Despite previously declaring she “hated being super skinny” and loves her curves, Christina Aguilera dropped 40 lbs ahead of her Las Vegas residency and looks leaner than she has in years.

However, the pop star’s yo-yo dieting is worrisome. “No one thinks it’s healthy,” an insider exclusively tells Life & Style, noting that the “Beautiful” singer, 43, began restricting her caloric intake to just 1,600 calories a day and has been working out “rigorously” seven days a week.

“Christina didn’t feel as energetic with extra weight,” says the source. “Some are concerned she’ll gain the weight back but she loves all the attention she’s getting.”

While Aguilera insists her slim-down was the result of a healthy diet, cutting out sugary snacks and a consistent exercise routine, others wonder if popular weight-loss drug Ozempic played a factor in her getting fit.

“She looks terrific,” Dr. Stuart Fischer, author of The Park Avenue Diet, tells Life & Style. “But if she’s using Ozempic, that’s not a good idea unless it’s for diabetes. Weight loss is a side effect of the medication, it’s not what it’s for. There are risks involved, including constant nausea, abdominal pain, constipation, diarrhea and vomiting. You should lose weight the old-fashioned way — through diet, nutrition and exercise.”

Sports medicine expert Dr. Gabe Mirkin agrees. “Yes, she is probably taking diet medication to help her lose weight,” he says. “But nobody should ever take these drugs without being schooled first on lifestyle changes to keep excess weight off.”

Still, the Burlesque star is taking snarky critics in stride, saying, “Being too thin. Being bigger. I’ve been criticized for being on both sides of the scale.”