November 11, 2005

Suddenly, the latest health crazes seems to be all about carbs. You can’t open a magazine or newspaper, turn on the television or walk into a grocery store without encountering “low-carb” claims. First, the Atkins Diet extolling the virtues of low-carb diets while dangling the promise of eating unlimited amounts of fats and proteins, then others like the Zone Diet and the South Beach Diet which emphasise eating “good” carbs and eliminating “bad” carbs, now advice turns to the glycaemic index of carbs.  Should you be jumping on the carb bandwagon? AFIC takes a look at the latest scientific understanding of carbohydrates and their role in a healthy diet…. 

Carb Classics

First of all, what are carbs (or carbohydrates) and what do they do?  

Carbohydrates are one of the three macro-nutrients (fat and protein are the other two) that we get most of our energy (measured in Kcalories or Kjoules) from. Examples of carbohydrates are sugars (as in table sugar as well as those found in fruit and grains) and starches (found in rice, wheat and cereal products like bread). Some carbohydrate containing foods such as whole grains, fruits and vegetables also contain dietary fibre which plays an important part in our health. The human body cannot digest fibre and hence fibre does not contribute any energy to the diet – but it does reduce the risk of weight gain, heart disease and bowel cancer. Most health authorities recommend consuming at least 25-30g of fibre in the diet and this amount is easily obtained by eating moderate amounts of carbohydrate containing foods in the form of whole grains, legumes, fruits and vegetables. 

Varying amounts of simple (such as sugars) and complex (such as those containing starch) carbohydrates are naturally present in fruits, vegetables, grain foods and many dairy products. Carbohydrates may also be added into processed foods for example as thickeners for gravies and sauces.  

Carb Quality

In addition to this classification of carbohydrates into simple and complex, more recently there has been a great deal of interest in understanding the impact of dietary carbohydrates on glucose levels in the blood. The glycaemic index is a classification system based on the ranking of carbohydrates, according to their immediate effect on blood sugar levels. Foods with a high glycaemic index (GI) increase blood sugar quickly after consumption; and those with a lower GI break down more slowly and have a more gradual and sustained effect on blood sugar. Foods with little or no carbohydrates (like meats and eggs) have no GI. 

Researchers have been investigating how GI may play a role in the body’s sensitivity to the hormone insulin and the production of other hormones which are involved in the regulation of appetite, blood fats and blood sugars. For example, foods with a low GI result in lower insulin levels overall, and enable people to feel full longer, delaying the return of  appetite after a meal. The lower levels of insulin also result in lower levels of fats, such as cholesterol and triglycerides in the blood.  

Promisingly, several large-scale epidemiological studies have shown an association between a low GI diet and a lower risk of Type 2 diabetes. While the reasons for the association are still not fully understood there is a growing body of evidence that those with or at high risk of developing Type 2 diabetes may benefit from a low GI diet. 

However there are a number of critics of the glycaemic index, in particular some health professionals believe GI may be too cumbersome to be a reliable tool as a dietary guide for consumers. For example, the GI values of individual foods are not good indicators of the impact these foods will have on blood glucose when consumed together. As most foods are consumed with a variety of other foods in meals and snacks, this information would be more useful than the individual food GI values but it’s clearly not feasible to determine GIs of all possible meal combinations. Also, the GI of a food differs slightly between individuals and can also be affected by how the food is cooked, the plant variety and method of growing. Take apples for example:  According to the University of Sydney’s GI website, apples from Denmark were found to have a GI of 28 and the GI value of those from the United States was 40.  

Curbing Carbs to Cut Weight?

Studies have shown that generally, people on low-carb diets do lose weight in the short term. In addition, after being on the low-carb diet for six months, study participants have better blood triglyceride levels, increased levels of “good” cholesterol and better blood sugar control. 

However researchers attribute the gains to actual weight loss rather than the reduction in carbs. Generally any diet that limits the amount of carbs one is allowed to eat will also reduce total calories consumed. Hence people on low-carb diets usually eat less total calories than they regularly do. Many researchers believe that the same results can be achieved by reducing the total amount of food consumed, irrespective of food group included or excluded. 

A study conducted by Dr Michael Dansinger, of Tufts-New England Medical Center, Boston published in the Journal of the American Medical Association in January this year showed that people lost weight on any diet – the length of time they stayed on a diet, not the type of diet – determined how much weight they lost. Also, the gains in blood triglyceride levels, cholesterol and blood sugar were seen in all diets whether they were low-carb, low fat or just calorie controlled.  

Furthermore some researchers point out that low-carb diets that are usually high in protein and fats may exert an unusual burden on the kidneys. Hence they may not be suitable for some one who is diagnosed with a kidney problem or equally importantly, anyone who may not be aware that they have kidney problems. Diets high in saturated fats and cholesterol may also increase the risk of heart disease and some forms of cancer in the long-term.  

Nevertheless all the focus on carbs have at least highlighted that not all carbs are created equal. If there is a lesson from the carb-craze, it is that optimal diets should feature whole grains, fruits and vegetables and be low in sweets, desserts, pastries and products made from refined white flour. 

Protein Power

The focus on carbs has also highlighted the role of protein in weight management and the feeling of satiety. A growing body of evidence points to eating more lean protein to help satisfy hunger better and reduce the amount of food consumed.  

In a study conducted at the University of Washington, participants given the freedom to eat as much or as little as they wanted of a high-protein diet actually ate fewer calories and lost an average of 11 lbs (5 kg) in 12 weeks.  

The CSIRO Total Well-Being Diet and the “Sensible Solution” diet proposed by Dr Donald Layman of the University of Illinois both adopt the same model – about 30% of total calories from protein, about 15% from fat and the rest from carbohydrates consisting of mainly whole grains, fruits and vegetables. 

Indeed recent studies are beginning to indicate that a diet based on moderate amount of carbs (comprising mainly whole grains, fruits and vegetables), lean protein (fish, lean poultry and beans) and low in fat may be the best mix for weight management and the prevention of chronic diseases like Type II diabetes and heart disease.  

Carbs in Perspective

Eating most of your carbs in the form of whole grains, fruits and vegetables not only helps to provide adequate amounts of energy in a healthful form, it reduces the insulin demand exerted on the body, and ensures adequate consumption of dietary fibre, vitamins and minerals that help to promote overall health. This is helpful in reducing the risk of weight gain, Type 2 diabetes and heart disease. 

Although evidence is emerging that replacing some of carbohydrate in the diet with protein may be helpful for weight management, increasing the amount of protein to say 30% of Kcaloric intake may not be safe for people with kidney problems, diabetes or glucose intolerance. 

Finally, at the risk of being repetitive, while the proportions of carbohydrates, protein and fat may matter in weight management, the cardinal rules of limiting total caloric intake and increasing physical activity are still the keystone for effective, long-term weight management.  

Further Reading

(1)   Comparison of the Atkins, Ornish, Weight Watchers, and Zone Diets for Weight Loss Dansinger et al. JAMA.2005; 293: 43-53.

(2)   CSIRO Total WellBeing Diet http://www.csiro.au/index.asp?type=division&id=Human%20Nutrition&style=division