The essential nutrients found in foods and drinks in the largest amounts are proteins, fats and carbohydrates, and consequently these are known as macronutrients.Carbohydrates are largely used as an energy source by our bodies, being the preferred fuel for our brains and nervous systems, and for our exercising muscles. They have many other roles however, including structural (within cells), genetic (RNA and DNA), and in the function of certain proteins. We must not forget that they add taste, texture and colour to our foods and drinks, providing that vital part of eating – pleasure. Carbohydrates are found in plant foods, milk and certain other dairy products. The three most common kinds of carbohydrate in foods are sugars, starches and dietary fibres.SugarsThe simplest form of carbohydrate are the monosaccharides fructose, glucose and galactose. Two monosaccharides joined together are known as disaccharides, and the most common varieties are sucrose, maltose and lactose. Sucrose is made up of fructose and glucose, and is the most common form of sugar in foods and drinks in most parts of the world. It occurs naturally in fruit, but two of the best sources are sugar cane and sugar beets, which are both used to make a wide variety of refined sugars from icing sugar to table sugar, and of course are commonly added to foods and drinks for taste, texture and colour (sugars help brown starches, for example). Maltose is made up of 2 units of glucose, and is found naturally in grains like barley, and is commonly added to foods as an ingredient. Lactose is made up of glucose and galactose and is found in milk, yoghurt and some other dairy foods, and is also added to some foods as an ingredient.StarchesAre comprised of long chains of the monosaccharide glucose, and are found naturally in a wide range of foods from grains (wheat, rice, etc…), legumes (beans, peas and lentils), starchy vegetables (potatoes, carrots, etc…) and nuts and seeds, and they are also added to many foods as an ingredient (e.g., thickeners). There are two main kinds – amylose and amylopectin. Amylose is like a string of glucose molecules that tend to line up in rows and form tight, compact clumps, whereas amylopectin is a string of glucose molecules with lots of branching points – a bit like a tree.Dietary fibresPerhaps surprisingly for some, most dietary fibres are in fact a form of carbohydrate. Dietary fibres come mostly (but not exclusively) from plants, and they are the poorly digested portions that pass through into the large intestine (bowel) and provide much of the bulk in our stools (along with water and bacteria, amongst a few other things). Needless to say, a diet that provides sufficient fibres is essential for long-term health and well-being.Digestion and metabolism of sugars and starchesWhether a sugar or a starch, most of the carbohydrate in foods that we eat is digested in the mouth, stomach and small intestine, absorbed into the bloodstream via the small intestine and, one way or another, converted to glucose.For most carbohydrates the process is relatively simple. Starches are simply chains of glucose joined together by bonds. Small proteins called amylases found in saliva and intestinal digestive juices (secreted by the pancreas) snip each bond between the glucose molecules, so that by the end of the digestive process you wind up with pure glucose in the small intestine, which is then transported through the intestinal wall into the bloodstream. Similarly, in order to be absorbed into the bloodstream, disaccharides need to be broken down into their constituent monosaccharides. Like starches, this is done by specific enzymes in our small intestine: maltase, sucrase and lactase. The glucose, fructose, and galactose molecules are then absorbed into the bloodstream via the small intestine.Glucose circulates throughout the body and can be absorbed directly into the cells of most of the body’s tissues and organs where it usually ends up as organic acids such as pyruvate plus adenosine triphosphate (ATP), which is our body’s main energy currency. Normally, pyruvate is also converted to ATP, producing more energy. Galactose and fructose molecules, however, require further processing. In the small intestine and liver, phosphor is added to fructose, and the resulting phosphorylated fructose enters what is known scientifically as the glycolytic pathway, where through a series of chemical reactions, it usually ends up as glucose, organic acids and ATP. Similarly, galactose is absorbed and converted to glucose in the small intestine and liver, and again converted to organic acids and ATP, just