Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you expect demand for consumer staples to keep growing as our population grows, the First Trust Consumer Staples AlphaDEX ETF (NYSE: FXG) could save you a lot of trouble. Instead of trying to figure out which companies will perform best, you can use this ETF to invest in lots of them simultaneously.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The Consumer Staples ETF's expense ratio -- its annual fee -- is 0.7%. That's a bit higher than many ETFs, but still much lower than the typical stock mutual fund.

This ETF has performed reasonably well, having beaten the S&P 500 over the past three years, on average. The fund is still young, though. As with most investments, of course, we can't expect outstanding performances in every quarter or year. Investors with conviction need to wait for their holdings to deliver.

What's in it?
Several of this ETF's components made strong contributions to its performance over the past year. Green Mountain Coffee Roasters (Nasdaq: GMCR) surged 97%, buoyed in part by deals such as the one to offer Dunkin' Brands (Nasdaq: DNKN) coffee in its K-Cups. Interested investors should be careful, though, as the stock is heavily shorted by those who think it's gotten ahead of itself, and the SEC is looking into the company's accounting.

Other companies didn't add as much to the ETF's returns last year, but could have an effect in the years to come. Food distribution giant Sysco (NYSE: SYY) slid 4%, operating in an environment of rising food costs and struggling restaurant traffic.

Walgreen (NYSE: WAG) sank about 7%, despite delivering strong earnings reports. Part of the problem is its dispute with Express Scripts (Nasdaq: ESRX) over the fees Express Scripts pays Walgreen for prescriptions. Considering that by acquiring Medco Health Solutions (NYSE: MHS) Express Scripts will become the nation's largest pharmacy benefits manager, this is a significant issue for Walgreen.

The big picture
Demand for consumer staples isn't going away anytime soon. (That's why they're staples.) A well-chosen ETF can grant you instant diversification across any industry or group of companies -- and make investing in and profiting from it that much easier.

Learn about the best dividend ETFs. And if you're looking for some great investments beyond ETFs, consider these 10 stocks for your retirement portfolio.