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Objectives:
Clinical trials of vitamin K supplementation have reported conflicting results on its effect on insulin sensitivity. Therefore, this meta-analysis (systematic review) has been conducted.

Does vitamin K supplementation improve insulin sensitivity?

Study design:
This review article included 8 RCTs involving 1,077 participants. A wide variety of participants were enrolled, including older men, postmenopausal women, prediabetic premenopausal women and participants with a history of diabetes, hypertension, or vascular disease.
The number of participants ranged from 42 to 452. The study duration varied from 4 weeks to 3 years.
5 RCTs evaluated vitamin K1 ranging from 500 to 1,000 µg/d. 2 RCTs used vitamin K2 (MK-4) 1.5 and 45 mg/d and 1 RCT assessed vitamin K2 (MK-7) 100 µg/d.

All the studies were regarded as high quality.

Heterogeneity was detected in the meta-analysis.

Results and conclusions:
The investigators found that vitamin K supplementation had no effect on homeostasis model assessment of insulin resistance (HOMA-IR), fasting plasma glucose (FPG) and fasting plasma insulin (FPI). The pooled mean difference = -0.14, [95% CI = -0.35 to 0.07, p = 0.19], 0.02 mmol/L [95% CI = -3.11 to 3.70, p = 0.86] and -0.34 µIU/mL [95% CI = -1.13 to 0.45, p =0.40] for HOMA-IR, FPG and FPI, respectively.

The investigators found that vitamin K supplementation had no effect on leptin [mean difference = 0.77 ng/mL, 95% CI = -1.32 to 2.86].

The investigators found that vitamin K supplementation failed to show a significant effect on IL-6 or CRP levels. The pooled mean differences were 0.14 pg/mL [95% CI = -0.69 to 0.97] and -0.49 mg/L [95% CI = -1.18 to 0.20] for IL-6 and CRP, respectively

The investigators found that vitamin K supplementation had no effect on adiponectin [pooled mean difference = 0.82 µg/mL [95% CI: -0.89 to 2.53].

The investigators concluded that vitamin K supplementation does not improve insulin sensitivity. Given the limited evidence available and the heterogeneity in the study results, further well-designed, large sample size randomized controlled trials are warranted. Different forms and doses of vitamin K should be explored in various populations and other surrogate markers for insulin sensitivity should be measured to better establish any beneficial effects and their clinical relevance.

Original title:
Effect of vitamin K supplementation on insulin sensitivity: a meta-analysis and Suksomboon N, Poolsup N and Darli Ko Ko H.

Link:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5422317/

Additional information of El Mondo:
Find more information/studies on vitamin K and diabetes right here.
An improved insulin sensitivity decreases diabetes risk.

Research question:
Does tea consumption decrease depression risk?

Study design:
This meta-analysis (review article) contained 11 observational studies (22,817 participants with 4,743 cases of depression).

The Begg rank correlation test and the Egger linear regression test indicated no evidence of publication bias [Begg’s test: Z = 0.06, p = 0.951; Egger’s test: t = -0.1, p = 0.925].

Results and conclusions:
The researchers found in the meta-analysis of 22,817 participants with 4,743 cases of depression, a significant 31% decrease [RR = 0.69, 95% CI = 0.63-0.75, I2 = 41.9%] in the risk of developing depression for higher tea consumption compared with low tea consumption. Significant means, there is a relationship at a 95% confidence interval.

The researchers found overall, higher tea consumption had a protective effect against depression in all subgroups except for study location.

The researchers found a significant reduced risk of 32% [RR = 0.68, 95% CI = 0.62-0.75] in Asia populations for higher tea consumption compared with low tea consumption. Significant because RR of 1 was not found in the 95% confidence interval ranging from 0.62 to 0.75. RR of 1 means no relationship or risk.

The researchers found in the dose-response analysis consisted of 10,600 participants with 2,107 cases of depression a significant 37% decrease [95% CI = 0.55-0.71, I2 = 0.00%] in the risk of developing depression for every 3 cups/day increment in tea consumption.

The researchers concluded that higher consumption of tea was associated with lower risk of depression. The dose-response analysis identified a linear association between tea consumption, and the risk of depression, with every 3 cups/day increment in tea consumption associated with a 37% decrease in the risk of depression.

Original title:
Tea consumption and the risk of depression: a meta-analysis of observational studies by Dong X, Yang C, […], Lu Z.

Link:
http://journals.sagepub.com/doi/full/10.1177/0004867414567759

Additional information of El Mondo:
Find more studies/information on tea consumption right here.