A small-scale study in the US has shown that a stringent calorie-restricted diet may produce large reductions in heart disease and diabetes risk.
The researchers, from the Washington University School of Medicine, St Louis, studied the effects following a calorie-restricted diet of between 10-25 percent less calorie intake than the average western diet. The study appeared in the online version of the journal The Proceedings of the National Academy of Sciences on 19 April and will be published in the 27 April 2004 edition. It offers further support to idea that eating a varied, balanced diet and staying at a healthy weight by being physically active will help to lower your risk of developing heart disease.
What is a calorie-restricted diet?
A calorie-restricted diet is one where a person eats 10-25 percent less calories than an average western person. In this study, the calorie-restricted diet individuals ate 1,112-1,958 kcal/day of energy.
Because there is the danger that people following such a diet might not get the full range of vitamins and minerals they need, individuals following a calorie-restricted diet eat nutrient-dense foods. To do this they eat a wide variety of vegetables, fruits, nuts, dairy products, egg whites, wheat and soy proteins and meat. In this way, they ensure that they get more than 100 percent of their RDI (recommended daily intake) for all essential vitamins and minerals.
In addition to a restricted calorie intake, the people avoid all processed foods containing trans-fatty acids and refined carbohydrates in the form of desserts, snacks and soft drinks.
How does a calorie-restricted diet compare with a normal western diet?
The average american takes in almost twice as much energy as a calorie-restricted dieter: roughly 1,976-3,537 kcal/day.
Also, because the average american diet contains less fruit and vegetables and more processed foods than a calorie-restricted dieter, the composition of what an average american eats is different too.
Normal american diet:
Total energy intake – 18 percent protein, 32 percent fat, 50 percent carbohydrates.
Total energy intake – 26 percent protein, 28 percent fat, 46 percent complex carbohydrates.
Why do some people follow a reduced-calorie diet? What benefits do they believe they will gain?
Some people believe that calorie-restricted diets will confer a longer life-expectancy and help to counter the effects of ageing.
These beliefs are based on the results of animal studies that show that calorie reduction can increase life expectancy by up to 30 percent. So far, these studies have been carried out in rats, mice, fish, worms and various insects.
These findings also have support from an observation during the Second World War. During the war some of North European countries experienced a food shortage and this resulted in rationing. In some of these countries, rationing was accompanied with a sharp fall in mortality from coronary artery disease (heart disease). When the war ended, mortality from coronary artery disease rose sharply.
For these reasons, a small number of individuals have been following calorie-restricted diets for a number of years to try to extend their lifespans beyond the usual range.
What were the researchers aiming to achieve with this study of calorie-restricted dieting?
The researchers wanted to conduct the first human study into the effects of a calorie-restricted diet. They were particularly interested in finding out whether or not such a diet really did have an effect on a person’s risk of developing heart disease.
One of the main causes of heart disease (and strokes) is the narrowing of a person’s arteries through a process called atherosclerosis. Atherosclerosis is the plaque build up on the arterial walls that can impair blood flow and leads to formation of a clot, which can cause a heart attack or a stroke. It is the leading cause of death in the western world.
For this reason, the researchers decided to measure the individual’s risk of developing atherosclerosis. To do this, they measured a person’s risk factors for this condition.
What are the risk factors for developing atherosclerosis?
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There are a number of risk factors for atherosclerosis, and many of them can be reduced by eating a healthy, balanced diet and being physically active.
The risk factors for developing atherosclerosis are:
- being overweight or obese (having a body mass index – BMI – of more than 25)
- high total cholesterol levels
- high levels of LDL (bad) cholesterol
- low levels of HDL (good) cholesterol
- high ratio of total cholesterol to HDL cholesterol
- high triglyceride levels
- high blood pressure
How was the study carried out?
The researchers decided to measure the risk factors for atherosclerosis in people who had been following a calorie-restricted diet for a number of years and compare them with those of people following a normal american (or western) diet. The researchers were interested to see if the people following a calorie-restricted diet had lower levels (and hence lower risk of atherosclerosis) than healthy people who ate a normal western diet.
Who was studied?
The study followed 18 individuals from Caloric Restriction Optimal Nutrition Society who were following calorie-restricted diets. They were aged 35-82 years old, with an average age of 50 years. They had been following calorie-restricted diets for 3-15 years, the average length of time was 6 years. None of them were smokers and none were taking medicines that would affect their blood pressure or cholesterol levels.
To compare the calorie-restricted dieters to people eating a normal western diet, the researchers used a comparison group of 18 healthy americans. These 18 people were matched against the 18 calorie-restricted dieters for age and socioeconomic status.
Could the effect of starting a calorie-restricted diet on an individual be followed?
Another interesting part to this research was that the effect of starting and then following a calorie-restricted diet could be studied. The researchers discovered that 12 of the calorie-restricted diet individuals had medical data for their blood pressure, serum lipid levels and lipoprotein levels from the time before they started calorie restriction. These levels could be compared with the results of starting and then maintaining a calorie-restricted diet.
