For people suffering from glaucoma, certain yoga positions - especially head-down postures - and other exercises like push-ups and lifting heavy weights may be dangerous, a team of US researchers has warned.

Glaucoma patients may experience increased eye pressure as the result of performing several different head-down positions while practicing yoga, claimed the researchers from New York Eye and Ear Infirmary of Mount Sinai (NYEE).

Four inverted yoga positions - facing dog, standing forward bend, plow and legs up the wall - were key to the research.

"While we encourage our patients to live active and healthy lifestyles, certain types of activities, including pushups and lifting heavy weights, should be avoided by glaucoma patients," said Robert Ritch, senior study author and Director, Glaucoma Research, NYEE.

Damage to the optic nerve occurs in glaucoma patients when fluid pressure inside the eye rises. Elevated intraocular pressure (IOP) is the most common known risk factor.

Certain yoga postures and exercises increase "the risk of increasing IOP and possibly damaging the optic nerve," Ritch noted.

In previous research, studies and case reports had tested only the headstand position which showed a marked two-fold rise in IOP.

In the new study, researchers asked healthy participants with no eye-related disease and glaucoma patients to perform four inverted yoga positions.

Both normal and glaucoma study participants showed a rise in IOP in all four yoga positions, with the greatest increase of pressure occurring during downward facing dog.

When the measurements were taken after the participants returned to a seated position and again after waiting for 10 minutes, the pressure in most cases remained slightly elevated from the baseline.

"As we know that any elevated IOP is the most important known risk factor for development and progression of nerve damage to the eye, the rise in IOP after assuming the yoga poses is of concern for glaucoma patients and their treating physicians," explained study first author Jessica Jasien at NYEE.

"In addition, glaucoma patients should share with their yoga instructors their disease to allow for modifications during the practice of yoga," Jasien added.

The research team emphasises the importance of educating glaucoma patients on all of the risks and benefits of relating to physical exercise and their overall vision health.

"The new study will help clinicians advise their patients on the potential risk associated with various yoga positions and other exercises that involve inverted poses," the authors concluded in a paper published in the journal PLOS ONE.

A new study has debunked the widespread belief that radiations from X-rays and CT scans can cause cancer.

Scientists said that it is time to hurl away an unproven, decades-old theoretical model that has led many people, including doctors to believe otherwise.

Scientists used a model known as linear no-threshold (LNT) to estimate cancer risks from low-dose radiation such as medical imaging.

But, risk estimates based on this model are only theoretical and, as yet, have never been conclusively demonstrated by empirical evidence, they said.

The use of LNT model drives unfounded fears and excessive expenditures on putative but unneeded and wasteful safety measures, according to James Welsh, a professor at the Loyola University Chicago Stritch School of Medicine in US.

The LNT model dissuades many physicians from using appropriate imaging techniques and "discourages many in the public from getting proper and needed imaging, all in the name of avoiding any radiation exposure," Welsh and colleagues wrote in the study published in the American Journal of Clinical Oncology.

The said that the LNT model assumes there is no safe dose of radiation, no matter how small. However, the human body has evolved the ability to repair damage from low-dose radiation that naturally occurs in the environment.

They said that studies of atomic bomb survivors and other epidemiological studies of human populations have never conclusively demonstrated that low-dose radiation exposure can cause cancer.

Any claim that low-dose radiation from medical imaging procedures is known to cause cancer "should be vigorously challenged, because it serves to alarm and perhaps harm, rather than educate," they said.

The researchers suggested that the LNT model "should finally and decisively be abandoned."

The researchers reexamined the original studies, dating back more than 70 years, which led to adoption of the LNT model.

US researchers have found a new sunscreen that guarantees no loss of vitamin D, rather allows the body to produce the essential vitamin, a deficiency or insufficiency of which causes major health problems in both adults and children.

Researchers from Boston University in the US developed Solar D, through a process in which the ingredients of a sunscreen are altered in such a way that it does not impact its sun protection factor (SPF), but does allow the body to produce vitamin D.

