If you're a regular at the gym, it's hard not to see the money to be made in the workout industry. Nike (NYSE:NKE) Dri-Fit clothing costs a small fortune, gym memberships can easily exceed $100 per month, and accessories like workout watches and bicycles can add up to thousands of dollars every year.

For investors, all that spending spells opportunity -- and sometimes from companies you might not expect. Here are my five top stocks for workout warriors to invest in. 

Garmin fenix 2. Image: Garmin.

If you've ever done a triathlon, an open water swim, or a marathon, you know about Garmin (NASDAQ:GRMN), the maker of GPS watches, golf range finders, and even fishfinders. Despite the growth of GPS capabilities on devices like Apple's (NASDAQ:AAPL) iPhone, Garmin still offers capabilities that competitors can't. 

As much as Apple and Google's Android want to overtake Garmin in smartwatches, Garmin has a niche that will be harder to overtake than it may seem. Garmin devices are rugged enough to endure events like the Ironman and specific enough for aviation and sailing. Despite its challengers, Garmin has a place with athletes for years to come.

It's tough to overstate how important Nike is to athletes. The company plays a part in nearly every major athletic market and it's become the de facto equipment provider for a lot of training gear. Dri-Fit is a common product in any gym, and Nike shoes, shirts, shorts, and compression gear are a staple in any workout warrior's wardrobe.

What's impressive about Nike is its ability to enter new markets and engineer a product that's equal to or better than competitors. Before Tiger Woods, Nike barely had a presence in golf and today it's one of the biggest players in golf apparel and equipment. In running it has always played second fiddle to brands like New Balance, but it has been getting better at designing products for running as well.

With the brand awareness Nike has and the engineering prowess to dominate any market it chooses, Nike should be a staple for any workout warrior's portfolio.

Under Armour
If any company could push Nike from its perch atop athletic gear, it would be Under Armour (NYSE:UAA). The company is a major competitor in athletic clothing and has had an impressive launch in the shoe business, where it started from scratch just a few years ago.

But the workout warrior also may recognize Under Armour's booming app business: a field where the company has invested hundreds of millions of dollars to become a leader. Over the last two years, Under Armour has bought MapMyFitness, My Fitness Pal, and Endomondo for $710 million.

Under Armour's apps are now a go-to for tracking everything from your workouts to what you're eating. No doubt, Under Armour will also be a big provider of apps to new devices like the Apple Watch. 

Fitness is built into the functionality of every Apple Watch. Image: The Motley Fool.

Every one of the companies I've mentioned so far had one thing in common: They all make apps for Apple products.

The iPhone has become a go-to fitness item whether you're downloading data from your Garmin watch or tracking a run on your iPhone. New products like the Health app and the Apple Watch will only integrate Apple further into the fitness scene.

For Apple, it helps that CEO Tim Cook is a fitness buff, so he understands what features and devices athletes want. And Apple doesn't have to intentionally design much fitness functionality in itself. It can rely on thousands of developers to do the hard lifting creating value in the fitness business and Apple will benefit from selling devices.

Whole Foods
When it comes to staying healthy, Whole Foods Market (NASDAQ:WFM) is the first name on many workout warriors' grocery lists. The company's business is built around providing fresh, organic, and healthy foods for its consumers, which people will pay a premium for. It's no coincidence that a resurgence in people thinking about their health has happened as Whole Foods has grown. The company is simply riding a larger health trend happening in the U.S. today.

Whole Foods will have its competitors in the health food space, but it has such a lead in terms of brand awareness and infrastructure that investors should put this stock first when it comes to food for before or after your workouts.

The health trend isn't going away
After decades of Americans getting less healthy, the country now seems to be swinging in the other direction. Marathons, triathlons, mountain climbing, and all kinds of other workout activities are growing in popularity and people are spending billions of dollars each year on the gear they need to work out. These are five of the best stocks for the workout warrior and they'll continue to ride this healthy trend for years into the future.