The science of blood sugar

January 31, 2006

19 Min Read
The science of blood sugar

As growing numbers of people struggle with obesity, glycaemic monitoring is catching on — and will be featured as a special topic at the Nutracon conference. But while the glycaemic index is gaining cachet on packaging, Patrick Holford shows that a better measure of how foods affect blood sugar is actually the 'glycaemic load'

People are eating less. Calorie consumption has gone down. Fat intake has gone down even more. Yet obesity continues to rocket. Why? The simple reason is that we are consuming more sugar, refined foods and caffeinated drinks. These upset the body's blood-sugar balance, which causes weight gain.

Dr Atkins was right in this regard. He maintained that the best way to stabilise weight was to stabilise blood sugar by limiting the amount of carbohydrates consumed, substituting protein and fat in their stead. This approach works, but the side effects aren't worth it — increased risk for kidney problems, osteoporosis, and breast and prostate cancers. All of these diseases have been linked to high-protein diets based on meat and dairy products.

There must be a better way — and there is. The 'GI' or glycaemic index of foods is a measure of how much of the sugar within a food raises sugar in the blood. It's an index, which means that each food is compared to glucose, with a GI score defined as 100. Oats, for example, score 49. This means that the sugar in oats raises a body's blood sugar level half as much as 50g of glucose. It is measured by feeding volunteers however many oats they need to eat to obtain 50g of available carbohydrate (fibre doesn't count).

Load trumps index
Diets based on eating low-GI foods have a major problem. They are unquantifiable because the GI of foods only tells about the quality of the sugars in a food with respect to raising blood-sugar levels. The GI score tells nothing about the quantity of carbohydrate within a food.

For example, carrots and chocolate contain a similar GI score. However, carrots contain a fraction of the amount of available carbohydrates compared to chocolate. Hence, you would need to eat a lot more carrots to end up with the same effect on blood-sugar levels.

On the other hand, by knowing both the quantity of carbohydrates eaten as well as the GI of the food in question, one can determine the total glycaemic load, or GL, of a food. It measures what each piece of food actually does to one's blood sugar and body weight. It is calculated by multiplying the amount of carbohydrate in, for example, a slice of bread, and the GI of that particular bread.

Low-GI diets are therefore only crude indicators of the benefits of low-GL diets because it is possible to eat low-GI foods in abundant quantity, resulting in a high-GL diet. GL is therefore a superior, quantifiable and accurate measure of a diet's effect on blood-sugar balance.

The formula is very simple: If a person eats 40 GLs a day, he or she will lose weight; 60 GLs a day assures the maintenance of an even energy level.

GL studies on weight
GL principles show exactly what foods do to blood sugar and, consequently, body weight. A tightly controlled animal study demonstrated the GL's benefit for weight loss. One group of rats was given a high-GL diet, the other a low-GL diet. They had exactly the same amount of calories.1 The low-GL rats gained no weight, whereas the high-GL diet rats gained weight, week on week. By the end of the 32 weeks, they were not only 16 per cent heavier, they had gained 40 per cent more body fat. Same calories — very different results.

Does the same thing happen to humans? One human study compared weight loss between dieters placed on a low-GL diet vs a conventional low-fat, calorie-controlled diet, each diet providing the same number of calories. Researchers in the Department of Human Nutrition at South Africa's University of the Orange Free State assigned 15 volunteers to a low-GL diet and 15 others to a normal, calorie-controlled diet for 12 weeks.

During the first 12 weeks, both groups lost weight, but those on the low-GL diet lost 1.9kg more weight (9.3kg vs 7.4kg). After the initial 12 weeks, seven subjects who had been on the low-GL diet and nine subjects who had been on the conventional diet switched diets for 12 weeks.

