Photos have revealed NASA astronaut Sunita Williams' shocking weight loss as she and her crewmate remain stuck on the International Space Station (ISS). 

Williams, 59, and Barry Wilmore, 61, have been living on the ISS for five months after Boeing's faulty Starliner spacecraft was deemed unsafe to return them to Earth. 

A 'gaunt' photo of Williams taken on September 24 recently sparked concerns due to her 'sunken' cheeks which suggested she'd rapidly lost weight.

More recent images also show her with a pronounced facial structure and a thinner frame - a concern given she still has three months until she returns.  

Publicly, NASA has said Williams' is 'in good health' and that she's been undergoing 'routine medical evaluations'.

But behind the scenes, the space agency has been scrambling to 'stabilize the weight loss and hopefully reverse it,' according to a well-placed source. 

The unnamed NASA employee who is 'directly involved with the mission' told the New York Post that Williams has been 'unable to keep up with the high-caloric diets that astronauts must consume' while on the ISS.

Long-stay space missions take a toll on astronaut's health, especially women, leading to weight loss, muscle breakdown, bone loss, heart and vision problems and kidney stones.

Female astronauts have been found to lose more weight - particularly through loss of muscle mass - than men during spaceflight.

'The pounds have melted off her and she's now skin and bones. So it's a priority to help her stabilize the weight loss and hopefully reverse it,' the NASA source told the New York Post. 

About a month ago, NASA doctors began working with Williams to help her regain weight, the source said, even before DailyMail.com's original story brought attention to her deterioration.

To stop and reverse her weight loss, Williams could have to eat up to 5,000 calories per day, the source added. 

For comparison, the average woman on Earth has to eat between 1,600 to 2,400 calories to maintain her weight. The source said that Williams started the mission at approximately 140 lbs.

Williams and Wilmore launched toward the orbiting laboratory on June 5 for what was supposed to be a 10 day mission, but have been stuck there for 156 days. 

Now, they are set to return to Earth on SpaceX's Crew-9 Dragon capsule. Williams, Wilmore and the four Crew-9 astronauts will journey home together in February 2025.

By then, the Starliner crew will have spent roughly eight to nine months on the ISS, depending on when in February they splash down. 

During that time, they will have endured microgravity, close quarters and high doses of space radiation.

Over the last five months, photos sent back to Earth have offered glimpses into the lives of Williams and Wilmore as they adapt to their unforeseen circumstances. 

Just hours before the Starliner launch on June 5, the crew was photographed beaming in their blue Boeing spacesuits as they geared up for their short mission.

In that photo, the astronauts - especially Williams - look strikingly different than in recent images, including the September snapshot. 

That image was one of several posted on the official ISS Instagram page that showed Williams, Wilmore and the other ISS astronauts making pizza together. 

It recently went viral as the public raised concerns over Williams' shocking appearance.  

In it, she appears 'gaunt,' Dr Vinay Gupta, a pulmonologist and veteran in Seattle, told DailyMail.com earlier this week.

'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,' he said. 

'Her cheeks appear a bit sunken - and usually it happens when you've had sort of total body weight loss.'

'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.'

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

The NASA source told the New York Post that they were shocked by the image. 'I gasped out loud when I saw the last picture,' they said.

'And it's something we're talking about, but not something we're obsessing about. It's a real concern and everyone is taking it seriously.' 

More recent photos, including one in which Williams and Wilmore pose for a group-shot with the other members of the Expedition 72 crew on October 4, show that Williams is still looking thin. 

NASA declined to comment on the statements made by the anonymous agency employee, and instead referred DailyMail.com back to their original statement which asserts that all astronauts aboard the ISS are 'in good health.' 

The body burns more calories in space as it adjusts to the changes in gravity and tries to maintain its body temperature in cold, harsh conditions.

'They have to eat about 3,500 to 4,000 calories per day, just to maintain their current weight,' the NASA source said. 

'And when you start falling behind, your weight drops fast.' 

To prevent muscle and bone loss, astronauts exercise about 2.5 hours a day, which burns even more calories. 

'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,' Dr Gupta said.

'Her body's probably working harder to do basic things, because the partial pressure of oxygen is lower than it would be on sea level.'

The problem of the body breaking down is a particular worry for women. 

A study assembled by NASA in 2014 found that women have greater loss of blood plasma volume than men during spaceflight.

And women’s stress response characteristically includes a heart rate increase while men respond with an increase in vascular resistance.

The loss of blood plasma causes your metabolic rate to temporarily increase while your body mobilizes resources to adjust to the loss of plasma.

And this response can slightly elevate your calorie burn, resulting in weight loss similar to what Williams may be experiencing.

Another study released by Ball University in 2023 also found that women lose more muscle than men in a microgravity environment such as spaceflight.

Williams and Wilmore still have to wait about three to four months until they can return to Earth on SpaceX's Crew Dragon. 

At this time, there is no evidence to suggest that Williams' alleged health decline will impact this timeline. 

Though the Starliner mission has extended significantly, Williams and Wilmore's ISS mission will not be the longest one ever completed by an astronaut.  

That record is held Russian cosmonauts Oleg Kononenko and Nikolai Chub, who returned to Earth aboard Russia's Soyuz MS-25 spacecraft on September 23 after 374 days on the ISS.

More recently, a crew of four NASA/SpaceX astronauts returned to Earth on October 25 after a 232-day stay on the ISS. 

All four crewmembers were taken to the hospital after splashing down. Three received medical evaluations and were discharged that same day, while one was kept for overnight observation. 

NASA has not disclosed any details about why the astronauts required medical attention, or which one was kept overnight.