Being obese can cause a deficiency in vitamin D, a nutrient that helps the body absorb calcium, which is essential for healthy bones, reports a new study.  However, the flip side isn't true: Boosting blood levels of vitamin D can't help people lose the excess weight.

According to the researchers, for every 10 percent increase in body-mass index (BMI), a person can expect to have 4.2 percent drop in blood levels of vitamin D. BMI is a measure of body fat that's based on height and weight.

The study is published today (Feb. 5) in the journal PLOS Medicine.

As many as three-fourths of Americans are deficient in vitamin D, according to a 2009 study published in the journal Archives of Internal Medicine. And 35.7 percent are obese, reports the Centers for Disease Control and Prevention.

Using genetic information from more than 42,000 people collected across 21 studies, American and European epidemiologists tracked 12 BMI-related genes and four vitamin D-related genes in people of all weight classes. The researchers found that people with a genetic predisposition for being heavier tended to have genes related to lower levels of vitamin D. As a result, no matter how much vitamin D an obese person gets, their genes are likely going to cause them to have lower-than-optimal levels of vitamin D.

However, having the genes associated with lower levels of vitamin D formation and metabolism does not necessarily mean a person is destined to be obese.

The researchers verified their observation using another data set of genes from 46 studies that included nearly 124,000 people.

“Obese individuals need to be mindful they are likely to be vitamin D deficient,” said study author Elina Hyppönen, a reader of epidemiology and public health at the University College London Institute of Child Health. “If you lower your BMI or reduce your body fat, then your vitamin D status probably will get higher.”

The study found that the connection between vitamin D and obesity went in one direction only: Obesity affected levels of vitamin D, but vitamin D had no effect on obesity.

Not everyone agrees with the latter finding.  “This study doesn't conclusively answer that vitamin D can't help in weight loss,” said Brant Cebulla, development director at the Vitamin D Council, a nonprofit advocacy group. “It may have a modest effect, but if you're looking for substantial weight loss, vitamin D isn't the answer.” Cebulla was not involved in the current study.

Previous research has examined whether boosting vitamin D levels by taking vitamin D supplements could help people lose weight by increasing the metabolism of fat cells or reducing inflammation that can lead to weight gain. In clinical trials, people who randomly took vitamin D versus a placebo showed a minor weight loss of a few pounds.

The Institute of Medicine, the health branch of the National Academy of Sciences, recommends that people ages 1 to 70 get 600 International Units (IUs) of vitamin D a day and people 70 and older get 800 IUs a day. (A glass of milk contains about 100 IUs.)

Around the world, experts have noted that people don't get enough vitamin D. Sunlight is humans' prime source of vitamin D but concerns about skin cancer have prompted many people to slather on sunblock, wear sun-blocking clothing and avoid the sun, limiting the amount of sun exposure they get. What's more, the angle of the sun during the winter months prevents people in northern latitudes from getting any sun-producing vitamin D from late fall through early spring.  It's also difficult to get enough vitamin D from food. Prime sources include fatty fish—such as salmon, tuna and mackerel — and vitamin D-fortified milk.

“You're probably not getting enough from your diet,” said Cebulla. “To get adequate vitamin D, you either need to be a sun worshipper and be out in the sun all the time or you need to take a supplement.”

Taking a supplement may be especially crucial for obese people. The  more obese a person is, the greater the need for vitamin D supplements, according to Hyppönen, who is currently investigating the association between vitamin D levels and blood pressure.

Pass it on: If you're obese, you are likely going to have a vitamin D deficiency.

Follow MyHealthNewsDaily on Twitter @MyHealth_MHND. We're also on Facebook & Google+.