Over the last few years, research has increasingly shown that a high carbohydrate diet is not as good for health as previously believed. In fact, studies now show a link between the quality and amount of carbohydrate in your diet and your risk of developing diabetes, heart disease and stroke, especially when combined with a high energy intake. For those who are obese, the risks of eating excessive carbohydrates are especially high as overweight reduces your body’s ability to handle the glucose derived from digesting carbohydrates.
Until as recently as 2004, healthy eating recommendations suggested eating at least 55% of calories as carbohydrate, emphasizing the quantity rather than the quality of carbohydrate consumed. Now, recommendations have started to focus on the type of carbohydrate consumed, and the development of the glycemic index (GI) and glycemic load (GL) has provided a measure to consider how foods that contain carbohydrate affect circulating glucose levels.
Carbohydrates are the main source of energy in your diet, and one of the most controversial food groups. As your body can make all the carbohydrate it needs from other sources, no dietary carbohydrates are deemed ‘essential’ in the same way as some amino acids, and the essential fatty acids.
Carbohydrates can be divided into simple carbs (sugars), complex carbs (starches) and indigestible carbs (fibre).
The simplest carbohydrates are single sugars (monosaccharides) of which the most important in the diet are glucose (grape sugar, also known as dextrose), galactose (a milk sugar) and fructose (a fruit sugar).
Two single sugars can join together to form double sugars (disaccharides) of which the best known example is sucrose – table sugar – made from glucose and fructose. Other examples include lactose (a milk sugar made from glucose and galactose) and maltose (a sugar found in cereals, made from two glucose molecules bound together).
Monosaccharides are absorbed directly from your intestines into the circulation. Glucose has a rapid impact on blood sugar levels, whereas fructose and galactose have much less effect as it takes time for the liver to take them up and convert them into glucose.
Because disaccharides are made up of two sugar molecules joined together, they are not absorbed directly into the circulation. First, they are broken down into their constituent monosaccharides by enzymes found in your saliva and gut. These digestive processes are carried out relatively quickly, and as the main dietary disaccharides contain 50% glucose, they do affect blood glucose levels, although the effect is slower than with monosaccharides.
Most of us eat too much sugar – new guidelines recommend that adults consume no more than 30g free sugars per day – the equivalent of around 7 sugar cubes.
Sugar molecules may also be ‘chained’ together to form complex carbohydrates (polysaccharides) such as starch (eg found in rice, wheat flour and potatoes) or glycogen (an energy storage molecule found in the liver and muscles).
Dietary fibre (what used to be known as roughage) consists of the non-digestible carbohydrates in your diet, and are mainly found in fruit, vegetables and whole grain products.
Fibre passes through the small intestines unchanged and act a bit like a sponge to slow the absorption of sugars in the small intestines and lower the glycemic impact of a food. Once fibre reaches the large bowel, it is fermented as a food source by bacteria.
The way in which different foods affect blood glucose levels can be assessed from their Glycemic Index (GI). This concept was first developed in 1981 by scientists from the University of Toronto, and has been expanded by researchers at the University of Sydney who maintain the International GI database.
The Glycemic Index compares how different foods affect blood glucose levels compared with eating a known amount (50g) of glucose, which is given an arbitrary GI value of 100. The index is based on volunteers eating whatever quantity of a particular food contains 50g of digestible carbohydrate. Blood tests are taken before and after, and a food that raises blood glucose levels by the same amount as the glucose is assigned the same value of 100, while a food that raises blood glucose levels by, for example, half as much as glucose, is assigned a GI value of half as much (ie 50).
Some measures have used white bread as the index food but as bread varies, this gave less accurate results.
Foods with a high GI (70 or above) have a rapid affect on your blood glucose levels (see yellow line on the graph). These foods are sometimes referred to as ‘gushers’.
Foods with a medium GI (56-69) produce a more sustained effect on blood glucose levels (see orange line on graph below).
Foods with a low GI (less than 55) contain few carbohydrates, or carbohydrates that break down slowly to produce only a minor effect on blood glucose levels (see blue line). These foods are sometimes known as ‘tricklers’.
Very low GI foods, with a value of less than 30, have little impact on your blood glucose levels. These foods include low carbohydrate foods such as butter, cheese, eggs, fish, grapefruit, green vegetables, meat, nuts, plums, seafood and pulses such as soy beans and kidney beans.
The simple sugar, fructose, has a surprisingly low GI value of 23, as it must be converted into glucose in the liver before it can affect blood glucose levels.
The Glycemic Index is not always helpful, as it is based on volunteers eating whatever quantity of food contains 50g of digestible carbohydrate. In the case of carrots, which contain only 7% carbohydrate, this involves eating several bunches in one go, which is not a realistic amount.
Another system, called the Glycemic Load (GL) was therefore developed by researchers from Harvard University to take into account the amount of carbohydrate present in a typical portion.
The Glycemic Load of individual foods is calculated by multiplying a food’s glycemic index (GI) value by the amount of carbohydrate found in a single serving, then dividing the result by 100.
The Glycemic Load provides more useful information about how eating a particular food will affect your blood glucose levels in real life. For example, an average serving of white pasta contains more than 50g digestible carbohydrate and has a larger impact on blood glucose levels than you might expect from its GI alone.
A food with a GL value of 20 or more is classed as have a high GL.
A food with a GL value of 11 to 19 is classed as having a medium GL.
Foods with GL values of 10 or less are classed as low GL foods.
Carrots, for example, have a high glycemic index (calculated from eating one and a half pounds of carrots) but a negligible glycemic load (calculated from eating a typical serving of 3 to 4 ounces) makes them perfectly acceptable for a low glycemic load diet.
