The stock market lost ground on Thursday, as investors sent the Dow Jones Industrials below the level at which the average began the year. Losses for the S&P 500 and Nasdaq Composite were modest at around a third of a percent, but they reflected a downbeat attitude among investors, who are struggling to predict what's likely to happen on the political and macroeconomic fronts for the remainder of the year and beyond.

In addition, there was some bad news from individual companies that had a negative effect on sentiment, and GNC Holdings (GNC), Aaron's (AAN), and Dean Foods (DF) were among the worst performers on the day. Below, we'll look more closely at these stocks to tell you why they did so poorly.

Image source: GNC Holdings.

GNC looks less healthy

GNC Holdings finished the day down 17% after analysts at Goldman Sachs downgraded the seller of nutritional supplements and other health-related products. Goldman reduced its rating on GNC stock from neutral to sell, cutting its price target by a third, to just $8 per share. The analyst company believes that GNC's efforts to restructure itself in its brand relaunch create risk that results won't be as strong as the company hopes.

For its part, GNC has been confident that eliminating multiple pricing structures, mobile-friendly innovation, and better customer-service efforts will all help regain customers' trust. With fourth-quarter results due in early February, investors will soon get initial signs of whether GNC's strategy will pay off.

Aaron's falls off

Aaron's dropped 10% in the wake of a downgrade. Analysts at Raymond James cut their rating on the rent-to-own specialist from strong buy to market perform, citing recent share-price gains that had sent the stock soaring by nearly a third since the end of October.

In addition, industry rival Rent-A-Center issued preliminary guidance for the fourth quarter, shocking investors by saying that it expects a loss due to plunging comparable-store sales at its core U.S. locations. Company-specific technology issues were part of Rent-A-Center's problems, but Aaron's investors believe that some of the problems have to do with the tough conditions in the current retail environment, and those could hurt Aaron's just as badly as its peers.

Dean Foods gives back some ground

Finally, Dean Foods dropped almost 7%. The egg and diary specialist got downgraded by analysts at Bernstein, cutting their rating on the stock from outperform to market perform and leaving their $22 per-share price target on the stock unchanged. Dean has also climbed dramatically in recent months, rising by a third during the fourth quarter of 2016 as business conditions strengthened and investors speculated that the company could benefit greatly from corporate tax reform. Yet investors seemed comfortable giving back some of those gains, and potential fundamental challenges, like rising costs for raw materials and falling demand for brand-name milk products, could weigh on earnings.