Eating a Mediterranean diet has been reported as beneficial in preventing metabolic syndrome, slowing down the progression of atherosclerosis and decreasing risk of myocardial infarction and stroke. Metabolic syndrome (MS) is a disorder of energy utilization and storage, which according to the American Heart Association, is characterized by at least three of the following five signs:
- Abdominal obesity: (excess body fat around the waist) waist circumference greater than 102 cm (40 inches) in men and greater than 88 cm (35 inches) in women
- High serum triglycerides level: equal to or greater than 150 mg/dl (1.7 mmol/L)
- Reduced high density lipoprotein (HDL): less than 40 mg/dL (1.03 mmol/L) in men and less than 50 mg/dL (1.29 mmol/L) in women
- Increased blood pressure: equal to or greater than 130/85 mmHg or use of medication for hypertension.
- Elevated fasting blood glucose level: equal to or greater than 100 mg/dL (5.6 mmol/L) or use of medication for hyperglycemia
I recently read a blog about MS, which referred to a study that reportedly found a 42 cm reduction in waist circumference as a benefit of a switch to a Mediterranean diet. Regarding the reference values provided above (and common sense), it seemed unlikely. My first reaction was to assume that the reporter had made a mistake, potentially due to unfamiliarity with the metric system. If he knew that 42 cm equals 16 inches, an enormous figure in terms of waist circumference, it certainly would have raised some warnings. When checking the source, however, it turned out that the reported mean change found by a review of 50 studies involving more than 534,906 people (Kastorini 2011), was actually only 0.42 cm.
Really? Scientists called four tenths of one centimeter, one sixth of one inch or less than one percent (0.4%) of reference value for waist circumference a significant benefit?
Findings of effects on other indices of MS were of similar magnitude. For this kind of literature review, so called meta-analysis, it is customary to combine measurements from many relevant studies and look at the overall direction of effect, regardless of size of the change. In that sense, the scientists were correct; however, when looking at included studies one at time, it is evident that only a few reported significant improvement, most reported no meaningful changes, and a few reported changes in the opposite direction. In my opinion, it would be more appropriate to call the overall findings inconclusive.
Regarding erroneous reporting of 42 cm instead of 0.42 cm, I guess the reporter naturally expected that effects qualified as significant by scientists would be within common sense range. Lack of familiarity with metric units may have contributed to this error.
Reference: Kastorini C.The Effect of Mediterranean Diet on Metabolic Syndrome and its Components. A Meta-Analysis of 50 Studies and 534,906 Individuals. Journal of the American College of Cardiology Vol. 57, No. 11, 2011. doi:10.1016/j.jacc.2010.09.073.
Learn More:
Four Healthy Lifestyle Behaviors Slow Down Progression of Atherosclerosis and Lower Risk of Death
Primary Prevention of Cardiovascular Disease with a Mediterranean Diet
Post written by: Petar Denoble, MD, D.Sc.
The Dive Lab 
The study suggests that the 0.42cm difference in mean circumference between people on the Mediterranean Diet and those not on the diet is statistically significant. This means that after running the data through a series of statistical tests, the authors could rule out random variability in the data as a cause for the difference in waist size between the two groups. I agree that .42cm is not a large difference, but the significance value in the paper does not reflect on the magnitude of the difference, rather whether that difference can be attributed to the Mediterranean Diet as opposed to other factors.
I understand the process and the meaning of the findings. However, the score of studies included with their results pulling in opposite directions seem rather inconclusive despite findings of reported statistical exercise. The more important point is that results were confusing for a lay reporter and that may be an invitation to scientists to write with more clarity.