(Web Desk) - A doctor has raised concerns about the health of a stranded Boeing astronaut after an unexpectedly-long stay aboard the International Space Station (ISS).
Sunita Williams, 59, flew to the ISS for an eight-day mission, but has now been on the orbital outpost for 153 days.
After Boeing's Starliner spacecraft was deemed too faulty for a manned return trip, Williams and crewmate Butch Wilmore, 61, have been stuck there.
Astronauts train for all the scenarios they may face while in space - including longer-than-expected stays.
The pair have put on a positive front, with Williams calling the space station her "happy place" back in September.
But Dr Vinay Gupta, a pulmonologist and veteran based in Seattle, says Williams' cheeks look "sunken" in a photo published 24 September.
"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 pressurised cabin, for extended periods," he told the Daily Mail.
"Her cheeks appear a bit sunken - and usually it happens when you've had sort of total body weight loss."
Astronauts generally burn up to 3,500 calories per day, much more than the average person, according to Nasa.
Space explorers consume a specialised, calorie-dense, diet because of the extra energy required.
Roughly 2.5hours per day is set aside for astronauts to exercise, which is the only way to fend off muscle loss and atrophy.
In the photo in question, Williams can be seen eating a pepperoni pizza and chips with her crewmates.
"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," Dr Gupta said.
"But I don't think you can look at that photo and say she has sort of healthy body weight.
"There's just things that the human body cannot adjust to, and one of which is, you know, she's probably losing more calories than she's intaking."
Women are at greater risk of losing blood plasma volume than men during spaceflight, a Nasa study found in 2014.
A loss of blood plasma can cause your metabolic rate spike temporarily while your body mobilises resources to adjust to the loss.
This biological response can slightly elevate your calorie burn, which may contribute to weight loss while women are in space, the Mail reported.