During the second 12-week period, those placed on the low-GL diet had an average further weight loss of 2.9kg more than those on the low-fat, low-calorie diet (7.4kg vs 4.5kg). That equates to 10 per cent weight loss compared to seven per cent — 40 per cent more weight loss on the GL diet. Of note, fasting insulin concentrations were significantly lower in the low-GL group. Lower fasting insulin is associated with decreased risk of obesity, diabetes and heart failure.2

Low-GL diets cause more weight loss on identical calories. Animals fed high-calorie diets providing the identical number of calories show less weight gain and fat gain on a low-GL diet. Two groups of rats were fed identical diets, except one group (n=10) ate low-GL and the other (n=11) ate high-GL food. The low-GL fed rats gained no weight. But the high-GL rats gained weight week on week. By the end of 32 weeks, the low-GL group was 14 per cent lighter and had 29 per cent less body fat.3

Two studies show that low-GL diets cause weight loss that is more likely to be sustained than conventional dieting. In a study published in the Canadian journal Cardiology, one group of people was put on a low-GL diet and another group on a conventional low-fat, low-calorie diet. Those following the low-GL diet lost significantly more weight compared to those following the conventional low-fat, low-calorie diet (2.8kg loss vs 0.2kg gain). Weight loss was sustained over 12 months. 4

Researchers at Harvard Medical School put a group of 14 obese teenagers on either a low-GL diet with no calorie restrictions or a low-fat, low-calorie diet for six months. After six months on the diet and a further six months 'off the diet,' the low-GL group had significantly reduced body mass index (BMI) while those on the low-fat diet increased their BMI (-1.3 vs +0.7).5

Low-GL diets can also cause rapid fat loss. Studies show that low-GL diets cause fat loss, while high-GL diets cause rapid fat gain, demonstrating that sugar and refined carbohydrates promote weight gain as fat. In a study published in Lancet, two groups of mice (n = 21) were fed either low-GL or high-GL diets for 18 weeks. The researchers controlled the rats' feeding to ensure that the mice from both groups maintained the same average body weight throughout. The group on the low-GL diet had almost half the body fat and significantly more lean body mass compared to the high-GL group.6

One of the main problems associated with dieting is that the body responds to a low-calorie intake by reducing the body's metabolic rate, hence burning fewer calories. This means that dieters find it increasingly difficult to lose weight. Low-GL diets cause half the reduction in metabolic rate compared to a typical low-fat diet.

In a study published in the Journal of the American Medical Association, 39 overweight or obese adults were assigned to one of two diets. One group was on a low-GL diet, the other group followed a conventional low-fat, low-calorie diet. They were then followed for two years while on these diets. Each person had his or her metabolic rate measured once 10 per cent of body weight had been lost. The group on the conventional low-fat, low-calorie diet had almost twice the reduction in metabolic rate compared with the low-GL group.7

As well as losing more weight and more fat on the same calories, people who eat a low-GL diet eat significantly less because they feel more satiated. Thirty-nine overweight or obese young adults, aged 18 to 40, followed a calorie-restricted diet, either low-GL or low-fat. Participants on the low-GL diet reported less hunger than those on the low-fat, low-calorie diet.7,8

It can also be said that GL diets are more effective than high-protein diets. High-protein diets are not significantly better at causing sustained weight loss when compared with a conventional low-fat, low-calorie diet. As shown earlier, low-GL diets are superior to conventional low-fat, low-calorie diets. Therefore, GL diets are superior to high-protein diets.

This was borne out when researchers at the University of Pennsylvania, where two groups of obese people were put on either a high-protein, low-carbohydrate, high-fat diet or a low-calorie, low-fat, high-carbohydrate (conventional) diet. Whilst the high-protein group had lost more weight after three and six months on the diet, after 12 months there was no significant difference between the groups.9

Researchers at Australia's CSIRO Health Sciences and Nutrition Centre put 100 overweight middle-aged women onto either a high-protein or a high-carbohydrate diet for 12 weeks. Both groups were restricted to 1,333 calories per day. Both groups of women lost the same amount of weight (7.3kg), showing that a high-protein diet is similar to a conventional low-fat, low-calorie diet.10

A review of all the high-protein/low-carbohydrate diet studies done to date concludes, "Weight loss was principally associated with decreased calorie intake and increased diet duration but not with reduced carbohydrate content."11

GL's other benefits
Many studies show that low-GL diets have immediate and long-term health benefits, including improving cholesterol levels, stabilising blood glucose levels and improving insulin sensitivity, and reducing diabetes and cardiovascular disease risk.