All 12 individuals had levels that were similar to normal americans before starting calorie-restricted diets. After starting a calorie-restricted diet, all 12 experienced their greatest changes in blood pressure, serum lipid levels, lipoprotein levels and weight loss in the first year of dieting. These levels were then maintained or decreased further with continued calorie restriction.
How did calorie restriction affect body mass index (BMI)?
Body mass index (BMI) was significantly lower in the calorie-restricted group when compared with the matched group; 19.6 compared with 25.9. The BMI values for the comparison group are similar to the mean BMI values for middle-aged people in the US.
All those on calorie-restricted diets experienced reductions in BMI after starting their diet. Their BMIs decreased from an average of 24 (range of 29.6 to 19.4) to an average of 19.5 (range of 22.8 to 16.5) over the course of their dieting (3-15 years). Nearly all the decrease in BMI occurred in the first year of dieting.
How did calorie-restriction affect cholesterol and triglyceride levels?
It was found that the average total cholesterol and LDL (bad) cholesterol levels for calorie-restricted individuals were the equivalent of those found in the lowest 10% of normal people in their age group.
It was found that the average HDL (good) cholesterol levels for calorie-restricted individuals were very high – in the 85th to 90th percentile range for normal middle-aged US men.
Because of these very healthy results, calorie-restricted dieters had remarkably low total cholesterol to HDL cholesterol ratios, a very positive sign for lowering the risk of heart disease.
These positive changes in calorie-restricted individuals were found to occur mainly in the first year of dieting. When the levels of the 12 individuals who had information for their cholesterol levels before they began a calorie-restricted diet were studied, it was found that they had close to average levels for normal middle-aged americans. However, following the calorie-restricted diet brought about large improvements in cholesterol levels and most of this occurred in the first year of dieting.
And finally, it was found that the calorie-restricted group had remarkably low triglyceride levels. In fact, they were as low as lowest 5 percent of americans in their 20s. This is more remarkable when it is noted that the calorie-restricted individuals were actually aged between 35 and 82 years.
How did calorie-restriction affect blood pressure levels?
Both systolic and diastolic blood pressure levels in calorie-restricted group were remarkably low, about 100/60, values normally found in 10-year-old children. In contrast, the average blood pressure for the normal diet group was about 130/80 (which is standard for the typical american).
For the 12 individuals who had medical data concerning their blood pressure before they started calorie restriction, it was found that their starting blood pressures were similar to those of the comparison group. Their large decreases in systolic and diastolic blood pressures occurred during the first year of following a calorie-restricted diet with a further decrease to extremely low levels in the period after that.
How did calorie-restriction affect insulin and glucose levels?
Fasting plasma insulin concentration was 65% lower and fasting plasma glucose concentration was also significantly lower in the calorie-restricted group when compared with the comparison group.
What do all these findings mean?
This means that the results show that calorie restrictions results in large, sustained, beneficial changes to a person’s risk factors for atherosclerosis: total cholesterol, LDL cholesterol, HDL cholesterol, triglyceride levels and blood pressure.
Calorie-restriction also provides a powerful protective effect against obesity and insulin resistance (a risk factor for developing type 2 diabetes).
Could the findings be due to vitamin supplements the people were taking?
It appears that taking vitamin supplements made little difference to the results of the study.
Many of the calorie-restricted individuals and comparison subjects took a wide range of supplements. While four calorie-restricted dieters and three comparison subjects did not take any supplements at all. The researchers found that serum lipid, lipoprotein and blood pressure levels of those not taking supplements showed no statistically significant difference from those who were taking supplements.
Can the results of the study be trusted?
While the results of the study appear to be compelling, it must be remembered that the results are based on a very small sample of people – only 18 people who followed the calorie-restricted diet. This means that it is difficult to be certain that these findings are definitely the result of calorie restriction and not some other factor. Much larger trials will be needed to confirm these findings and the study’s researchers are running a larger controlled, prospective study to do this.
Also, the current study only followed people on a calorie-restricted diet for an average of six years. This means that the long-term effects of calorie restriction are unknown and its long-term safety is still unclear. For this reason, the researchers will be running their new study for a longer period.
Is a calorie-restricted diet recommended?
Because there is still uncertainty surrounding the effects and safety of a calorie-restricted diet, the medical community is cautious about recommending this approach as a way of reducing heart disease and diabetes risk. However, the results of this study have been welcomed by UK doctors as further support of the health benefits of eating a healthy, balanced diet and maintaining a healthy weight by being physically active.
What can we do to reduce our risk of heart disease, stroke and type 2 diabetes?
The current advice is:
- keep at a healthy weight or lose excess weight if necessary (you should aim for a BMI of 25 or less)
- reduce the amount of salt you eat (to less than 6g a day)
- eat five portions of fruit and vegetables a day
- give up smoking
- learn to relax and reduce stress
- cut down on alcohol (Men should aim to drink no more than 3-4 units of alcohol per day with a maximum of 21 units a week. Women should aim for no more than 2-3 units per day with a maximum of 14 units a week.)
- aim to be physically active for about 30 minutes on five or more days of the week