By Noel Randewich

SAN FRANCISCO (Reuters) - Shares of GNC jumped from record low levels on Thursday after its interim chief executive invested $5 million in the vitamin and supplement retailer.

The nearly 600,000 shares bought by Robert Moran added to a smaller GNC stake he already owned, according to a U.S. Securities and Exchange Commission filing.

Stock purchases by senior executives and other company insiders are often seen by investors as a positive sign.

GNC's stock surged 10.6 percent, on track for its sharpest one-day gain since Tuesday, when Moran made his purchase.

On Friday, GNC closed at its lowest level ever after falling 20 percent so far in 2017.

Short interest in GNC spiked to a record high in January of 22.6 percent of outstanding shares, up form 13.3 percent in December, according to Thomson Reuters data.

After taking over in July, Moran revamped GNC's branding and product pricing to try to win back customers and repair what he described as a "broken business model." But investors worry those steps are not enough to turn around falling sales.

GNC and rival Vitamin Shoppe <VSI.N> have both been struggling with competition from online retailers and a shift toward organic and "natural" foods rather than vitamins and other supplements. GNC has also faced trouble with regulators about selling supplements made with misbranded and dangerous ingredients.

The company suspended its dividend last week after reporting a 12-percent slump in same-store sales at domestic company-owned stores during the December quarter.

(Reporting by Noel Randewich; editing by Grant McCool)