KATIE HOLMES continues to shed pounds and is now looking frighteningly thin.

Once a healthy 118 pounds, a source says the 5-foot-9 actress has dropped to just 99, and experts warn low weight can trigger severe medical problems for women.

Katie’s bony back and gaunt face were shockingly evident at the May 6 Cos­tume Institute Gala in New York City.

“Katie has become skin and bones,” said a source. “She’s obsessive about watching her weight, counting ev­ery calorie and exercising at least four times a week.”

Orlando-based sports medicine expert Dr. Gabe Mirkin has not treated the 34-year-old star but believes pictures show she’s extremely thin.

“You can see every bone in her back because she has almost no fat un­derneath her skin,” noted the expert, adding that she looks to have less than 8 per­cent body fat when the average American woman has more than 30 percent.

“Less than 10 percent is un­healthy and causes low levels of female hormones, cessation of men­strual periods, osteoporosis and infertility,” Mirkin explained.

New York fitness expert Dr. Edward Jackowski, author of “Escape Your Shape,” who does not treat Holmes, warns that women who are too thin can break bones in a fall due to a lack of calcium, and sug­gested that Holmes “needs to put on 10 to 15 pounds.”

In April 2012, before her divorce from Tom Cruise, Katie had al­ready begun to look run-down, with prom­inent dark circles under her eyes.

Over the months, The ENQUIRER has chronicled how the stress of her divorce, career pressures and being a great mom to 7-year-old Suri wreaked havoc on her weight.

Celebrity plastic surgeon Dr. Anthony Youn doesn’t treat Katie but has a suggestion for her: “EAT a sandwich!”