Media mogul Oprah Winfrey is making serious cash in her new venture with Weight Watchers.

According to Market Watch, the former talk show queen raked in $18 million ($US12 million) in just over an hour after tweeting a video about how she lost weight while still eating bread.

"I lost 26 pounds (11.7 kilograms) and I have eaten bread every single day," she declares in the footage.

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The ambassador for Weight Watchers saw her ten per cent stock in the company soar after the tweet went out to her 31 million followers.

The 61-year-old is ranked by Forbes as the 211th richest person in America, making $US70 million after she announced she was taking a stake in the dieting company.