28 August 2006

Sugar's effect on cholesterol

A toffee here, a jelly baby there. Do these foods, and how much sugar you eat, have an effect on your blood-fat levels? DietDoc recently reviewed a number of relevant studies.

I recently had the privilege to do a presentation at the '2006 Sugar & Health Symposium', held in Phalaborwa. One of the papers I presented dealt with the subject of 'sugar and hyperlipidaemia'.

I had undertaken a study of the available scientific literature to see if sugar intake has an effect on blood-fat levels.

This was done as there is a belief among the public and the nutrition community that sugar increases certain blood fats and that this, in turn, increases the risk of developing heart disease.

What are triglycerides and HDL-cholesterol?
Triglycerides (TGs) are fats that, as the name implies, consist of three fatty acids attached to a molecule of glycerol. Information on the role these TGs play in heart disease is conflicting.

Because changes in TG levels in the blood are always associated with changes in HDL-cholesterol (HDL-C) levels, it is difficult for scientists to determine which one of these fats is important in preventing heart disease. Generally speaking, it is a good idea to have low TG levels.

HDL-C is regarded as 'good cholesterol' and it is desirable to have high HDL-C levels to prevent the risk of heart disease.

Researchers have found that when TG levels increase, HDL-C levels decrease – a sort of seesaw effect. So, they are not able to say which one of these changes increases the risk of heart disease.

At present, experts caution that we should have low TG levels and high HDL-C levels to protect us against heart disease.

The study
By scanning the scientific literature for the period 1997 to 2005, I found 29 publications referring to studies on TGs and HDL-C.

However, only five of these studies had actually determined how much sugar the participants had been eating.

The remaining 24 studies measured carbohydrate intake in terms of high and low glycaemic index (GI) (a diet with a high GI does not only contain sugar, but also a variety of highly processed starchy foods like white bread, white rice, cakes, biscuits, sweets and cold drinks), or in terms of simple and complex carbohydrates (a diet high in simple carbohydrates does not only contain sugar, but also all the foods listed for the high GI diet).

The small amount of evidence (only five studies) about the direct effect of sugar on TGs and HDL-C levels made it rather difficult to try and get a clear picture of what is happening when people eat a lot of sugar.

In addition, most of the factors in these 29 studies varied a lot. Factors such as the age, sex, weight and health status of the subjects, the length of the studies (this varied between a few days and many months), and the types of effects that were measured (all blood fats, just TGs or HDL-C, weight loss or the incidence of heart attack), varied from study to study.

This meant that comparisons were tricky because I couldn't compare "apples with apples".

The results
After I studied the 29 papers, I could basically make the following deductions:

a) High-carbohydrate or high-GI diets
These diets, which are rich in simple carbohydrates (refined sugars and starches), have the following effects:

  • they seem to increase blood triglyceride levels;
  • they seem to decrease protective HDL-cholesterol levels.

b) Divergent results
Because there was so much variation in the important factors that influence the results of these studies (the age, health, weight of subjects; the length of the studies; the outcomes that were measured), very divergent results were obtained that made it difficult to come to solid conclusions.

c) Larger populations for long periods
Those studies that used large study populations and monitored blood fat levels for long periods (exceeding six months), found that high carbohydrate diets, which initially raised TG and decreased HDL-C levels, did not do so after a few months.

This seems to indicate that human beings initially react to a high-carbohydrate diet with increased TG and lowered HDL-C levels, but that this effect wears off as the individual continues to eat the high-carbohydrate diet, so that TG levels return to normal after about six months.

One of the larger studies (CARMEN Study in the UK, 2000), with approximately 400 subjects, even found that over a period of six months, diets that were actually rich in simple sugars had no effect on blood fat levels.

In all the studies where the experimental subjects ate a diet that was rich in carbohydrates (some used refined starches and sugars, while other studies used unrefined starches only), and the subjects lost weight, the TG levels did not increase.

Furthermore, when the experimental subjects did 60 minutes of brisk exercise when they were eating a high-carbohydrate diet, their TG levels did not change.

At this stage, there are many aspects of the relationship between TG production and metabolism and its supposed link with heart disease that have not yet been explained.

The diet which is usually prescribed for heart patients with high TG levels is a low-fat, high-carbohydrate diet with a low GI, for example the American Heart Association Diet. Patients who suffer from heart disease and have high TG levels should consult a clinical dietician for such a diet prescription.

Finally, my literature study indicated that the Prudent Diet Recommendations, to eat a diet low in total and saturated fat and high in complex carbohydrates, with a moderate amount of simple carbohydrates such as sugar to increase the palatability of the low-fat diet, are still valid.

If you suffer from high TG levels and are at risk of heart disease, consult a dietician to obtain the correct diet prescription, which will be based on a low-fat, low-GI diet with small amounts of sugar to make the diet palatable.

Also remember to do 60 minutes of exercise a day to stabilise your weight and blood-fat levels. – (Dr Ingrid van Heerden, DietDoc, August 2006)

(Van Heerden, IV (2006). Sugar & Hyperlipidaemia. Paper presented at the 2006 Sugar & Health Symposium, Phalaborwa.)

Read more:
Cholesterol Centre
What happens to the fat you eat?


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