Vitamin D went up in every metabolic syndrome trial. Blood pressure, HDL and blood sugar mostly stayed put

Metabolic syndrome is the name for a cluster: a wide waist, high triglycerides, low HDL cholesterol, raised blood pressure and raised fasting glucose. Any three of the five count, and together they push up the risk of heart disease and type 2 diabetes. People with the syndrome are more often short of vitamin D, and vitamin D receptors sit in the pancreas, muscle and fat tissue, so it is a tempting idea that a daily capsule might nudge several of those numbers at once. A systematic review from the University of Sharjah in the United Arab Emirates, published on 27 October in Frontiers in Nutrition, looked at whether the trials bear that out.

The authors searched four databases and turned up 1,240 records. After screening, eight randomised controlled trials remained, with 634 adults aged 30 to 65 in Brazil, Iran, Pakistan, Greece, India and Iraq, followed for 8 to 36 weeks. Doses ranged from 1,000 to 2,000 IU a day, through 50,000 or 60,000 IU once a week, to two single 200,000 IU doses a month apart. The trials were too different to pool into one number, so the team compared them side by side instead.

Every trial that took blood found that vitamin D levels went up, so the capsules were being absorbed. Beyond that the picture breaks into pieces. Triglycerides fell in two trials, including the largest and best-run one, 160 postmenopausal women in Brazil on 1,000 IU a day for 36 weeks, whose triglycerides fell by 12% and whose insulin resistance score (HOMA-IR) fell by 18%. Waist size and BMI dropped in one Indian trial, inflammation markers improved in two, and in an Iraqi trial cholesterol fell only in the group that also exercised. Blood pressure, HDL cholesterol and fasting glucose showed no consistent change across most of the trials that measured them.

Seven of the eight trials reported at least one significant improvement, which sounds better than it is: each measured many things, and the improvements were rarely the same thing twice. Only the Brazilian trial was judged at low risk of bias; four had some concerns and three were at high risk, mostly because participants or staff knew who was taking vitamin D. One of the eight is also a later analysis of the same 80 Iranian volunteers as another, looking at inflammation rather than metabolism, so the trials cover seven separate groups of people.

The authors see a hint that people who start out deficient, and trials using larger doses (the equivalent of 4,000 IU a day or more), did better. They label that pattern exploratory, since no trial was designed to test it. Their conclusion is that vitamin D shows suggestive but not conclusive benefit for some parts of the syndrome, mainly triglycerides, insulin resistance and inflammation, and that larger, standardised trials are needed before it can be recommended as part of treatment. That matches earlier meta-analyses, which also disagreed with one another. Correcting a real deficiency is worthwhile for its own sake; as a treatment for metabolic syndrome, vitamin D is still unproven.