How to strike the right note when picking out a theme Impax is a specialist asset manager with several funds available to British investors around resource scarcity, with portfolios focused on water and food supply. These are structured as investment trusts. Sarasin runs several established portfolios, including the wide-ranging EquiSar fund, which aims to profit from numerous and evolving themes, and AgriSar, which focuses on agriculture and food production. He said: "Health care can be accessed through specialist funds such as Axa Framlington Biotech, which offers direct exposure to the sector but has had long periods of poor performance until recently. With regard to agriculture, this sector is still in its early days and investors should consider long-term performance. For commodities, look at a better-known fund such as First State Global Resources or Artemis Global Energy . Beware themed funds with narrow remits The more specific a fund's theme or remit, the smaller a part it should play in your portfolio. Algy Smith-Maxwell, a co-manager of the Merlin fund of funds range at Jupiter Investment Management, said: "Investors who want to play themes should do so as part of a broader portfolio and also pay very close attention to them. There comes a time when themes will run out of steam or when share prices simply get too high. And, as the tech bust proved, you do not want to be exposed to these areas when they go wrong. Other financial commentators also urge caution. Jason Hollands of Bestinvest, the broker, said: "Investors need to treat funds anchored around supposedly long-term 'mega trends" with a degree of scepticism. The history of the industry is littered with examples of disappointment. The dotcom bubble is an easy target, but don't forget the raft of 'climate change" funds that were launched six years ago - and have since bombed. While the narrative of 'positive demographics" is used to set out a convincing case for emerging markets, it isn't assured that the benefit will accrue to companies listed in these markets. Yes, the Chinese may well buy a lot more cars or mobile phones over time, but the beneficiaries could well be developed market companies that manufacture them. Choose 'ordinary" funds that follow a themed approach Mainstream, highly respected fund managers at established organisations such as Invesco Perpetual and Standard Life Investments take wider, global themes into account when picking stocks in their portfolios. For more investment tips: sign up to our weekly email here.