The recession of 2008 taught investors to focus on lean companies that had enough cash around keep the lights on. But as the economy continues to improve, does it make sense to part from the pared-down approach to investing?

It can. Companies that struggled under the burden of heavy debt loads have been ignored for a while now, offering a potentially great value opportunity for investors who are willing to add a bit of risk to their portfolios. To be sure, not all of these underwater companies are worth taking a stake in – but the ones that aren't are still worth watching. After all, a simple debt restructuring could potentially be all that stands in the way of a more reasonable fundamental picture.

Underwater AND Profitable
Here's a look at a handful of companies that are saddled with huge debt loads but still manage to operate profitably.

Healthcare Rising
Things are looking up for hospital operator Tenet Healthcare (NYSE:THC). Following a restructuring in 2009 that saw the company divest itself of a number of underperforming facilities, THC is starting to reemerge as a key player in the healthcare arena. Debt isn't the only problem that was on the company's plate. In 2002, a Medicare fraud and kickback scheme were uncovered, slashing share prices by 72% in the last five days of October that year.

Now, with investors' attention once again, and a rebounded reputation, the company's leverage remains its biggest hurdle. Tenet paid down a reasonable portion of its debt as part of its restructuring program, a good sign that management is turning the corner. A profitable year in 2008 echoed that bullish sentiment. Hopefully for Tenet, that sentiment can continue. (Learn more in Investing In The Healthcare Sector.)

Risky Real Estate
One of the worst hit sectors in the past few years has been real estate. As the real estate market crumbled, so too did the financial positions of property management companies and real estate investment trusts (REITs). Among this group is Home Properties (NYSE:HME), a REIT that owns and operates apartment communities. Like Tenet, Home Properties has focused on improving its financials in recent years by selling off underperforming assets in favor of properties with more profit potential.

But that change-up has done little to counter the adverse effects of an ailing real estate market – a fact that sent the REIT's shares down double digits in 2008. With an already-leveraged balance sheet, Home Properties' chances to increase its property portfolio through borrowings could be restricted by a lack of available credit. (Learn more in Basic Valuation Of A REIT.)

CBL & Associates Properties (NYSE:CBL) faces similar challenges. The Tennessee-based REIT owns and operates commercial real estate, including shopping malls, community centers, and office buildings. Because of CBL's connection to consumer spending through its mall business, the REIT has been doubly hurt by cutbacks in consumer spending and real estate devaluations in recent years. And with interest rates expected to rise in coming years, the company's debt service could reach unmanageable proportions; currently, interest alone accounts for nearly 30% of revenues.

Should Republic Be Grounded?
Regional airline Republic Airways (Nasdaq:RJET) has been in the headlines lately as one of the best-in-breed air carriers following November earnings that impressed Wall Street. Much of the airline's performance has been thanks to significantly lower fuel costs in 2009 and better efficiency than the competition. But a highly leveraged balance sheet could be bad news for investors who are already on relatively thin ice.

The future for airline stocks is still very uncertain at the onset of 2010; fuel costs and the cost of capital both remain relatively volatile and have substantially impacted bottom lines in the last several years. Republic's relatively recent acquisition of former partner Frontier Airlines increases the former's debt load even more. With Republic required under current agreements to retire around $750 million in borrowings during the next few years, the company will need to tighten its belt hard to stay on investors' radar.

Don't Discount Liquidity
While many investors and economists believe that better economic days are ahead of us, it's important to plan for how a second pullback could affect your portfolio. Although many underwater stocks trade at a perceived discount right now, there's a reason why investors favored companies with cash-filled coffers in 2009. Don't get caught with an over-leveraged portfolio.