One of the hottest categories of mutual funds in recent years inflicted the harshest losses on investors in 2015, the result of a toxic combination of sliding oil prices and rising interest rates. A league table of the worst-performing US funds is dominated by so-called energy limited partnership funds, which were sold as a high-yield investment that would not be as exposed to commodity prices as direct investments in energy producers. The funds invest in master limited partnerships, stock market-listed companies that run oil and gas pipelines and other infrastructure businesses. MLPs are mandated to pay out most of their profits in dividends, making them popular with income investors. American savers poured more than $20bn into energy limited partnership funds in the two years before 2015, according to the research firm Morningstar, only to suffer losses of up to 35 per cent in some cases this year.