(Story updated to add that Goldman Sachs downgraded Whole Foods Market to "neutral" from "buy" on valuation concerns.)BOSTON ( TheStreet) -- Stock market experts are saying there's are still a lot of economic issues to worry about despite the first quarter's stellar performance. That should have investors thinking about rebalancing their portfolios to preserve some of those gains. So with a view toward turning to more conservative, less volatile stocks, I reviewed the portfolios of eight of Morningstar's most highly rated big mutual funds to search out stocks that those funds have held the longest. Such longevity imparts a greater degree of confidence in them on the part of the fund managers. Given most mutual fund managers' reputations as quick-on-the-draw traders, these funds have held some stocks for a surprisingly long time, including one 28-year-old pick. But it's worth noting that many fund managers change the size of their allocation to a particular stock or it changes due to other variables. And some funds may close out positions in long-held stocks only to reinitate them at a more opportune share price. The $21 billion American Funds American Mutual Fund ( AMRMX) has owned computer-industry bellwether International Business Machines ( IBM) since 1984. Its next-oldest holding is drugmaker Bristol-Myers Squibb ( BMY), which it bought in 1993. Clearly, the fund's managers think the old fellas still have room to run as both stocks are currently in the fund's top 10 holdings. The $5.5 billion Sequoia ( SEQUX) isn't far behind in the longevity race, as it has owned Warren Buffett's Berkshire Hathaway ( BRK.B) since 1990. And the fund's managers still have faith in its value as Berkshire is currently the second-largest portfolio holding at 7.7% of the fund. Sequoia owns only 37 stocks. It has an average annual return of 24% over three years. Given its 931-stock portfolio, a stock could potentially hide out for decades in the $36 billion Fidelity Low-priced Stock Fund's ( FLPSX) portfolio, but that's not the case, given this fund's sterling track record. Although it has owned the aviation and marine fuel services firm World Fuel Services ( INT) since 1991 and curling iron maker Helen of Troy Ltd. ( HELE) since 1995, both stocks are earning their keep. The Fidelity fund is up 14% this year and has a three-year average annual return of 28%. On the other end of the spectrum of portfolio size, the focused, 28-stock portfolio of the FMI Large Cap Fund ( FMIHX) has held a stake in Kimberly-Clark ( KMB), the maker of consumer products such as Kleenex and Kotex, for a decade, and it's now its eighth-largest holding at 4% of the portfolio. The $5 billion fund has a three-year average annual return of 22%.