Two groups of overweight or obese people followed either a low-GL diet or a low-fat, low-calorie diet for two years. After each person had lost 10 per cent of body weight, other health measures were taken. Those on the low-GL diet had greater improvements in insulin resistance (blood-sugar control), triglycerides (fat circulating in the blood), inflammation and blood pressure compared with those on the conventional low-fat low-calorie diet. The researchers concluded that a reduction in glycaemic load may aid in the prevention or treatment of obesity, cardiovascular disease and diabetes mellitus.7

One group of people followed a low-GL diet while another group followed a conventional low-fat, low-calorie diet. Those following the low-GL diet not only lost more weight, they also had greater improvements in HDL cholesterol, triglycerides and fasting glucose compared to those on the conventional low-fat, low-calorie diet after six months. These health gains were sustained or improved upon after 12 months. The researchers concluded that 'implementation of a low-GL diet is associated with substantial and sustained improvements in abdominal obesity, cholesterol and blood-sugar control.'4

Diabetes markers can also be positively changed with a low-GL diet. Researchers at the Harvard School of Public Health monitored the health of 42,000 middle-aged men for six years and found that those who ate a high-GL diet were one-and-a-half times more likely to develop diabetes than those who ate a low-GL diet. This positive association between GL and diabetes risk was after taking into account other factors such as age, BMI, smoking, physical activity, family history of diabetes, alcohol consumption, cereal fibre and total energy intake.12 Similar results were seen in women.13

In obese adolescents, Harvard researchers found the same. Two groups of obese adolescents followed either an unlimited low-GL diet or a low-calorie, low-fat diet. In addition to losing more weight, those on the low-GL diet reduced their insulin resistance while those on the conventional low-fat, low-calorie diet worsened theirs. Increased insulin resistance is associated with increased risk of obesity, diabetes and cardiovascular disease.5

Comments on the science
The following points should be borne in mind when analysing this data. Participants in diet surveys and diet studies often under-report food intake. In comparative studies between two groups, this is unlikely to change the comparative effect because under-reporting may occur in both groups. The same applies for diet surveys comparing year on year.

Regarding calorific expenditure, it is difficult to accurately estimate energy expended in all physical activity. Therefore, it is plausible that decreased overall physical activity, despite apparent increase in actual exercise, may be a significant factor in the rate of obesity increase, at least in the UK.

Some of the studies referred to are animal studies and may not necessarily be extrapolative to humans. However, the animal studies are easier to control.

Some of the studies are small in number and therefore provide preliminary evidence. There is a need for larger controlled studies.

The studies refer to low-GL diets. Low-GL intake can be achieved in three ways: by reducing the amount of carbohydrates, reducing the intake of high-GI carbohydrates, or a combination of both. Which of these approaches is most effective is not yet known.

The studies referred to put people on a variable amount of GLs. So, 'low GL' describes any diet below 90 GLs. Some people call for a low-GL diet as being below 45 GLs. Further understanding of the ideal level of GLs for weight loss will become apparent as more studies are published.

Patrick Holford, BSc, is founder of the Institute for Optimum Nutrition. He published the first UK book on low-glycaemic load diets, The Metabolic Diet, in 1987. His most recent book is The Holford Low GL Diet, published in the UK (Piatkus, 2004) and in the US (Atria, 2006). Respond: [email protected]. All correspondence will be forwarded to the author.

1. Brand-Miller J et al. Glycemic Index and obesity. Am J Clin Nutr 2002; 76;281S 5S.
2. Slabber M et al. Effects of a low-insulin-response, energy restricted diet on weight loss and plasma insulin concentrations in hyperinsulinemic obese females Am J Clin Nutr 1994; 60(1):48-53.
3. Pawlak D, et al. Long-term feeding with high glycemic index starch leads to obesity in mature rats. Proc Nutr Soc Australia 2000; 24:215.
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10. Noakes M, et al. Effect of an energy-restricted, high-protein, low-fat diet relative to a conventional high-carbohydrate, low-fat diet on weight loss, body composition, nutritional status, and markers of cardiovascular health in obese women. Am J Clin Nutr 2005 Jun; 81(6):1253-4.
11. Bravata DM, et al. Efficacy and safety of low-carbohydrate diets: a systematic review. J Am Med Assoc 2003; 289(14):1837-50.
12. Salmeron J, et al. Dietary fiber, glycemic load, and risk of NIDDM in men. Diabetes Care 1997; 20(4):545-50.
13. Salmeron J, et al. Dietary fiber, glycemic load, and risk of non-insulin-dependent diabetes mellitus in women. J Am Med Assoc 1997; 227(6):472-7.

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