At their most basic, they're both low-carb diets. But they're not exactly the same.
The biggest difference between the keto diet and the Atkins plan might be their origin stories, says dietitian and educator Claudia T. Felty, PhD, RD. “Atkins was designed for weight loss, and keto was designed, it its strictest form, for seizure prevention.” (Really! It was a tool in the treatment of epilepsy, and has only recently been adopted as a slim-down strategy.)
When you crunch the numbers for the two diets, things shake out a little differently too. People on the keto diet usually get 2% to 5% of their daily calories from carbs; while Atkins followers are typically getting around 10% of their calories from carbs (at least at first). Both diets use this ultra-low carb approach to trigger ketosis, a state in which the body burns fat for fuel instead of stored carbs, leading, in theory, to weight loss.
Another difference: The Atkins approach to carbs changes over time. “Atkins has what’s known as the ‘induction phase,’ which is the first phase of the diet. It allows 20 grams of net carbs—total carbs minus fiber. As the diet progresses, the carb amount allowed goes up," Felty explains. "Keto counts all carbs—not just the net—and the amount tends to be much lower long-term than that of Atkins.”
Adding in more good-for-you carbs as you reach, and then maintain your goal weight brings you out of ketosis. And that might be a good thing: Ketosis can trigger ketoacidosis, which is when excess ketones–a byproduct of fat metabolism–build up in the blood. Left untreated, ketoacidosis can be fatal.
When it comes to losing weight, both plans can help you shed pounds, especially at first. Low-carb diets are often successful weight-loss plans in the short-term as you shed water weight. But the results might not last: In its reviews of the two diets, U.S. News and World Report pointed to longer-term studies that haven't found much difference between low-carb diets and low-fat diets. There simply hasn't been enough research to determine if lasting weight-loss success on a low-carb plan is due to cutting carbs, or simply cutting calories.