Friday October 30 2009
Fructose syrup is used to sweeten some soft drinks
A common sweetener is a “silent killer” according to the Daily Express, which today claimed that new research on fructose sugar shows that millions of Britons are at risk of high blood pressure from consuming it.
The claim that fructose is a “silent killer” is not justified based on this research. The study in question has so far only been presented at a scientific conference and only limited details of its methods have been made available. It is, therefore, too early to draw firm conclusions from its results. However, even at this stage, it has obvious limitations. For example, this type of study (called a cross-sectional study) cannot establish whether one factor causes another. It is also difficult to single out the effects of an individual substance in the diet, and a high intake of fructose may simply indicate unhealthy food choices.
This study is not a reason to panic about fructose. Fructose is found in fruit, which is an important part of a balanced diet. The message that we should eat a healthy, balanced diet is not altered by this study, and fructose can form part of this diet, although cakes, confectionary and sugary drinks that contain it should be eaten in moderation.
Where did the story come from?
Dr Diana I Jalal and colleagues from the University of Colorado carried out this research, which was presented at the American Society of Nephrology’s Renal Week conference. No sources of funding for the study were reported in the abstract of this presentation, which is available online. The study has not yet been published in a peer-reviewed journal.
What kind of scientific study was this?
This was a cross-sectional study which looked at the relationship between fructose intake and blood pressure. Fructose is a simple sugar found in many foods. For example, table sugar contains a sugar called sucrose, which is made up of one molecule each of glucose and fructose bound together. Fructose occurs naturally in foods such as fruit, but may be added as an ingredient in products such as soft drinks or cakes.
This research has not yet been fully published and only limited details of its methods were available in the form of an abstract to accompany a conference presentation. From this abstract, the study’s methods cannot be fully appraised.
The study looked at 4,528 adults who had not been diagnosed with high blood pressure. These adults were taking part in a large survey called the National Health and Nutrition Examination Survey, which was conducted in the US. They completed a questionnaire about their diets, including various items containing fructose, such as fruit juices, soft drinks, bakery products and sweets. Answers were used to calculate their fructose intake. Although fruits contain fructose, they were not included in the calculations as “their high content of ascorbate, antioxidants and potassium … counter the effects of fructose”. The participants’ blood pressure was also measured.
The authors used a technique called “multivariate logistic regression analysis” to look at the relationship between fructose intake and blood pressure. This analysis took into account other factors that may affect results, including demographics, comorbidities, physical activity, total calorie intake and dietary confounders such as salt and vitamin C intake.
What were the results of the study?
The researchers found that, on average, people consumed 74g of fructose a day (median), which they say is the equivalent of about two-and-a-half sugary soft drinks. In analyses that had not been adjusted to account for other factors, fructose intake at this average level or above was associated with a 33% increase in the odds of having an elevated blood pressure measurement, defined as 140/90mmHg or higher [odds ratio 1.33, 95% confidence interval 1.09 to 1.62].
After the researches took into account factors that could affect their results, this increased level of fructose intake was still associated with higher odds of elevated blood pressure. These adjusted analyses found that the odds of having a measurement of 140/90mmHg were increased by 36%, and of having a measurement of 160/100mmHg by 87%.
What interpretations did the researchers draw from these results?
The researchers concluded that “these results indicate that high fructose intake in the form of added sugars is significantly and independently associated with higher blood pressure levels in the US adult population with no previous history of hypertension.”
What does the NHS Knowledge Service make of this study?
Limited details of the methods of this study were available from the study abstract. Therefore, its methods could not be fully appraised. Until it has been fully peer reviewed and published, it is not possible to draw any firm conclusions based on its results. However, there are a number of points to note:
- The abstract does not say how blood pressure was measured. Blood pressure can vary depending on how and when it is measured, and the reliability of the measurement will depend on how well it was measured. Also, a single elevated blood pressure measurement would not be sufficient to diagnose high blood pressure (hypertension). This measurement would need to be repeated on a different day to confirm the diagnosis.
- The exact details of the factors taken into account in the analysis were not clear. If important factors were not taken into account, the differences seen in blood pressure could be due to these other factors and not just fructose. It is difficult to single out the effects of an individual nutritional component, and a high intake of fructose may simply have been an indicator of a more unhealthy diet.
- Fructose intake was based on the participants’ self-reported diets. These intakes would be affected if there were inaccuracies in their reporting.
- The study was cross sectional, which means it is not possible to establish whether one factor causes the other as they are both measured at the same time.
- The abstract did not contain figures that show exactly what proportion of people had high blood pressure measurements. The number of people affected could be very small, and presenting only the increase in odds could make the increase look larger than it actually is.
- The abstract reports that previous studies have had inconsistent findings regarding the link between excessive fructose intake and high blood pressure.
This study is not a cause to panic about fructose. Fructose is found in fruit, which is an important part of a balanced diet. The message that we should eat a healthy, balanced diet is not altered by this study, and fructose can form part of this diet, although pastries, cakes, confectionary and sugary soft drinks should be consumed in moderation.