I was surprised to see a paper entitled ‘Yogurt Intake Reduces All-Cause and Cardiovascular Disease Mortality‘ in the current issue of the Chinese Journal of Integrative Medicine. My surprise turned into disbelief when I read the abstract. Here it is in its unabbreviated beauty:
To assess the relationship between yogurt intake and mortality risk from prospective cohort studies.
The PubMed, EMBASE, and Web of Science databases were searched for all records related to yogurt intake and mortality risk [all-cause or cardiovascular disease (CVD) or cancer mortality] before October 1, 2018. The Newcastle-Ottawa Quality Scale was used to estimate the quality of all eligible articles. The results of the highest and lowest categories of yogurt intake in each study were collected and the effect size was pooled using a random effects model. The dose-response analysis was calculated using the generalized least squares trend estimation model.
Eight eligible cohort studies were included in this meta-analysis. There were 235,676 participants in the 8 studies, and the number of deaths was 14,831. Compared with the lowest category, the highest category of yogurt intake was not significantly related with all-cause mortality [hazard ratio (HR)=0.93; 95% confidence interval (CI): 0.85, 1.01], CVD mortality (HR=0.92; 95% CI: 0.81, 1.03) and cancer mortality (HR=0.97; 95% CI: 0.83, 1.12). These studies were homogenous, since the homogeneity test showed that I2 was 28.7%, 15.1% and 11.8%, respectively. However, yogurt intake ⩾200 g/d was significantly associated with a lower all-cause mortality (HR=0.88; 95% CI: 0.80, 0.96) and CVD mortality (HR=0.87; 95% CI: 0.77, 0.99) in the subgroup analysis. The dose-response analysis showed that yogurt intake of 200 g/d was inversely associated with all-cause mortality (P=0.041, HR=0.95, 95% CI: 0.92, 1.00) and CVD mortality (P=0.009, HR=0.92, 95% CI: 0.86, 0.98), and all of which were linear relationship (P>0.05).
This review provided the evidence regarding yogurt intake can reduce all-cause and CVD mortality. Although some positive findings were identified, more high-quality cohort studies and randomized controlled trials are warranted on a possible protective effect of yoghurt on health.
As the above three graphs indicate (and as researchers learn in their first lesson of statistics),
CORRELATION IS NOT CAUSATION.
We all know that, of course – except those of us who work in the so-called alternative medicine (SCAM), it seems – and except the editors of Chinese Journal of Integrative Medicine or the people who do the peer-reviewing for this publication. How else would we explain that a conclusion such as yogurt intake can reduce all-cause and CVD mortality can get published (not to mention the title of the paper)?
But what is the explanation of the intriguing association discovered by the authors of this paper? As far as I can see, there are several possibilities:
- The investigators could be correct, and the link could indeed be causal (in this case, they need to prove it).
- The finding could be coincidental, due to random noise (there are hundreds of epidemiological studies, and they tend to report mainly positive associations [big fishing expeditions really]; thus, it is conceivable that 8 of them found an association where in truth none exists).
- It could be that people who eat a yoghurt tend to not eat a ‘full English’ for breakfast, thus avoiding the harm of unhealthy nutrition)
- Similarly, it might be that yoghurt consumers care more about their health in general (including exercise, body weight, smoking, etc.) than those who shun yoghurt.
And how do we decide which explanation applies? The answer is simple: by doing proper research and drawing appropriate conclusions from it.
Perhaps something like this?
Background: Although a link between regular yogurt consumption and mortality appears plausible, data are sparse and have yielded inconsistent results.
Objectives: We examined the association between regular yogurt consumption and risk of all-cause and cause-specific mortality among US women and men.
Methods: A total of 82,348 women in the Nurses’ Health Study and 40,278 men in the Health Professionals Follow-Up Study without a history of cardiovascular disease (CVD) and cancer in 1980 (women) or 1986 (men) were followed up until 2012. Yogurt consumption was assessed by updated validated FFQs.
Results: During 3,354,957 person-years of follow-up, 20,831 women and 12,397 men died. Compared with no yogurt consumption, the multivariable-adjusted HRs (95% CIs) of mortality were 0.89 (0.86, 0.93), 0.85 (0.81, 0.89), 0.88 (0.84, 0.91), and 0.91 (0.85, 0.98) for ≤1-3 servings/mo, 1 serving/wk, 2-4 servings/wk, and >4 servings/wk in women (P-trend = 0.34), respectively. For men, the corresponding HRs (95% CIs) were 0.99 (0.94, 1.03), 0.98 (0.91, 1.05), 1.04 (0.98, 1.10), and 1.05 (0.95, 1.16), respectively. We further noted inverse associations for cancer mortality (multivariable-adjusted HR comparing extreme categories: 0.87; 95% CI: 0.78, 0.98; P-trend = 0.04) and CVD mortality (HR: 0.92; 95% CI: 0.79, 1.08; P-trend = 0.41) in women, although the latter was attenuated in the multivariable-adjusted model. Replacement of 1 serving/d of yogurt with 1 serving/d of nuts (women and men) or whole grains (women) was associated with a lower risk of all-cause mortality, whereas replacement of yogurt with red meat, processed meat (women and men), and milk or other dairy foods (women) was associated with a greater mortality.
Conclusions: In our study, regular yogurt consumption was related to lower mortality risk among women. Given that no clear dose-response relation was apparent, this result must be interpreted with caution.
I rest my case.