The unflattering photo of the 32-year-old "Home Alone" and "Uncle Buck" actor sparked fans' concern when it was posted Thursday on the Web site of "Entertainment Tonight."
But Culkin's publicist told UsMagazine.com there is no cause for alarm.
"Macaulay Culkin is in perfectly good health. For 'Entertainment Tonight' or any other media to speculate otherwise is thoughtless, irresponsible and destructive," the representative said.
"He is thin, so when he loses one pound it just looks like more," an unnamed source close to the actor told UsMagazine.com.