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If a specific pattern of eating could propel you toward PRs and boost your performance—all the while playing to your body’s DNA—you’d stick to the plan, right?

This past week, the respected —which promised to do all of the above. The fad suggested that being A, B, AB, or O should impact your food choices, and was popularized by the book .

But researchers say there’s absolutely no evidence to support it.

So why did the diet garner so much hype? We looked into the issue last year. And as we noted in that story, some of the science is sound. With new research, doctors have been able to ID and test tons of blood-based indicators that affect your health: cholesterol, calcium, and vitamin D, to name a few. And doctors then use these indicators to supposedly enhance performance, raise your metabolism, and boost your mood.

Where companies who market “blood-based diets” fall short is here: It’s not about your blood type,” says Stacy Sims, Ph.D., and co-founder of Osmo Nutrition. It’s about how your body reacts to certain foods—markers of which can be seen through your blood. Certain people feel better eating certain foods—some feel worse. The association between that idea and blood type is just that: an association, she explains.

What we do know: “Blood is very telling—it’s the main fluid and transporter in your body,” she says. If you have an inflammatory response to something you eat, it’ll show up in your blood. If you’re low on vitamin D, for example, that can affect how your muscles repair themselves,” says Sim. D levels can be discovered through a blood test since the vitamin circulates there. Blood tests—in this sense—make sense.

More importantly, though, blood tests need context. Take C-reactive protein: a marker of inflammation. Elevated levels could mean that you had a hard workout, that you’re highly stressed, or that you have heart problems. But without context, a test can be misleading.

There’s something to be said for personalized medicine and learning more about what’s going on in your body. But your health shouldn’t revolve around a “diet,” Sims emphasizes. “It’s about fueling your body with functional foods,” she says.