We've all heard the axiom that everything is OK in moderation.

But nutrition science doesn't really agree with this saying. A cookie here, a salad there — turns out that's not quite the same as actually being healthy.

Dr. David Ludwig, a Harvard Medical School nutrition and obesity expert, talked to Tech Insider about what scientifically sound dieting looks like. His book, "Always Hungry?" outlines an evidence-based eating plan that focuses on what you eat — not just how much.

Ludwig called the moderation mantra "useless."

He said:

There [are] some things you [should] eat a lot of, and I would put things like olive oil, avocado, nuts in that category. There are other things you [should] really minimize, especially if you're dealing with pre-diabetes or some other metabolic problem. You don't want to go moderate with sugar — whatever that means. You want to get rid of as much of it as you can.

Ludwig referenced a study published in the New England Journal of Medicine that gave a group of more than 7,000 people at risk for heart disease either a Mediterranean diet with lots of olive oil, a Mediterranean diet with lots of nuts, or advice to eat a low-fat diet as a control.

The researchers actually had to stop the study early because the rates of heart disease dropped so low in the groups eating lots of healthy fats that Ludwig said "it would have been unethical to keep the control group eating the low-fat diet."

The results suggested that eating especially healthy foods like olive oil and nuts "in moderation" was not enough. There are some foods we need to pile on, and others we should cut back on — something the Mediterranean diet is famous for.

Other scientific research has disproved the moderation idea, too.

A study in the American Journal of Clinical Nutrition found that eating a "high variety of sweets, snacks, condiments, entrées, and carbohydrates" but a low variety of vegetables made people fatter.

Another study found that type 2 diabetes risk decreased when people ate a high variety of fruits or vegetables, especially when they ate a lot of the latter. So eating plenty of vegetables — not just a moderate amount — seems to be the best idea.

And a study published in PLOS ONE last year analyzed the diets and relative risks of obesity and diabetes for a diverse group of about 5,000 American adults. The results were a pretty strong argument against one of nutrition's most persistent axioms.

"Our results challenge the notion that 'eating everything in moderation' leads to greater diet quality or better metabolic health," the authors concluded. "Our findings support the importance of diet quality, independent of diversity."

In fact, the very metric of diet quality — used by the government and researchers to assess healthy eating patterns — is grounded in the idea that we need a lot of certain foods, and less or none of others.

The takeaway? The quality of the foods you're eating matters more than the relative quantity. In other words, what you eat matters — not just the amount.

So it's probably time to stop saying that everything is OK in moderation, since some things just aren't.

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