The Evidence That Low Glycaemic Index Highcarbohydrate Diets Are Superior

It is now well established that both the type and amount of carbohydrate influences the degree of post-prandial glycaemia (34). The type of carbohydrate is best described by its glycaemic index, a ranking of foods according to their immediate effect on blood glucose levels (Figure 11.1). Per gram of carbohydrate a food with a GI of 80 (e.g. potato) has twice the glycaemic impact of a food with a GI of 40 (e.g. pasta) and this applies even in mixed meals (35,36) (Figure 11.2). The proportions of starch, sugar, fat or fibre in foods are not a good guide to GI. Many common starchy foods (even wholemeal versions) such as bread, rice and breakfast cereals have surprisingly high GI values, while foods containing sugars often have a relatively low GI (37). Reducing the overall GI of the diet involves substitutions within those food groups that contribute most of the dietary carbohydrate (Table 11.1).

The GI of foods is highly relevant to the management of Type 1 and Type 2 diabetes. In nine well-designed long-term studies in diabetic subjects, low-GI diets (GI values <55%) were shown to reduce glycosylated proteins (HbA1c and/or fructosamine) by an average of almost 11% over periods ranging from two to 12 weeks (38). At the end of the low-GI, high-carbohydrate diet, urinary C-peptide levels (a measure of endogenous insulin demand) fell by an average of 20%, daytime blood glucose levels decreased by 16%, and total cholesterol and TG were reduced by 6% and 9%, respectively (39). Triglyceride levels fell to a much larger extent (by up to 20%) in patients with overt hypertriglyceridemia. In a recent, randomised, cross-over study, clotting factors were normalised in patients with Type 2 diabetes by a low-GI, high-carbohydrate diet, but unchanged by a high-GI diet containing similar amounts of energy, protein, fat, carbohydrate, starch and fibre (40).

Studies comparing the effects of high-GI versus low-GI carbohydrate-rich diets have been longer (four to 12 weeks) than the high-MUFA studies, and unlike the latter, have been able to document beneficial changes in HbA1c and/or fructosamine levels. In the few studies that have directly compared high-carbohydrate, low-GI diets with high-MUFA diets, HDL levels were increased on both (compared to the

Figure 11.1 The derivation of the glycaemic index

high-carbohydrate, high-GI diet) but insulin secretory function and sensitivity appeared to be better only on the low-GI diet (41,42). Reductions in HbA1c or fructosamine levels of 10% on the low-GI diets have been criticised as being 'modest', yet changes of this magnitude are commonly seen with oral hypoglycaemic drugs. Furthermore, these 'modest' changes were achieved in patients in free-living conditions, not in a controlled metabolic ward situation. Although the high- and

□ Gins le food H Mixes Tisal

□ Gins le food H Mixes Tisal

Bmad Potatoes Fasta Rioo I citils I ontilE

Figure 11.2 The GI of single foods predicts the GI of mixed meals in subjects withType 2 diabetes

Source: Redrawn from Bornet etal. (35).

Bmad Potatoes Fasta Rioo I citils I ontilE

Figure 11.2 The GI of single foods predicts the GI of mixed meals in subjects withType 2 diabetes

Source: Redrawn from Bornet etal. (35).

Table11.1 A low-GI diet is achieved by substituting high-GI foods with GI alternatives. Breads, breakfast cereals and potatoes contribute the majority of carbohydrate in Western diets. Changes within these food groups have the biggest impact on the diet's overall GI

High-GI food

Low-GI alternative

Bread, ordinary wholemeal or white

Most breakfast cereals

Potato (all varieties) Most varieties of rice Cakes, biscuits and muffins Tropical fruits such as bananas

Bread containing a high proportion of wholegrains ('granary' breads), sour dough breads, stone ground breads

Unrefined cereal such as oats (muesli or porridge). Some processed cereals (e.g. All-Bran) Sweet potatoes, pasta, noodles, legumes Basmati or other high amylose rices Versions made with fruit, oats, wholegrains Temperate climate fruits such as apples and stone fruit low-GI diets were usually designed to be similar in macronutrient composition, in some studies the low-GI diet contained more fibre. Indeed in one outpatient study, dietary education with emphasis on low-GI foods resulted in higher carbohydrate and fibre intakes and less saturated fat intake than achieved by patients given 'traditional' dietary counselling (43). However, on the whole, studies comparing high- and low-GI diets have contained much less fibre (<50g per day) than the earlier studies that provided the basis for recommending high-carbohydrate diets in diabetes (>75g per day). Lastly, low-GI, high-fibre diets may be the only strategy (diet or drug) that enables HbA1c to be improved while simultaneously reducing the incidence of hypoglycaemic episodes in Type 1 diabetes (44).

These findings suggest that any adverse effect of high-carbohydrate diets on blood lipids is almost certainly linked to the high GI of most such diets. Indeed, any strategy that slows down the rate of digestion and absorption of carbohydrate (e.g. nibbling versus gorging, alpha-glucosidase therapy or purified supplements of viscous fibre, as well as low-GI diets) has been shown to improve glucose and lipid metabolism in diabetes (45).

Dieting Dilemma and Skinny Solutions

Dieting Dilemma and Skinny Solutions

The captivating thing about diets is that you don't get what is researched or predicted or calculated but rather, you get precisely what you expect. If the diet resonates with you then it will likely work, if it doesn't resonate, it won't.

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