Activist hedge fund Clinton Group has upped its stake in Nutrisystem Inc. to 5.2 percent, calling the weight loss product company's stock undervalued and asking for an increase in dividend, according to a filing.

"Given our enthusiasm for all that you are doing and for the company's assets and opportunities, you can imagine how bewildered we are by the stock price," Clinton Group said.

In 2012, Clinton Group pushed Nutrisystem to appoint a new member to its board of directors.