"We think it's possible these people don't have increased risk of fracture but that their low bone mass is related to the fact that they are lighter because they take in fewer calories," Dr. Luigi Fontana, who led the study, said in a statement.

"Raw food vegetarians believe in eating only plant-derived foods that have not been cooked, processed, or otherwise altered from their natural state," Fontana's team wrote in this week's issue of the Archives of Internal medicine.

"Because of their low calorie and low protein intake, raw food vegetarians have a low body mass index (BMI) and a low total body fat content. It is well documented that a low BMI and weight loss are strongly associated with low bone mass and increased fracture risk, while obesity protects against osteoporosis."

Fontana's team studied 18 strict raw food vegans aged 33 to 85. All ate a diet that included unprepared foods such vegetables, fruits, nuts, and sprouted grains. They had been on this diet for an average of 3.6 years.

The team compared them to 18 more average Americans. The raw food group had an average body mass index of 20.5, while the average group were slightly overweight with a BMI of 25.

BMI is an internationally accepted measurement of height to weight, and a BMI of 18.5 to 24 is considered the healthy range.

Fontana expected the vegans to have low vitamin D levels because they avoid all animal products including dairy. But in fact their vitamin D levels were "markedly higher" than average.

Vitamin D is made by the skin when the body is exposed to sunlight and is key to keeping strong bones. It is added to milk and other foods because it is so important.

"These people are clever enough to expose themselves to sunlight to increase their concentrations of vitamin D," Fontana said.

And the vegans had low levels of C-reactive protein, an inflammatory molecule that is becoming linked with the risk of heart disease, diabetes and other chronic disease.

Furthermore, they had lower levels of IGF-1, a growth factor linked to risk of breast and prostate cancer.