“A diet rich in green leafy vegetables may reduce the risk of developing diabetes,” reported the BBC. It said that one-and-a-half portions a day “cuts type-2 diabetes risk by 14%”.
This news story was based on a systematic review and meta analysis that pooled data from six prospective cohort studies investigating diet and the risk of developing type 2 diabetes. The analysis found that people who ate around 120g of green leafy vegetables per day were 14% less likely to develop the condition than people who ate the least amount of this type of vegetable.
On its own, this study is not convincing evidence that simply eating green leafy vegetables reduces the risk of developing type 2 diabetes. It is not possible to say whether the small decreased risk this study found was due to particular compounds found in these vegetables or because the people who ate more vegetables tended to have a healthier diet and lifestyle.
In combination with other lifestyle choices however, a healthier diet may help to reduce the risk of diabetes. In people at risk, reducing the intake of total and saturated fat, increasing their intake of vegetables, fruit, and wholegrain cereals, and increasing physical activity is known to reduce the risk of diabetes by about 60%. This is thought to be mainly because these factors all work towards reducing weight.
Where did the story come from?
The study was carried out by researchers from the University of Leicester and was also funded by the university. The study was published in the peer-reviewed_ British Medical Journal._
This research was covered well by The Daily Telegraph and the BBC. The Daily Express focused on the magnesium content of these vegetables being key to these findings, but this is not supported by the current study. The papers quote a linked editorial on the topic that says, “we must be careful that the message of increasing overall fruit and vegetable intake is not lost in a plethora of magic bullets.” It seems sensible to promote a balanced overall approach to lifestyle change that does not just focus on specific food types.
What kind of research was this?
This was a systematic review and meta analysis of six large prospective cohort studies from the United States, China, and Finland, which had looked at whether eating a large amount of fruit and vegetables affected people’s risk of developing diabetes. It also analysed the data by type of vegetable and vegetable and fruit separately.
What did the research involve?
The researchers searched various medical and scientific databases to find prospective cohort studies that had looked at fruit and vegetable intake and the risk of developing type 2 diabetes. These studies were assessed for their quality using criteria such as whether the participant’s fruit and vegetable consumption had been measured with a validated tool (like a standardised questionnaire) or if the statistics used in the paper were adjusted for factors that may influence the results such as age, BMI and a family history of type 2 diabetes.
The researchers pooled data from the research articles that had looked at the risk of developing type 2 diabetes associated with eating more or less fruit and vegetables (Hazard ratio).
What were the basic results?
The search identified 3,346 articles and of these only six met the inclusion criteria. The combined population in these six studies was 223,512, however only two of the studies included men. The age of participants ranged from 30 to 74. The studies had followed the participants for between 4.6 and 23 years.
None of the papers met all the criteria for being high-quality. Two papers had a quality score of four out of six, two had a score of three and two had a score of one or two.
The meta analysis of the pooled data did not show that there was a statistically significant change in the risk of developing type 2 diabetes with increased consumption of fruit, vegetables or fruit and vegetables combined (Hazard ratio 1.00, 95% confidence interval 0.92 to 1.09).
However, the pooled data from four studies that assessed the consumption of green leafy vegetables and the risk of developing diabetes showed that 1.35 servings a day (the highest intake) compared with 0.2 servings (lowest intake) resulted in a 14% reduction in risk (Hazard ratio 0.86, 95% confidence interval 0.77 to 0.96).
How did the researchers interpret the results?
The researchers say that their meta analysis supports “recommendations to promote the consumption of green leafy vegetables in the diet reducing the risk of type two diabetes”. The researchers had used 106g as a standard portion size, however they said that the current UK recommendation suggests a serving size of 80g. They therefore said that increasing consumption of green leafy vegetables by one and a half UK portions a day (121.9g) could result in a 14% reduction in type 2 diabetes.
They balance this advice by saying that “the potential for tailored advice on increasing intake of green leafy vegetables to reduce the risk of type 2 diabetes should be investigated further”.
This was a well-conducted systematic review and meta analysis assessing whether fruit and vegetable intake affects the likelihood of developing type 2 diabetes. It found that increased green leafy vegetable intake was associated with a reduced risk of developing type 2 diabetes. One limitation of pooling data from these types of diet cohort studies is that they may have measured diet differently, potentially affecting the results.
- The researchers did not detail other aspects of the participants’ diets such as the amount of sugar that they consumed. This observed positive effect of eating greens may not be due to the vegetables themselves, but actually a result of people who eat lots of green leafy vegetables having a healthier diet or making other healthy lifestyle choices in general.
- The researchers say that not all of the studies that investigated green leafy vegetables used the same criteria. Two of the papers included spinach, kale and lettuce, another included Chinese greens, greens and spinach. The other paper did not provide a definition. Owing to the different criteria used to assess leafy vegetable consumption, it is not possible to say whether one particular leafy vegetable decreases risk more than others.
- Only one study was from Europe, highlighting the lack of specific research in this area.
At this point, it is not possible to say whether the reduced risk of type 2 diabetes associated with eating more green leafy vegetables is due to compounds found in these vegetables or because people who eat more leafy vegetables have a healthier diet in general.
Lifestyle changes such as adopting a healthier diet may help to reduce the risk of diabetes. In people at risk, reducing the intake of total and saturated fat, increasing intake of vegetables, fruit, and wholegrain cereals, and increasing physical activity are known to reduce the risk of diabetes by about 60%. This is thought to be mainly because these factors all work towards reducing weight in people at risk (four times this relative risk reduction seen with eating leafy vegetables). It seems sensible to promote a balanced overall approach to lifestyle change, one that does not just focus on specific food types.