A new meta-study has concluded that a higher daily dosage of supplemental vitamin D is associated with significant cardiometabolic benefits.

The researchers, from China and the US, looked specifically at the positive impact of vitamin D supplementation.

These included reductions in systolic and diastolic blood pressure, overall cholesterol, hemoglobin A1C – a marker of type 2 diabetes – and fasting blood glucose and insulin. 

The finding comes from a new meta-study of 99 randomised controlled trials (RCTs). The review attempts to explain and distill findings of sometimes inconsistent studies regarding vitamin D.

According to Medical News Today, a “key component of the meta-study was a cataloguing of differences between the RCTs”.

This “might explain their varying conclusions”.

Once those differences were understood, “the authors of the meta-study could re-analyse and compare the RCTs’ data in a more balanced, apples-to-apples manner”.

A key finding was that vitamin D supplementation provided the greatest benefit to:

• Non-Westerners
• People with lower levels of circulating vitamin D in their blood
• People with a BMI of under 30
• Those aged 50 and older.

This suggests that ethnocultural background, age, body weight and study participants’ circulating vitamin D levels at enrolment may have confounded results in previous studies.

They also suggest that these differences could mean a more personalised approach to dosage would be more efficacious.

The authors of the meta-study found that taking a daily median dose of 3320 International Units (IU) of vitamin D, or about 83 micrograms, was most beneficial.

This is toward the top end of safe dosage. It may be harmful to take more than 100 micrograms of vitamin D a day.

Vitamin D studies not always so definitive

During the Covid-19 pandemic, it didn’t take long before researchers wondered if vitamin D might protect against infection.

It was inevitable because vitamin D has emerged as a potential ‘wonder drug’  for all sorts of diseases and conditions.

But, as it almost always goes, the evidence was mixed and confusing.

Some studies found that vitamin D was protective against severe illness.

Others found there was no protective effect, while also finding that low levels of vitamin D in the blood serum were associated with higher risk of the disease.

It’s well established that vitamin D plays a vital role in bone metabolism. This is the dynamic process by which bone is regenerated and repaired over its lifetime.

Turning that around, metabolic bone disease is a weakening of the bones caused by abnormal levels of the bone’s ‘building blocks’. These include calcium, phosphorus or vitamin D.

More simply put, vitamin D deficiency can lead to osteoporosis, and increase the risk of falls and related fractures in older people.

It’s also clear that vitamin D is involved in ‘innate immunity’, the defence system with which you were born. It’s the first response of the body’s immune system to a harmful foreign substance.  

In May, a meta-analysis from Italy, adjusting for confounding factors, supported the idea that low-serum vitamin D ‘‘can predict the onset of type 2 diabetes in older adults’’.

Two-way relationship

According to a fascinating overview from Frontiers, low-serum vitamin D is associated with the incidence of several metabolic diseases and conditions.

These include obesity, insulin resistance, metabolic syndrome, dyslipidemia (unhealthy levels of fat), diabetes, non-alcoholic fatty liver disease, and cardiovascular disease.

However, inconsistent findings muddy the issue “and cause/effect relationships still need to be confirmed”.

The relationship of vitamin D with metabolic disorders “seems to be bidirectional”.

For example, obesity can worsen vitamin D deficiency – while vitamin D deficiency can aggravate obesity and related metabolic complications.

These complications occur “by multiple mechanisms, many of which still remain undiscovered and unclarified”.

In other words, the overall big-picture relationship between vitamin D and metabolic disorders is consistent. The trouble is in the details.