"Solar D was designed with compounds with differing filter compositions to maximise vitamin D production while maintaining its sun protection for reducing erythema or burning of the skin," said Michael F. Holick, professor at Boston University.

Sun exposure has been the major source of vitamin D for most children and adults worldwide.

However, the increased risk of skin cancer led to the widespread usage of sunscreens.

As a result, an SPF of 30 when properly applied reduces the capacity of the skin to produce vitamin D by almost 98 percent, the researchers said.

There are several chemical compounds that are typically used in a sunscreen that efficiently absorbed varying wavelengths of UVB radiation, the researchers explained in the study published in the journal PLOS ONE.

After removing certain ingredients the researchers compared Solar D, which has an SPF of 30, to a popular commercial sunscreen with the same SPF, and found Solar D allowed for up to 50 percent more production of vitamin D in-vitro.

Every extra hour a day spent sitting could increase the risk of developing diabetes by about a fifth, new study has warned.

The study by Julianne van der Berg of Maastricht University in the Netherlands and colleagues investigated cross-sectional associations of total duration and patterns of sedentary behaviour with glucose metabolism status and the metabolic syndrome.

Each extra hour of daily sedentary time (for example spent sitting at a computer) was associated with a 22 per cent increased risk of developing type 2 diabetes, researchers said.

The study participants used a thigh-worn accelerometer, which classifies sedentary behaviour using data on posture, as this has shown to be an accurate means of assessing sedentary behaviour.

The study included 2,497 participants (mean age 60 years, 52 per cent men) who were asked to wear their accelerometer 24 hours per day for 8 consecutive days.

Researchers calculated the daily amount of sedentary time, daily number of sedentary breaks, number of prolonged sedentary periods (of 30 minutes or more), and the average duration of these sedentary periods.

To determine diabetes status, participants underwent an oral glucose tolerance test.

Overall, 1,395 (56 per cent) participants had a normal glucose metabolism, 388 (15 per cent) had an impaired glucose metabolism and 714 (29 per cent) had type 2 diabetes.

Participants with type 2 diabetes spent the most time sedentary, up to 26 more minutes per day in comparison with participants with an impaired or normal glucose metabolism.

The increased risk of diabetes per additional hour of sedentary time was 22 per cent.

No significant associations were seen for the number of sedentary breaks, the number of prolonged sedentary periods or average duration of these sedentary periods with diabetes status.

The study is the largest in which this type of posture-identifying accelerometry has been used to objectively measure total duration and patterns of sedentary behaviour in a cohort of people with type 2 diabetes, impaired glucose tolerance, and normal glucose metabolism, researchers said.

"An extra hour of sedentary time was associated with a 22 per cent increased odds for type 2 diabetes," they said.

The study was published in the journal Diabetologia.

A new study has suggested that taking drugs for anxiety and sleep problems is not associated with an increased dementia.

These results from the University of Washington (UW) and Group Health in Seattle do not support a direct (causal) association between benzodiazepine (widely used drugs to treat anxiety and insomnia) use and dementia, say the researchers.

However, the researchers suggest that healthcare providers should avoid benzodiazepines in older adults to prevent important adverse health outcomes.

Benzodiazepines are widely prescribed among older adults to manage sleep, anxiety and depressive disorders. Some studies have suggested that benzodiazepine use could be associated with an increased risk of dementia, but results are conflicting.

The study involved 3,434 participants at Group Health aged 65 and older without dementia at study entry, who were followed for an average of seven years.

During follow-up, 797 participants developed dementia, of whom 637 developed Alzheimer's disease.

The team found no association between the highest level of benzodiazepine use (the median level of use in this group was equivalent to about one year of daily use) and dementia or cognitive decline.

Contrary to expectations, they found a small increased risk for dementia in people with low (up to one month) or moderate (one to four months) use.

The study appears in The BMJ.