There is a good reason why restaurants and food manufacturers are catering to millennial consumers: They're a massive generational force with an equally tremendous capacity for spending money.

Millennials started coming into adulthood around the year 2000 and are the biggest generation in U.S. and world history. Following on the heels of Gen Xers, or the so-called slacker generation, millennials are a group on the move, expected to spend as much as $200 billion annually by 2017 and as much as $10 trillion over their lifetime -- and that's just in the U.S.!

So what millennials want, restaurants and food companies are smart to give to them. But with that demand comes some odd preferences. QSR Magazine pegged millennial food habits as including a desire for diverse, unusual, customized fare that touts health as well as social consciousness. The following four food trends could be the next huge opportunity, even if some of them seem really weird.

1. Bone broth
Kale has the distinction of being among the best-known and most widely available of the so-called superfoods, a group of foods that pack a wallop in terms of their near-magical health qualities. Even McDonald's is using it to attract consumers. But the leafy green is being pushed aside in favor of a new superfood craze: bone broth.  

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Putting meat on the bones of the superfoods trend, millennials can't get enough of bone broth. Photo: Kevin Teague.

Although it's been around for hundreds of years and you're likely to have seen and tasted it in your grandmother's kitchen, bone broth is enjoying a resurgence in popularity as celebrities and sports figures like Kobe Bryant, sign onto its cure-all reputation. 

While it's name is exactly what it sounds like -- broth made from boiling animal bones in water -- prepackaged versions are making their way onto the shelves of Whole Foods Market (NASDAQ:WFM). Although science isn't completely sold on just how much health can be supped out of a bowl of bone broth, it still represents part of a larger movement by millennials to adopt products and practices that have a perceived natural, healthy, and organic pedigree. Food Business News noted earlier this summer that millennials like soup because it is an affordable comfort food that can be made with fresh, local ingredients.

2. Tree water
Water is big business, which is why SodaStream International (NASDAQ:SODA) shuns its former association with carbonated soft drinks and touts itself instead as a sparkling water company.

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Coconut water has been a go-to thirst quencher for millennials, but will it be replaced by the newest fad, tree water? Photo: Mike Mozart.

The International Bottled Water Association says bottled water consumption grew to 34 gallons per person in the U.S. last year, a 7% increase over 2013, with the possibility it could surpass soda as soon as 2017.

But in keeping with the millennial way to up the health aspects of a product, plain water is just not enough. According to Coca-Cola's (NYSE:KO) Zico brand, coconut water is growing 30% annually, and becoming a $1 billion business in the process. Coconut water is a clear liquid tapped from the center of young, green coconuts.

And millennials are helping the trend branch out in new directions. Tree water, which is tapped just as you would when gathering maple syrup, is now stocked at Whole Foods in flavors like birch and maple. It also has a long history of healthfulness in eastern Europe, and if it catches on here we may see this niche product rival coconut water in availability. 

3. Meat in your energy bar
For anyone who's wanted to buy a steak recently -- heck, just wanted to make a hamburger -- the record-high price of beef is a testament to what surging demand at a time of diminished cattle herd size can cause. But with high-protein diets like paleo contributing to sustained interest, it's causing food manufacturers to consider fortifying everything with protein.

Coca-Cola, for example, thinks consumers will pay a hefty premium for protein-infused milk, despite milk consumption being in a decades-long decline. And now energy and protein bars are getting a meat makeover. Think of them as PowerBars with organic and grass-fed beef, along with a few spices and dried fruit thrown in for good measure.

Eatclean.com rounded up some of the contenders in this category and described the meat bars as providing "clean, unprocessed protein, real meat flavor, and a soft and chewy [texture] ... with a hint of natural sweetness from the fruit." One of the companies featured in the post is EPIC, which aims to treat animals and land well and targets health and taste with its bars, all things close to your stereotypical millennial's heart. 

Meat bars could become a thing, but then again Greek yogurt went from phenom to mature category very quickly, and unit sales have turned negative.

4. Insects
I said millennials had some weird tastes, and though bugs have been used as a source of protein in many cultures for eons -- they're high in minerals and vitamins, too -- getting beyond the ick factor in Western society has been extremely difficult. The trick, it seems, is disguising them as more familiar foods.

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Ring the dinner bell; insects are on the menu! High in protein, vitamins, and minerals, the key to gaining acceptance as a meal choice is disguising what exactly they are. Photo: liz west.

Protein bars from Exo, for example, are made from cricket flour and are described as a dense, moist, and chewy alternative to the meat bars above. Next Millennium Farms provides cricket flour, mealworm flour, and more to substitute in baking. Banana cricket bread anyone?

The hook for millennials aiming to be kind to the Earth is that raising insects instead of livestock is a much more sustainable enterprise that consumes far less of the world's limited resources.

Opening up a whole new can of worms
Millennials seem willing to try anything once -- and more than once by the looks of it. Whether any of these trends have any staying power may be ultimately determined by Generation Z, the cohort of kids who are only just now coming into their own.

John Mackey, co-CEO of Whole Foods Market, is a member of The Motley Fool’s board of directors. Rich Duprey has no position in any stocks mentioned. The Motley Fool owns and recommends Whole Foods Market. The Motley Fool owns shares of SodaStream and has the following options: long January 2016 $37 calls on Coca-Cola, short January 2016 $43 calls on Coca-Cola, and short January 2016 $37 puts on Coca-Cola. The Motley Fool recommends Coca-Cola. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.