like glucose.When glucose enters the bloodstream, our pancreas releases insulin, which signals most of the body’s organs and tissues to absorb the glucose from the blood. As we now know, people with diabetes produce either essentially no insulin (type 1), or not enough insulin (type 2), and this is why their blood glucose levels can rise too high after consuming too much of a high-carbohydrate meal or drink.Glycemic index (GI)As explained previously, essentially all sugars and starches in food are digested, absorbed and metabolised into the monosaccharide glucose. However, some foods break down quickly during digestion, are absorbed quickly and don’t require further metabolism, so they cause the glucose in our blood to increase rapidly. On the other hand, some foods break down slowly during digestion, and/or take longer to be absorbed and/or metabolised, and so the glucose is consequently released gradually into our blood streams.The glycemic index, or GI, is simply a number between 1 and 100 that provides us with a way of estimating how fast our body is going to digest, absorb, and metabolize carbohydrate containing foods and how high our blood glucose levels will rise as a consequence. Simply, foods that are digested, absorbed and metabolised into glucose quickly have a high GI and those that are digested, absorbed and metabolised into glucose slowly have a low GI.By definition, high GI foods and beverages have a GI value of 70 and above; medium GI foods and beverages have a GI value between 56 and 69; and low GI foods and beverages have a GI value of 55 and under. On the other hand, low GI diets have an average dietary GI less than 45.Glycemic load (GL)The glycemic load (GL) is the mathematical product of the glycemic index (as a percentage) and the amount of available carbohydrate (i.e., all carbohydrate except fibres) found in a typical serving of food. For example, a typical slice of white bread has a GI of 71 and contains 15 grams of available carbohydrate. Therefore, its glycemic load is 71 × 15 ÷ 100 = 10.6 g. If you eat 2 slices of bread as part of a sandwich, the overall GL of the meal is 21. There is very good evidence from clinical trials that the GL of a food or meal is the best predictor of a person’s blood glucose and insulin levels – better than the GI or amount of available carbohydrate alone.So, how do we define quality carbohydrate?There is currently no universally accepted definition of carbohydrate quality, and therefore it’s an issue that we regularly come back to at GI News.In this month’s WHAT’S NEW? Dr Kate Marsh discusses the study by Andres Korat and colleagues. In this study, the carbohydrate intake variables were total carbohydrates (e.g., starches, sugars and fibres), refined carbohydrates, high-quality carbohydrates (sum of carbohydrates from fruits, vegetables, whole grains, and legumes (but not milk or yoghurt)), carbohydrates from starchy vegetables, and dietary fibre intake, including fibre from fruits, vegetables, and cereals.Most food and nutrition scientists would agree that minimally processed fruits, vegetables, whole grains, legumes, milk and yoghurt are the best sources of carbohydrate (i.e., starches, sugars and fibres (except dairy)) and essential vitamins, minerals and other important bio-active substances. The best tools for measuring the effects of carbohydrate-containing foods and beverages on blood glucose and insulin levels are the GI and GL – not the amounts of starches, sugars or fibres in a serve of food, per se.For optimal digestive health, carbohydrate-containing foods and/or meals should also ideally be a good source of fibres. That means providing at least 4 g of fibre per serve. In total, women should aim to consume at least 25 g of fibre and men at least 30 g each day.For the prevention of chronic conditions like Type 2 diabetes, the average adult should aim to consume less than 95 g of GL in a typical day. A meal has a low GL if the value is less than 32 g, and it is high GL if the value is greater than 64 g.Therefore, a diet based on high quality carbohydrate should be low GI, provide less than 95 g of GL per day and more than 25 g of fibre per day for women and more than 30 g of fibre per day for men.Read more:
Dr Alan Barclay, PhD, is an Accredited Practicing Dietitian, nutritionist and chef with a particular interest in carbohydrates, diabetes and food law. He is author of Reversing Diabetes, and co-author of 40+ scientific publications, The Good Carbs Cookbook (Murdoch Books), Managing Type 2 Diabetes (Hachette Australia) and The Ultimate Guide to Sugars and Sweeteners (The Experiment Publishing).
