abdominal obesity, is known to worsen insulin resistance and increase the risk of Type 2 diabetes (46).
The degree of insulin sensitivity is also affected by the energy content and macronutrient composition of the diet.
Epidemiological and dietary intervention studies in humans indicate that a high-fat, energy-dense diet promotes weight gain and the development of obesity (47), impairs insulin sensitivity and increases the risk of developing Type 2 diabetes (48).
Relatively high intakes of saturated fat appear to worsen insulin resistance and are also associated with higher blood levels of LDL-cholesterol and a greater risk of atherosclerosis (49,50).
Questions still remain about the optimal diet for improving insulin sensitivity in Type 2 diabetes.
It is well recognised that higher carbohydrate intakes are related to improved insulin sensitivity in non-diabetic individuals (26).
This is likely to be true in the early stages of Type 2 diabetes, but as pancreatic beta-cell function declines, higher carbohydrate intakes could compromise remaining insulin secretory capacity.
There are few studies that document changes in insulin sensitivity directly in diabetic subjects and these tend to be inconclusive.
Low-GI diets appear to improve insulin sensitivity in coronary heart disease patients (51,52) and animal models (53).
Indirect evidence suggests that the fibre content and GI of the diet may influence insulin sensitivity, weight gain and the risk of developing Type 2 diabetes.
In the CARDIA study of young adults, low fibre consumption predicted 10-year weight gain and fasting insulin levels (a measure of insulin resistance) more strongly than did total or saturated fat consumption (54).
Fibre but not amount and type of fat was associated with 2-h insulin levels.
Two other large- scale prospective studies in healthy subjects showed that diets based on low-fibre, high-GI foods doubled the risk of developing Type 2 diabetes, after controlling for known risk factors such as age and body mass index (55,56).
Importantly, the total carbohydrate and refined sugar content of the diet, and the amount and type of fat consumed, were not found to be independent risk factors in these studies.
DIETS FOR WEIGHT CONTROL: IS THE AMOUNT AND TYPE OF CARBOHYDRATE IMPORTANT?
Weight loss is usually a major treatment goal in Type 2 diabetes, but the ideal dietary composition for weight control is still the subject of debate.
Many health professionals are concerned that high-fat diets, irrespective of the type of fat, might promote weight gain.
The prevalence of obesity is often lower in people with high carbohydrate consumption (expressed as a percentage of energy) than in those with high fat intakes (but this is not always true).
In animal studies, high-fat diets induce faster weight gain and greater insulin resistance compared with high-carbohydrate diets, whether fed ad libitum or isocalorically (50).
In humans, several studies have shown that ad libitum
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Figure 11.3 Reduction in glycosylated proteins (glycosylated haemoglobin or fructosamine) on low GI in nine studies comparing high-carbohydrate, low- versus high- GI diets Source: Redrawn from Brand-Miller et al. (38).
consumption of high-carbohydrate diets, even one high in refined sugar, was more effective in promoting long-term weight loss than higher fat diets (47,57,58).
Despite this, results from several recent intervention trials have indicated that high-MUFA diets are just as effective as high-carbohydrate diets in producing weight loss in diabetic subjects (59,60).
However, in these studies MUFA was used in the context of a strictly controlled low-energy diet.
By directly controlling energy intake, any spontaneous reduction in energy intake and body weight associated with the high-carbohydrate or high-fibre diet was unlikely.
The long-term effect of ad libitum consumption of Western diets enriched in MUFA is currently not known.
There is concern that the promotion of energy-dense, high-MUFA foods to diabetic subjects [as recommended by the American Diabetes Association (20)] may lead to gradual weight gain.
THE SATIETY VALUE OF HIGH-CARBOHYDRATE DIETS
The satiating capacity of high-carbohydrate diets may be the major explanation for weight control benefits. The energy density of foods strongly influences the amount of food people consume and consequently influences body weight (61). High-fat foods are energy dense, very palatable and less satiating, a combination which makes them easy to ‘passively overconsume’
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(62).
On the other hand, less refined, ‘natural’ high-carbohydrate foods (legumes, wholegrains, fruits and starchy vegetables) are more bulky and difficult to overeat.
In laboratory studies comparing the short-term filling powers of equal-calorie portions of different foods, the weight of food per 1000 kJ was the strongest determinant of short-term satiety (63).
However, many new reformulated low-fat foods on the market (e.g. snack products, biscuits, ice cream, yoghurt) are as energy dense as their full-fat counterparts and unlikely to offer weight control benefits.
Low-GI diets may be particularly beneficial for weight control in people with diabetes.
Low-GI foods are more satiating, calorie for calorie, than their high- GI counterparts [reviewed by Ludwig (64)].
Low-fat, energy-restricted diets based on low-GI foods have been found to result in greater weight loss in overweight people than conventional reduced-fat diets with a higher GI rating (64,65).
Higher satiety resulting from the prolonged digestion and absorption of carbohydrate in the small intestine and the reduced post-prandial insulin secretion may explain these findings.
