Americans have a bit of a reputation when it comes to food and exercise. They really like food, especially fast food. They're less keen on exercise - although they do like the trappings (activewear in the wardrobe; unused exercise bike in the garage).

With an eye to this and an eye to the stats (almost one-third of Americans are obese; the problem is getting worse) Janus Henderson had the insight and the requisite light touch to list The Obesity ETF (SLIM) back in 2016.

The fund - among the more edgy thematic offerings - invests in companies "positioned to profit from servicing the obese, including, but not limited to, biotechnology firms, healthcare, weight loss programs and supplement companies," the factsheet says. It is one of many Janus Henderson thematic funds, which includes the aging population (OLD) and organic food ETFs (ORG).

While SLIM may play like a joke, it's performance certainly isn't. The fund has made fat returns for its investors, beating the Vanguard Total Stock Market ETF since inception. (SLIM has returned 44% in that time, VTI has returned 36%).

But it's also beat the raft of other colourful thematic funds - including MAGA, which tracks companies with close ties to the Republican Party; MJ, the marijuana ETF; FANZ, an ETF that tracks companies that sponsor sports; and GENY, which tracks Generation Y's consumer habits.

Despite its success, investors don't appear to be biting. SLIM sits on a relatively small $10 million.