Intriguing Cochrane Study Findings on Cardiovascular Disease
This week I stumbled upon a new publication in the Cochrane Library. This particular publication looked at the link between following a low glycaemic index diet and cardiovascular disease rates. The reason this study caught my eye was because it appeared to contradict usual dietary advice on this topic.
Conventional thinking would associate low glycaemic index carbohydrates with improved cardiovascular outcomes but this isn’t always how research pans out. In fact, this is the beauty of combining evidence based research with practical experience. Doing this prevents anecdotal opinions and helps inform healthcare professionals like myself when advising patients.
Avoiding cardiovascular complications should be high on the agenda for people with diabetes. This is because glucose can damage the blood vessels leading to complications over time. Therefore, people with diabetes are at an increased risk of cardiovascular complications.
The Cochrane Library is one of the more credible publications in health care. They consist of various research groups dedicated to publishing high quality research. This helps people like me make more informed decisions on healthcare and diet based on evidence based literature. This is why when Cochrane publish something I tend to listen.
If you’re unsure what glycaemic index is I’ve blogged about this previously here. However, I’m a nice guy so I’ll give you a quick summary too.
Glycemic index is a measure of how quickly carbohydrates increase blood glucose levels. High glycaemic index carbohydrates enter your system very quickly. Whereas, low glycaemic index carbohydrates are absorbed quite slowly.
We can test this response by measuring glucose levels after eating carbs. The faster blood glucose levels increase the higher glycaemic index rating that food will receive. Each food receives a score between 0-100, with 100 being the highest score. These are then categorised into high, medium or low glycaemic index.
The first tables of glycaemic index were published in 1995. Recognition from the World Health Organisation soon followed in 1997. Since then glycaemic index has become an important component of type 2 diabetes care. Most patient education courses will likely teach glycaemic index. This is because low glycaemic index diets are beneficial in type 2 diabetes care because they help to lower glucose levels.
Cardiovascular disease is a group of conditions. These affect your heart and blood vessels and so impact your circulation. You’re probably familiar with several of the diseases. Coronary heart disease, cerebrovascular disease and peripheral arterial disease are just some examples.
We know about the association of dietary fat and cardiovascular disease. Contrary to media reports this is actually well documented. We can see eating high levels of saturated fat and/or trans fat increases the risk of cardiovascular problems in susceptible individuals. However, good fats like olive oil, oily fish, nuts and seeds help to reduce cardiovascular complications.
Research on low glycaemic index diets
There has also been a fair amount of research into the effect of low glycaemic carbohydrates and cardiovascular risk. So far the results appear to show us low glycaemic index foods help to reduce the risk of cardiovascular problems.
However, most of the research up until now is from observational studies (researchers watch from the sidelines). This is quite typical in studies on nutrition. The reason is because diet takes many years before it begins to impact health. Therefore, you need to follow patients in the study for many years. Observational studies usually manage this by asking participants to self report their dietary intake. This makes follow up easy. However, you can no doubt see this impacts the accuracy of data but it does give us an idea of what is happening.
Ideally we’d use a different kind of study called an intervention study. An intervention study inserts a controlled intervention, like a medication, and assesses the effect against a group without the intervention. Therefore, we can look at cause and effect. This is hard to do in nutrition because you cannot control peoples diets for years at time in large numbers. It just isn’t practical.
Observational studies cannot examine cause and effect because other factors may have caused the result. You are watching people in their natural habitat. Some will smoke, some will not, some will exercise, some will not. So there are many factors other than what you are studying which may influence the results. You can try to limit these factors at the start or when analysing the data but it is much harder in an observational study. Therefore, in order to get a valid result, you usually need large participant numbers.
Although the evidence from observational studies on this subject looks promising, the Cochrane review wanted to see if intervention studies mirrored the observational studies findings. In other words, can we say for sure low glycaemic index carbohydrates reduce the chances of developing cardiovascular problems?
The Cochrane review looked at a total of 21 studies with 2538 people included. Included studies must have compared high glycaemic index diets against low glycaemic index diets. They must have also lasted at least 12 weeks.
The study looked at death from cardiovascular disease, cardiovascular events, diagnosis of a cardiovascular complication or any other adverse event. They also looked at other variables such as weight, blood lipids, blood pressure and quality of life.
What they found
They found no reduction in cardiovascular disease when people ate a lower glycaemic index diet. In fact, no health marker improved including blood lipids or body weight.
When I initially read the abstract of the this article I was quite startled at this finding. This goes against most things I have assumed to be true through my training and professional career.
I’m glad I read the full article. Though the Cochrane review was conducted well, they say a workmen is only as good as his or her tools. The studies available for the Cochrane review to compare were mostly poor quality.
Most studies were only around 6 months in duration. It surprised me actually when they set the minimum duration for inclusion for 12 weeks. For diet to impact health to the point of disease it usually takes years. So I’m not too sure what they were expecting to find. I suspect they were unaware of the available studies upon starting but soon realised it was slim pickings.
The studies in this review also used differing amounts of low glycaemic index foods. This makes it difficult to compare against one another and makes findings inconsistent.
The numbers of participants wasn’t too bad when added together. Over 2000 participants is fairly reasonable in an intervention study. However, this would be more powerful if they were all in the same study as opposed to many different studies with different designs.
It didn’t surprise me either that the participants didn’t lose weight. Though lower glycaemic index foods are more nutritious they remain the same calories as their high glycaemic index counterparts.
This may also explain the lack of improvement in the participants blood lipids. Weight gain causes increased blood lipids as well as many other health problems. Therefore, if their weight was the reason for their elevated lipids weight loss would be the solution.
I think in summary, the best we may do with this type of question is to use obversataonal studies. Although we can’t establish true cause and effect with this type of research with a good study design and enough participants conclusions can be made. I’m still of the opinion lower glycaemic index options are better for you, particularly in diabetes care. However, we may need to wait longer before we can say for certain it reduces cardiovascular disease incidents.