ultimately, all carbohydrate is converted to ATP (adenosine triphosphate), which the body uses as energy. Carbohydrates provide the least number of kilojoules per gram (16.75 kJ/g), followed by protein (17 kJ/g), alcohol (29 kJ/g) and fats (37 kJ/g). Some may be surprised to learn that sugars provide less kilojoules per gram (16 kJ/g) than starches and maltodextrins (17.5 kJ/g).

The glycaemic index (GI) is a ranking of carbohydrate in foods from 0–100 according to their effect on blood glucose levels. Foods that are digested, absorbed and/or metabolised quickly are high GI foods and have a GI value ≥ 70, while those that are digested, absorbed and/or metabolised slowly are low GI foods and have a GI value ≤ 55.

Carbohydrates in Foods and Drinks

Foods high in carbohydrate include fruit, vegetables (e.g. potatoes, corn, peas), legumes (e.g. beans, lentils, chickpeas), breads and cereals (e.g. rice, pasta, breakfast cereals), dairy foods (milk and yoghurt) and processed foods and drinks made from these (e.g. soft drinks, lollies, chips, crisps and other savoury snacks).

Minimally processed foods that are naturally high in carbohydrate are good sources of vitamins B,C and E, minerals such as magnesium and potassium, and polyphenols (biologically active substances that are good for our health), and most are also a good source of fibre (with the notable exception of dairy foods). The dietary fibre found in some high carbohydrate foods can help lower the total amount of “bad” LDL cholesterol. Traditionally, high carbohydrate foods have been consumed around the world as a good source of energy.

In modern times we have too much energy in our diets relative to our low physical activity levels. Unsurprisingly, there is strong evidence that foods high in refined carbohydrate (sugars, malto­dextrins and starches) contribute to weight gain because they provide unwanted kilojoules. There is also evidence that consuming large amounts of pure added fructose (>104 g/day on top of what you are already eating) contributes to weight gain, but very few humans eat pure fructose, let alone in these quantities. There is also strong evidence that choosing mostly low GI carbohydrates will help you to lose weight and keep it off, and growing evidence that drinking less sugar-sweetened beverages will help you to lose weight. Importantly, there is no convincing evidence in humans that sugars are addictive.

Large quantities of refined carbohydrate with a high GI raise blood glucose levels and are associated with an increased risk of developing Type 2 diabetes. High consumption (1–2 cans per day) of sugar-sweetened soft drinks is also associated with increased risk of Type 2 diabetes, but so are diet soft drinks and other non-nutritive beverages, so more research is needed. Large amounts of pure fructose (>50 g/day) eaten on top of what you normally eat will also raise blood triglyceride (fat) levels.

Finally there is evidence that foods high in refined sugars and maltodextrins contribute to tooth decay if consumed frequently, particularly among those with poor dental hygiene.

Carbohydrate and Food Labels

With a few exceptions, like alcoholic drinks and foods in small packages, all processed foods and drinks sold in Australia must provide nutritional information on the packaging. This includes the amount of total carbohydrate and sugars in 100 grams and one serve of the food or drink. Unfortunately, starches and maltodextrins do not need to be listed in the information panel, although they can be estimated by subtracting sugars from total carbohydrate.

Nutritional claims about carbohydrate in foods and drinks are regulated by Food Standards Code 1.2.7. A food can claim to be reduced in carbohydrate if it contains at least 25% less carbohydrate than the same quantity of a similar reference food, and it can claim to be increased in carbohydrate if it contains at least 25% more carbohydrate than the same quantity of a reference food.

Similarly, a food can claim to be reduced in sugar if it contains at least 25% less sugars than the same quantity of a reference food. In addition, foods can claim to be low in sugar if they contain no more sugars than 2.5 g/100 mL for liquid foods or 5 g /100 g for solid foods. Finally, a food can claim to contain no added sugars if it does not contain any “added” dextrose, fructose, glucose, sucrose, lactose, maltose, honey, malt, malt extracts, concentrated or deionised fruit juice. Perhaps surprisingly, there are no claims permitted about high or low starch or maltodextrin content.

Low GI claims are allowed on foods if the food’s GI is 55 or less and if the food meets Food Standards Australia NZ’s nutrient profiling requirements (ensuring it is an all-round healthy food or drink). It can also carry the low GI symbol if its GI is 55 or less and it meets the GI Foundation’s nutrient criteria (again, ensuring it is an all-round healthy food or drink).

How Much Carbohydrate Should We Eat?

Australia’s nutrient reference values recommend that Australians consume 45–65% of their total energy from carbohydrates. For an “average” Australian adult this is equivalent to 245–325 grams each day. This means you can eat a Mediterranean diet that is higher in fat and moderate in carbohydrate, or an Asian diet that is high in carbohydrate and low in fat, depending on your personal and cultural preferences.

Australia’s most recent National Nutrition Survey (1995) found that the average Australian was eating 259 g/day of carbohydrate, or 47% of total kilojoules. In other words, most Australians already eat a relatively low carbohydrate diet. Our guidelines for sugar are 10–25% of energy for added (not total) sugars, which equates to 55–135 g/day (13–32 level teaspoons) of added sugars. Estimates from the 1995 National Nutrition Survey indicate that adults were consuming around 10% of kilojoules as added sugars, or 59 g/day. While there are no specific recommendations for starch and maltodextrins, we consumed 146 grams (or 26% of energy) in 1995. Over the past few decades, Australians have been consuming more alternative sweeteners and consequently less added sugars. However, starch consumption has increased.

Results from the latest (2012) National Nutrition Survey will be released soon and will provide further insight into our carbohydrate consumption patterns since 1995.

Conclusion

Carbohydrates are an essential nutrient, and minimally processed carbohydrate-containing foods are good sources of vitamins, minerals and many are also a good source of dietary fibre. All of these are essential for good health. Eating too much highly refined carbohydrate, particularly foods and drinks with a high GI, can contribute to weight gain, increased risk of Type 2 diabetes and tooth decay. Most Australians do not eat a high carbohydrate diet, but many eat too much refined carbohydrate with a high GI. You should consume minimally refined, low GI carbohydrates in moderation for optimal health.

Dr Alan Barclay is Head of Research at the Australian Diabetes Council and is Chief Scientific Officer for the GI Foundation.