"This finding may have public health significance, as vitamin D levels can easily, and cheaply, be increased by a modest increase in sun exposure or vitamin D supplementation," wrote lead author Robert Scragg in the American Journal of Hypertension. "However, first it needs to be confirmed by large, well-designed intervention studies," he cautioned.
In the UK alone, there are an estimated 10 million people with hypertension, defined as having blood pressure higher than 140/90 mmHg. The condition is a major risk factor for cardiovascular disease (CVD), which causes almost 50 per cent of deaths in Europe, and is reported to cost the EU economy an estimated €169 billion ($202 billion) per year.
The researchers, from the University of Auckland, University of Michigan, Hunter New England Area Health Service (Australia), used data from Third US National Health and Nutrition Examination Survey (NHANES III). The analysis was confined to 12,644 people (aged 20 or over, 6547 women) and those on hypertensive medication were excluded.
Vitamin D status was measured using blood levels of 25- hydroxyvitamin D (25(OH)D).
Vitamin D refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol. The former, produced in the skin on exposure to UVB radiation (290 to 320 nm), is said to be more bioactive. The latter is derived from plants and only enters the body via the diet, from consumption of foods such as oily fish, egg yolk and liver.
Both D3 and D2 precursors are hydroxylated in the liver and kidneys to form 25- hydroxyvitamin D (25(OH)D), the non-active 'storage' form, and 1,25-dihydroxyvitamin D (1,25(OH)2D), the biologically active form that is tightly controlled by the body.
The researchers report that non-Hispanic whites had the highest blood levels, followed by Mexican Americans, while non-Hispanic blacks had the lowest vitamin D levels.
"Age- and gender-adjusted mean blood pressures were higher in non-Hispanic blacks than in Mexican Americans and non-Hispanic whites, consistent with results from the first phase (1988 to 1991) of NHANES III," stated the researchers.
"The finding that ethnic differences in vitamin D status explained about half of the increased prevalence of hypertension in non-Hispanic blacks, compared with non-Hispanic whites, supports the previous suggestion that low vitamin D levels in non-Hispanic blacks may be a factor in their increased hypertension prevalence," they added.
When the subjects were split up into five groups depending on blood levels, the researchers calculated that people with the highest average 25(OH)D levels (at least 85.7 nmol/L) had systolic diastolic and blood pressure 3.0 and 1.6 mm Hg lower, respectively, than people with the lowest levels (40.4 nmol/L or less).
After adjusting the results to account for BMI the associations remained statistically significant for systolic blood pressure.
"These results from a nationally representative US sample show that systolic BP and pulse pressure are inversely associated with serum 25OHD... The inverse association between serum 25OHD and systolic BP has clinical significance, because the latter variable is a better predictor of coronary heart disease risk than diastolic BP, particularly in older people," wrote the researchers.
Calls to increase vitamin D intake have been growing. Indeed, only recently fifteen experts from universities, research institutes, and university hospitals around the world called for international agencies to "reassess as a matter of high priority" dietary recommendations for vitamin D because current advice is outdated and puts the public at risk of deficiency (The American Journal of Clinical Nutrition, Vol. 85, pp. 860-868).
A recent review of the science reported that the tolerable upper intake level for oral vitamin D3 should be increased five-fold, from the current tolerable upper intake level (UL) in Europe and the US of 2000 International Units (IU), equivalent to 50 micrograms per day, to 10,000 IU (250 micrograms per day).
Source: American Journal of Hypertension (Elsevier)
July 2007, Volume 20, Issue 7, Pages 713-719
"Serum 25-hydroxyvitamin D, Ethnicity, and Blood Pressure in the Third National Health and Nutrition Examination Survey "
Authors: R. Scragg, M. Sowers and C. Bell
Note: According to statistics released in 2004, there are 50 million Americans who have hypertension, with 600 million people suffering worldwide. Some of the complications of long-term hypertension - such as stroke and end stage kidney disease - are life threatening. Hypertension is easily preventable with the proper nutrients, yet this aspect is never given emphasis.
mmHg: The medical abbreviation for millimeter of mercury, a unit used to measure blood pressure. One mmHg is equal to the pressure exerted by a column of mercury at 0 degrees C one millimeter high at sea level
UVB: a type of sunlight. The ozone in the stratosphere, high above the Earth, filters out ultraviolet B rays and keeps them from reaching the Earth. Ultraviolet B exposure has been associated with skin cancer, eye cataracts and damage to the environment. Thinning of the ozone layer in the stratosphere results in increased amounts of ultraviolet B reaching the Earth.
One nmol/L: 6.02 x 10 14 particles per liter so, for example: 1000 nmol/L = 6.02 x 10 17 particles per liter.
IU stands for "International Units" and is usually used in the measurement of vitamin potency. There is no fixed definition for IU, as there is for grams or milligrams. The actual definition of IU depends upon the specific substance that you are measuring. For example, 1000 IU of Vitamin A has a different mass (weight) than 1000 IU of Vitamin E.