Why do many high-fibre foods still have a high GI value?
Dietary fibre is not one chemical constituent like fat and protein. It is composed of many different sorts of molecules and can be divided into soluble and insoluble types. Soluble fibre is often viscous (thick and jelly-like) in solution and remains viscous even in the small intestine. For this reason it makes it harder for enzymes move around and digest the food. Foods with more soluble fibre, like apples, oats, and legumes, therefore have low GI values.
Insoluble fibre, on the other hand, is not viscous and doesn’t slow digestion unless it’s acting like a fence to inhibit access by enzymes (eg the bran around intact kernels). When insoluble fibre is finely milled, the enzymes have free reign, allowing rapid digestion. Wholemeal bread and white bread have similar GI values. Brown pasta and brown rice have similar values to their white counterparts.
After reading your book I thought the idea of finding and eating low GI carbs seems very logical. So I was surprised to find that the American Diabetes Association has taken a position that is not supportive of this research. The comments I read in a Guest Editorial of their Sept 2005 issue of Diabetes Forecast suggest your approach may be more of a fad than a useful tool.
Our response: We found this Guest Editorial rather odd because the American Diabetes Association has given cautious endorsement to the glycemic index. In its September 2004 Statement on carbohydrates one of the final recommendations is ‘the use of this technique can provide an additional benefit over that observed when total carbohydrate is considered alone.’ (Nancy Sheard et al. Diabetes Care 2004; 27: 2266).
Our advice? In Low GI Eating Made Easy, dietitian Kaye Foster-Powell says: ‘On a day-to-day basis, low GI foods can minimise the peaks and troughs in blood glucose that make life so difficult when you have diabetes. Since they are slowly digested and absorbed, low GI foods reduce insulin demand—lessening the strain on the struggling pancreas of a person with type 2 diabetes and potentially lowering insulin requirements for those with type 1 diabetes. Lower insulin levels have the follow-on benefit of reducing the risk of large blood vessel damage, lessening the chance of developing heart disease. There isn’t any one optimum diet for all people with diabetes. Whether you eat higher fat, low fat, high protein, high carb or whatever, certain characteristics are desirable. They are to eat:
- Regular meals and choose slowly digested carbs with a low GI
- Plenty of vegetables and fruits
- Only small amounts of saturated fat
- A moderate amount of sugar and sugary foods
- Only a moderate quantity of alcohol
- A minimum amount of salt and salty foods