Summary

Low vitamin D levels, represented by serum 25-OH values below 15 ng/mL, are associated in hypertensive individuals with a 2-fold increased risk of a cardiovascular event within about 5 years.

Introduction

One third to one half of middle-aged and elderly US citizens have low levels of 25-hydroxy-vitamin D (25-OH D), which is the storage form of vitamin D in the body. Yet these people appear otherwise healthy. The commonly recognized effects of vitamin D deficiency involve the mineralization of bone, leading to rickets in childhood, osteomalacia, and contributing to osteoporosis. However, it's becoming evident that vitamin D is involved elsewhere in the body, one area of which is the cardiovascular system.

Boston researchers have now reported findings in their analysis of data from Framingham Heart Study enrollees in the journal Circulation. Here's a summary of what emerged.

What was done

Volunteers in the second generation of the original Framingham Heart Study - the Framingham Offspring Study - were examined between 1996 and 2001. This was part of one of their routine 4-yearly exams. There were 1739 of them that were free of prior cardiovascular disease. Blood levels of 25-OH D blood levels were estimated, and the participants classified according to the results; 28% had levels below 15 ng/mL, and 9% had levels below 10 ng/mL. Information on vitamin D intake (as supplements or in the diet) was obtained from detailed food frequency questionnaires.

Those participants with hypertension were identified by having a systolic blood pressure reading of 140 mm Hg or more, a diastolic reading of 90 mm Hg or more, or taking antihypertensive medication. Diabetics were identified by a fasting blood glucose of 126 mg/dL or greater or the use of antidiabetic mediation (including insulin).

Medical records provided information on cardiovascular events - heart attack (myocardial infarction or MI), coronary artery insufficiency (prolonged chest pain with ECG changes), angina, stroke, transient ischemic attack (TIA), severe peripheral artery narrowing (leading to calf pain on walking, or claudication), or heart failure.

What was found

At baseline, the average age of participants was 59 years; 55% of them were women. The average follow-up period was 5.4 years, during which 120 participants had a first cardiovascular event.

Those with baseline serum levels of 25-OH D below 15 ng/mL at the outset were 1.62 more likely to have a cardiovascular event, compared with people who had higher serum levels. Those with 25-OH D levels below 10 ng/mL were 1.8 times as likely to have a cardiovascular event in the subsequent 5 years or so.

Further analysis showed that the likelihood of a cardiovascular event in hypertensive subjects with 25-OH D levels below 15 ng/mL was 2.13 times that of those with 25- OH D levels above 15 ng/mL. Non-hypertensive subjects had no such link between 25-OH D levels and risk.

There was no evidence that other factors - age, gender, diabetes, or body mass index (BMI) - modified the association between 25-OH D deficiency and cardiovascular risk.

What the findings mean

This study shows that serum 25-OH levels below 15 ng/mL are associated in hypertensive subjects with a 2-fold increased risk of a cardiovascular event over the next 5½ years. The findings were obtained in an epidemiological study, i.e. they lack the power of a prospective study. However, they represent strong circumstantial evidence, suggesting that moderate to severe vitamin D deficiency is a risk factor for developing cardiovascular disease.

A finding like this one joins others that strongly suggest the need to ensure a higher vitamin D supply to the body. Although milk, cod liver oil, or supplements can provide vitamin D, solar radiation is still the main source for humans, even at high latitudes. Vitamin D seems to protect against cancers, including breast, colon, kidney, and ovarian cancers, as well as cardiovascular disease, as shown in this study. It appears very likely that the overall benefit from increased sun exposure (sun-bathing) may outweigh the increased risk of skin cancer. And, of course, supplements represent an easy, safety, and low cost way of ensuring adequate vitamin D levels.