What are the effects of zinc supplementation compared with placebo or no treatment for the prevention of type 2 diabetes in adults with insulin resistance?
Some studies have shown that zinc improves glucose levels (glycaemic control) in people with diabetes. As a consequence of diabetes long-term complications may develop, such as kidney, nerve and eye disease. Also, the risk of cardiovascular complications like heart attacks and strokes is raised. Type 1 diabetes is a form of diabetes where the body cannot produce insulin any more. The risk of developing type 2 diabetes increases with age, obesity and lack of physical activity and is characterised by an increasing inability of the body to make good use of insulin (insulin resistance). The mineral zinc plays a key role in the action of insulin and theoretically zinc supplementation used by people with insulin resistance could prevent the onset of diabetes.
We included three randomised controlled studies with a total of 128 participants in this review. The duration of zinc supplementation ranged between four and 12 weeks.
No study reported on our patient-important key outcomes (new onset of type 2 diabetes mellitus, side effects, health-related quality of life, all-cause mortality, diabetic complications, socioeconomic effects). The effects of zinc supplementation are uncertain regarding insulin resistance and lipid levels in the blood (mainly cholesterol and triglycerides).
Quality of evidence
The overall quality of the included studies was unclear because study authors did not provide important information for us to judge how the studies were performed (unclear risk of bias in most cases). In addition, the number of studies and participants is low and the study authors did not investigate important outcomes such as new onset of type 2 diabetes mellitus or side effects of zinc supplementation.
Currentness of evidence
This evidence is up to date as of March 2015.
There is currently no evidence on which to base the use of zinc supplementation for the prevention of type 2 diabetes mellitus. Future trials should investigate patient-important outcome measures such as incidence of type 2 diabetes mellitus, health-related quality of life, diabetic complications, all-cause mortality and socioeconomic effects.
Diabetes is associated with long-term damage, dysfunction and failure of various organs, especially the eyes, kidneys, nerves, heart and blood vessels. The risk of developing type 2 diabetes increases with age, obesity and lack of physical activity. Insulin resistance is a fundamental aspect of the aetiology of type 2 diabetes. Insulin resistance has been shown to be associated with atherosclerosis, dyslipidaemia, glucose intolerance, hyperuricaemia, hypertension and polycystic ovary syndrome. The mineral zinc plays a key role in the synthesis and action of insulin, both physiologically and in diabetes mellitus. Zinc seems to stimulate insulin action and insulin receptor tyrosine kinase activity.
To assess the effects of zinc supplementation for the prevention of type 2 diabetes mellitus in adults with insulin resistance.
This review is an update of a previous Cochrane systematic review published in 2007. We searched the Cochrane Library (2015, Issue 3), MEDLINE, EMBASE, LILACS and the ICTRP trial register (from inception to March 2015). There were no language restrictions. We conducted citation searches and screened reference lists of included studies.
We included studies if they had a randomised or quasi-randomised design and if they investigated zinc supplementation compared with placebo or no intervention in adults with insulin resistance living in the community.
Two review authors selected relevant trials, assessed risk of bias and extracted data.
We included three trials with a total of 128 participants in this review. The duration of zinc supplementation ranged between four and 12 weeks. Risk of bias was unclear for most studies regarding selection bias (random sequence generation, allocation concealment) and detection bias (blinding of outcome assessment). No study reported on our key outcome measures (incidence of type 2 diabetes mellitus, adverse events, health-related quality of life, all-cause mortality, diabetic complications, socioeconomic effects). Evaluation of insulin resistance as measured by the Homeostasis Model Assessment of Insulin Resistance (HOMA-IR) showed neutral effects when comparing zinc supplementation with control (two trials; 114 participants). There were neutral effects for trials comparing zinc supplementation with placebo for total cholesterol, high-density lipoprotein (HDL) cholesterol, low-density lipoprotein (LDL) cholesterol and triglycerides (2 studies, 70 participants). The one trial comparing zinc supplementation with exercise also showed neutral effects for total cholesterol, HDL and LDL cholesterol, and a mean difference in triglycerides of -30 mg/dL (95% confidence interval (CI) -49 to -10) in favour of zinc supplementation (53 participants). Various surrogate laboratory parameters were also analysed in the included trials.