Plant-based versions of classic menu items are taking over fast food, from Burger King's Impossible Whopper to Subway's Beyond Meatball Marinara sub.

These menu items are nutritionally pretty similar to the classics that they are named after.

Companies like Beyond Meat and Impossible Foods aren't aiming to produce a product that is necessarily better for you. Right now, the focus is simply on swapping meat for plants — and it's paying off.

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Plant-based meat is taking over fast food. But that doesn't necessarily mean your lunch is getting healthier.

Fast-food menu items created in partnership with companies like Impossible Foods and Beyond Meat are typically pretty similar nutritionally to the originals they are imitating.

The Impossible Whopper has 630 calories; the Whopper has 660. The Impossible Whopper has 12 grams of saturated fat; the Whopper has 11. The Impossible Whopper has 1,240 grams of sodium; the Whopper has 980.

Read more: Impossible Foods' and Beyond Meat's plant-based burgers aren't actually healthier than the fast-food originals

"Processed foods, whether they're meat-based or plant-based, aren't a nutritional need in our diet, especially when they involve low-quality oils," Whitney Stuart, a board-certified and licensed dietitian-nutritionist, recently told Insider of KFC's new Beyond Chicken.

While some people are seeking plant-based products because they want healthier options, nutrition isn't at the core of these new menu items. As one chain after another rolls out new menu items, the core similarity is clear: These aren't intended to reinvent the menu, but imitate it.

Beyond Meat and Impossible Foods aren't trying to make fast food something it isn't

The Impossible Whopper. Antonio Villas-Boas/Business Insider

A few years ago, chains were adding the Impossible Burger and the Beyond Burger to their menus. Now, the companies are creating new versions of fast-food classics, keeping the branding.

Jose Cil, the CEO of Burger King's parent company, Restaurant Brands International, recently told Business Insider that there was a lot of internal discussion about whether the chain should roll out a plant-based "Whopper" or a new burger entirely to avoid accidentally tarnishing the Whopper's reputation.

"When we use the Whopper brand, it needs to be a really exceptional product," Cil said. "The Whopper brand and heritage is so strong. People know it so well and associate it with a good, high-quality, great-tasting burger. It's really hard to put that label on anything else."

The quality of the product persuaded executives to roll out the new version of the Whopper, which the chain has advertised as identical to the meat-based version.

When imitation is the goal, pushing for a lower-calorie or lower-fat option comes second to perfectly recreating textures and flavors. The new menu items don't have to be healthier or even 100% vegan — they just need to pull off the magic trick of swapping meat for plants.

For many people, plant-based meat isn't tied to health at all, but reducing meat consumption to combat climate change and factory farming. Others are just curious about whether chains can pull off the trick of turning meat into plants without anyone noticing.

Chains see people's curiosity and have translated it into sales. The Beyond Taco and the Impossible Whopper have significantly boosted sales at Del Taco and Burger King. Down the line, chains may see another benefit of plant-based meat: lower prices.

While making plant-based menu items is more expensive, Beyond Meat CEO Ethan Brown said that in the next five years the company plans to create products that cost less than comparable animal proteins. If meat prices rise, having a plant-based option on the menu could be a key way for fast-food chains to keep prices low and win over budget shoppers.

The makers of plant-based "meat" aren't trying to turn fast food into something it isn't. Instead, with plant-based copycats, Impossible Foods and Beyond Meat are doing the opposite: betting that people want fast food that tastes exactly the same, without changing up much beyond the meat.