In animal studies, high-GI diets promoted faster weight gain, higher body fat, higher adipocyte volume and hyper- triglyceridaemia than low-GI diets providing similar amounts of energy and macronutrients (66,67).
High-GI diets were also associated with increased myocardial infarction in the Nurses’ Health Study (68).
EVIDENCE FOR SUCROSE RESTRICTION IN DIABETIC DIETS
Many randomised, controlled trials have shown that the isocaloric substitution of moderate amounts of refined sucrose for starch in diabetic diets has no adverse effects on blood glucose or lipid levels in people with diabetes (69, 71).
In fact, several studies show improved glycaemic control, especially in children with Type 1 diabetes (72).
This makes sense when we consider that most foods containing sugar have a GI less than 60, while that of most modern starchy foods is over 70 (37,73).
Many diabetes associations now officially recognise that sucrose restriction is not necessary in diabetic diets, although some put an upper limit of 30 g per day (the average intake in the non-diabetic population is about 60 g per day).
Unfortunately, the dietary dogma of sucrose avoidance in diabetic diets is so well entrenched in the mind of the public and most health professionals that little change has occurred in practice.
Intense sweeteners and low-joule soft drinks are almost universally recommended in diabetic diets in the belief that this will enhance both glycaemic control and weight loss.
This often detracts from more important dietary messages for people with diabetes (e.g. reduced saturated fat, increased high-fibre and low-GI foods).
The belief that sucrose facilitates excessive energy intake is one reason for continued use of intense sweeteners.
However, there is little evidence that the
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long-term use of artificial sweeteners is particularly useful for weight loss , reducing sugar intake saves fewer calories than reducing dietary fat by the same amount (74).
Some large-scale dietary surveys have shown that people who consume higher amounts of sugar and less fat tend to have lower body weights (75, 77).
Refined sucrose consumption correlates inversely with fat intake in both non-diabetic and diabetic populations.
In addition, research shows that a moderate, high intake of sugar is not associated with a reduced intake of vitamins and minerals (78).
One of the reasons for this is that sucrose increases the palatability and intake of nutritious foods such as cereals and dairy products.
Sucrose also satisfies an instinctual desire for sweetness and has many functional roles in foods that extend beyond its sweetening power, including preservative, textural and flavour-modifying qualities.
Fructose has also been used as a sweetener in diabetic diets because it has a smaller blood glucose (GI ¼ 20) and insulin-raising effect than isocaloric amounts of sucrose.
Concerns about its potential to raise TG and LDL- cholesterol levels have limited its use (20), but in amounts up to 12% of energy, no untoward effects have been seen in subjects with diabetes (79).
Other nutritive/calorie-containing sweeteners such as maltodextrins, corn syrup, fruit juice/concentrate, honey, molasses, dextrose and maltose do not offer any advantage over sucrose in terms of energy content or glycaemic response.
Indeed, post-prandial glycaemia is higher after maltodextrins and corn syrup than after sucrose.
Sugar alcohols (sorbitol, mannitol, xylitol) and isomalt used as sweeteners in sugar-free confectionery produce a lower glycaemic response than sucrose and inhibit dental caries formation.
Excessive consumption (420, 30 g per day) should be avoided because of their laxative effect.
REALISTIC DIET PRESCRIPTIONS
Weight loss and weight control are arguably the most challenging aspects of managing diabetes, yet are likely to offer the most immediate and obvious benefit.
People with diabetes find it more difficult to lose weight and maintain the loss compared with those without the disease.
Fortunately, it is now clear that they do not need to reach their ideal body weight in order to improve their metabolic status; as little as a 5, 10% reduction in body weight is sufficient to result in clinically relevant benefits (80).
Long-term weight control requires a comprehensive approach involving lifestyle changes, not just food and energy restriction.
A modest reduction in energy intake (about 250 to 500 calories from the daily energy intake) and an increase in daily physical activity by 250 to 500 calories are realistic.
A combination of strategies may help promote weight loss
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Emphasis on low saturated fat, low-GI, high-carbohydrate foods , to promote satiety and reduce hyperinsulinaemia. .
Modest caloric restriction not extreme , to prevent excessive hunger. .
Distribution of carbohydrate intake throughout the day , smaller more frequent meals to reduce post-prandial hyperglycaemia. .
Increased physical activity , even incidental activity , to promote higher energy expenditure. .
Behaviour modification techniques and relaxing activities , to reduce stress- related eating. .
Support from family and other professionals , to increase compliance.
No single dietary approach will be suitable for all patients.
Meal plans and dietary modifications need to be tailor-made to suit each patient’s needs and lifestyle.
Current medical status (HbA1c, blood lipid levels, home blood glucose monitoring results, nutritional status, body weight, medication) needs to be assessed before any dietary modifications are recommended.
Dietitians should reinforce that the dietary and exercise ‘prescription’ is an essential component of diabetes management, irrespective of medication.