Before there was Whole30, before everyone started going keto, and even before Paleo was a thing, there was the South Beach Diet. (Hey, Miami isn’t exactly known for modesty, so it’s no surprise the rest of the country was itching to know what the bikini-clad ate all day to feel so confident.)

Founded by cardiologist Arthur Agatston, MD, to give his patients a plan for heart-healthy living, it became a full-on brand in the mid-aughts. His advice in a nutshell: Eat high-fiber, low-glycemic carbs, unsaturated fats, and lean protein.

What exactly does the South Beach Diet mean 15 years later? And how does Dr. Agatston respond when people call it a (gasp!) outdated fad diet? To find out, I called him up to talk diet trends and what people get wrong about weight loss—no matter what year it is.

Keep reading for a no-holds-barred convo with South Beach Diet founder Dr. Arthur Agatston.

Here we are, 15 years after you developed the South Beach Diet. Has anything changed?

The principles of good fat, good carbs, healthy protein, and plenty of fiber have really stayed the same. Over time, there have been diet trends like low-fat or low-carbs, but we’ve always maintained that it’s more about the quality of fats and carbs.

Something that has changed is our guidelines about dairy. We added full-fat dairy to the “good fats” list because so much scientific evidence has come out about it being satiating—it won’t leave you feeling deprived or like you missed out on anything.

High fiber is a staple of the South Beach Diet, what do people need to know about adding the nutrient to their diet?

It’s a little different depending on how people react, but certainly fruits, vegetables, and whole grains are good sources of fiber. It’s not really about getting a certain amount of fiber; it’s more about where you’re getting it from. You want to make sure you’re eating whole foods, which have plenty of fiber but are also absorbed by the body slower. Carbs that are absorbed quickly wreck havoc on blood sugar levels. The key is eating slow-absorbing carbs, such as sweet potatoes and quinoa, which aren’t digested as fast and keep you full longer.

What do you say when people call the South Beach Diet a “fad diet”?

I would argue that the principles of good fats and good carbs are the principles of all the newer diets—it’s just said in a different way to be trendy. Also, the South Beach Diet was developed to be a lifestyle. I originally came up with it for cardiac prevention, which really takes eating a certain way all the time. This isn’t something I came up with so people could lose weight right before their high school reunion.

Speaking of fad diets, what do you think of the ketogenic diet? Everyone seems to be into it right now.

The first phase of the South Beach Diet is nearly the ketogenic diet. And the original ketogenic diet was the Atkins diet. People lose weight on it, certainly. That’s why people love the first phase of the South Beach Diet so much—they notice a difference. When you do it in the short-term, keto is good for brain function. But I don’t think it is safe nor sustainable to stay on it longterm.

Is there anything people repeatedly get wrong about weight loss?

Sure, that it should be fast. We are constantly battling the quick weight loss solutions. For heart patients, weight loss does nothing for you if you gain it back. Again, it needs to be a lifestyle.

Speaking of weight loss, find out if exercise if diet is the more important component. And for the record, yes, you can be body positive and still want to lose weight.