Wholewheat pasta, on the other hand, has a low glycemic index when perfectly cooked (so it is still slightly chewy or al dente) as only a small amount is needed to supply 50g carbohydrate. But, because the number of carbs present in a typical portion is much higher than 50g, it has a high glycemic load. Eating too much pasta can definitely lead to weight gain.
Glycemic load is therefore starting to replace glycemic index as a tool to help you eat healthily, as it takes portion size into consideration and gives a truer indication how a particular food affects your blood glucose level.
In general, you can eat foods with a low GL value freely, but need to moderate your intake of foods with a moderate GL value, and take care not to eat too many foods with a high GL.
|White rice, boiled||23||64|
|Brown rice, boiled||18||55|
|White spaghetti, boiled||18||44|
|Wholemeal spaghetti, boiled||16||37|
|New potatoes, boiled||14||50|
|Unsweetened orange juice||12||52|
|Unsweetened apple juice||11||40|
|Wholemeal rye bread||8||58|
|Kidney beans, cooked||7||28|
|Mixed grain bread||6||49|
High glycemic foods are bad for health
Humans evolved on a low carbohydrate, high protein diet. When agriculture developed after the last Ice Age, our intake of carbohydrate increased, but most was in the form of low glycemic grains that produced only modest increases in insulin levels.
As a result of the industrial revolution, milling increased the glycemic index of dietary carbohydrate leading to an estimated three-fold increase in the rise of blood glucose levels after a meal (postprandial glycemia) and insulin secretion. With the advent of fast foods, confectionary and super-sized meals, the glycemic index of the typical Western diet increased even further, leading to widespread ‘epidemics’ of obesity, glucose intolerance, insulin resistance and type 2 diabetes.
Simple sugars are quickly absorbed into your circulation and can produce a rapid rise in blood glucose levels after eating which can damage artery lining and increase the long-term risk of heart disease and stroke. In contrast, unrefined carbohydrates, such as the starches found in wholegrains, must first be broken down into their simpler sugars before they can enter your circulation, and have less of an impact on blood glucose levels after eating.
Foods that produce a rapid spike in blood glucose trigger the release of insulin hormone, which is the main fat-storing hormone in your body. Insulin quickly lowers blood sugar levels by pushing excess glucose into fat and muscle cells where it is either used as a fuel or stored for leaner times.
If you follow a high glycemic diet, increased insulin secretion can cause your blood sugar levels to rebound too low an hour or two after eating, so you feel tired, drained of energy and have difficulty concentrating. This is one reason why you may feel drowsy or in need of a mid-afternoon snooze after eating a carbohydrate-rich lunch.
High glycemic diet and heart disease
A pivotal study published in the American Journal of Clinical Nutrition assessed dietary the glycaemic load, carbohydrate intakes and risk of coronary heart disease in 75,521 women, aged 38-63 years, who were followed for ten years. After adjusting for other factors, such as age, weight, smoking status, alcohol intake, exercise levels, calories, family history, use of HRT, blood pressure and cholesterol levels, and multivitamin use, those following the highest glycemic load diet were twice as likely to have a heart attack during the 10 year follow-up than those following a low glycaemic load diet. The link between carbohydrate intake and heart disease risk was most marked in women who were overweight.
Dutch and Finnish studies found that a higher total carbohydrate intake, glycemic index and glycemic load is also associated with a higher risk of heart disease in men compared with those following lower GI/GL diets.
These results have consistently been verified in later meta-analyses with the results of 14 studies, involving over 229,200 people, showing that following a high GI/GL diet increases the risk of cardiovascular disease (heart attack and stroke) by around a quarter (23% overall) with greater risks for women (35% increased risk) than for men (10% increased risk).
The conclusion is that following a high glycaemic load (GL) diet is an independent risk factor for coronary heart disease. This is because eating a high glycemic diet promotes weight gain, obesity and insulin resistance, raises triglyceride levels, and worsens cholesterol balance (raising bad LDL-cholesterol and lowering good HDL-cholesterol). These adverse changes are associated with hardening and furring up of the arteries and increased risk of coronary heart disease and stroke.
High glycemic diet and diabetes
As it is linked with obesity and insulin resistance, following a high GI/GL diet also increases the risk of developing fatty liver disease, metabolic syndrome and type 2 diabetes.
The results from over 205,000 men and women who were initially free from diabetes, and who were followed for more than twenty years showed that, after adjusting for energy intake, those whose diets had the highest glycemic index (GI) were 33% more likely to develop type 2 diabetes than those whose diets and the lowest GI. When the glycemic load of their diet was assessed, those whose diets had the highest energy-adjusted GL were 10% more likely to develop type 2 diabetes than those eating the lowest GL diets. Those whose diets were assessed as both high in GI or GL and low in cereal fibre had the highest risk overall, and were 50% more likely to develop type 2 diabetes.
High glycemic diet and weight
Following a diet that is lower in carbohydrates and therefore relatively higher in protein and good quality fats (eg from oily fish, olive oil, nuts and seeds) and can help you lose weight. Low carb diets are certainly more effective than low-fat diets for many people, although they don’t suit everyone.
Eating a carbohydrate-rich diet, especially one that is high in free sugars, will raise glucose levels, trigger insulin release and lead to a down-swing in circulating blood glucose levels. Low glucose levels will make you feel hungry again, and can produce sugar cravings, so you find yourself reaching for the biscuit tin or a bar of chocolate. Unfortunately, eating more carbohydrate will trigger the glucose-insulin cycle all over again and one of the most obvious side-effects is a slowly expanding waistline.