NEW YORK ( TheStreet) -- During the past 12 months, the consumer price index rose 3.6% and oil prices climbed. Seeking to benefit from rising prices, investors scrambled to buy Treasury Inflation-Protected Securities. Inflation-protected funds -- which invest in TIPS -- returned 8% in the past year, outpacing the Barclays Aggregate Bond Index by nearly 3 percentage points, according to Morningstar.Should you join the crowd and buy inflation funds? Perhaps. Many financial advisers argue that the inflation funds make good choices for investors who seek to protect their purchasing power. But now some advisers have become lukewarm about TIPS, arguing that they have gotten a bit expensive. A vocal minority says investors should avoid TIPS altogether. In some respects, TIPS resemble Treasury bonds. Like Treasuries, TIPS make fixed-interest payments that are guaranteed by the government. When interest rates rise, the prices of TIPS and Treasuries can fall. But TIPS are unique because the principal value rises along with inflation. If the CPI climbs 3%, then the principal value of TIPS will climb by that amount. During the past year, everything worked in favor of TIPS investors. Interest rates fell, and bond prices rose. In addition, the rising CPI boosted TIPS. But as TIPS prices rose, yields sank to near record lows. The yield on 10-year securities is a measly 0.58%. On five-year TIPS, the yield is -0.48%. The negative yield has occurred because the securities sell for a premium price of 102. So an investor must pay about $1,020 to buy a security that will only be worth $1,000 on maturity. Are investors guaranteed to lose money on five-year TIPs? Not necessarily. If you buy now, you will receive regular annual interest payments of 0.15%. That is not enough to compensate for the drop in value from 102 down to 100. But if the CPI rises, the principal of TIPS will rise, and investors could come out ahead. Say the CPI continues rising at 3.6% for the next five years, an outcome that is entirely possible. Then TIPS would outperform conventional Treasuries by a comfortable margin. "TIPs are expensive, but they can still provide value for long-term investors," says Peng Chen, president of Morningstar's global investment management division.