NASA today insisted stranded astronaut Sunita Williams is safe and healthy amid growing public concern for her health.

But the space agency again refused to give any details about four astronauts who were mysteriously hospitalized late last month after returning from a separate space mission.

A doctor raised concerns about Williams this week after a photo showed her looking 'gaunt', suggesting she had lost a significant amount of weight after spending more than 150 days stuck on the International Space Station.

But NASA holds firm that Williams and the rest of the astronauts aboard the ISS are doing fine.

'All NASA astronauts aboard the International Space Station undergo routine medical evaluations, have dedicated flight surgeons monitoring them, and are in good health,' NASA spokesperson Jimi Russell told DailyMail.com.

Concern over Williams came less than two weeks after the hospitalization of four NASA/SpaceX Crew-8 astronauts, who splashed down off the coast of Florida on October 25. The crew had spent 232 days aboard the ISS. 

One of them was hospitalized overnight due to a 'medical issue', while the other three were discharged that same day after undergoing medical evaluations. 

NASA refused to provide any details about why the crew was hospitalized, which astronaut had to complete an overnight stay, or whether the medical issues were related to their return to Earth.

The agency instead deferred DailyMail.com to a blog post from October 26.

The statement said that one of the astronauts stayed overnight at Ascension Sacred Heart Pensacola in Florida, but was 'in good health and will resume normal post-flight reconditioning with other crew members' after being discharged.

NASA's decision not to disclose who that astronaut was or reveal details about his or her condition was made 'to protect the crew member's medical privacy.'

While the agency has repeatedly expressed confidence in the wellbeing of its astronauts, studies have shown that  living in space takes a toll on human health.

This is especially true of long-term ISS missions, which typically last six months.

Space is a harsh environment that causes astronauts to lose body fat, muscle mass, bone density, and endure high doses of radiation.

This can lead to a range of health issues, including vision problems, kidney stones and even causing astronauts to become more vulnerable to infections on the ISS.

Williams and her crewmate, Barry Wilmore, have been aboard the ISS for six months as of today, and will remain there until February 2025 at the earliest.

By then, they will be able to catch a ride back to Earth on SpaceX's Crew-9 mission Dragon Capsule. 

The photo of Williams, taken on September 24, shows her tucking into a pepperoni pizza and chips while surrounded by condiments and other treats.

'Her cheeks appear a bit sunken - and usually it happens when you've had sort of total body weight loss,' Dr Vinay Gupta, a pulmonologist in Seattle, told DailyMail.com.

'I think what I can discern by her face and her cheeks being sunken in is that [she] has probably been at a significant [calorie] deficit for a while.'

Weight loss is not uncommon among astronauts during extended ISS missions. In fact, most lose about five percent of their bodyweight during a four to six month stay aboard the orbiting laboratory, according to the Lunar and Planetary Institute. 

Dr Gupta told DailyMail.com: 'What you're seeing there in that picture is somebody that I think is experiencing the natural stresses of living at a very high altitude, even in a pressurized cabin, for extended periods.'

'Based on what I'm at least seeing in the photo, I don't think she's quite at a... place where I say her life's in danger,' he added.

'But I don't think you can look at that photo and say she has sort of healthy body weight.

As for Crew-9, which included NASA astronauts Matthew Dominick, Michael Barratt, and Jeanette Epps, as well as Roscosmos cosmonaut Alexander Grebenkin, it's possible that their health suffered during their ISS stay as well. 

The four astronauts spent 232 days - or nearly eight months - aboard the ISS. It's reasonable to believe that the stress their bodies endured during that time may have resulted in the need for medical attention upon their return to Earth. 

Alternatively, some have speculated that something went awry during the crew's descent, despite NASA's statement that the crew 'safely splashed down aboard their SpaceX Dragon spacecraft.' 

The fact that all four astronauts received some form of medical attention does suggest that the health issues were somehow related to their return flight.