Jennifer Lopez's Extreme Weight Loss Scares Family

Gene Young / Splash News

It's no secret Jennifer Lopez is attacking her baby bulge head-on: the superstar is participating in a gruelling triathlon on Sept. 14 to get her body back to pre-baby shape, and the results of her hard work are evident.

But one British tabloid reports that Jennifer's extreme focus on weight loss is concerning her family and friends. A source tells Now magazine, Jennifer gained 75 pounds while pregnant with twins Max and Emme and has lost 50 of them so far—but has hit a wall and can't seem to lose any more weight, no matter how much training she does.

Despite her busy schedule, the source reveals, Jennifer refuses to lessen her demanding exercise regime and rarely eats more than 1,000 calories per day, just half of what's normal for a woman of her size.

Concerned with her extreme dieting, Jennifer's husband Marc Anthony reportedly ordered her to consult with a doctor, who diagnosed her with an underactive thyroid gland, causing her metabolism to slow down.

The pressure to get back into top form has caused tension between Marc and Jennifer, according to the insider. "Let's just say some of their discussions have been pretty explosive," the source reveals.

We know Jennifer is one determined—and stubborn—lady, but we doubt she would be so stupid as to put her health in danger to get back into her pre-baby jeans. Besides, is Marc really one to worry about someone being too skinny?