Most health-conscious people are familiar with the saying, "You are what you eat".

Here's the good news: That's likely not true.

As Alan Levinovitz writes in a new book, "The Gluten Lie," the real principle we should be following has little to do with what types of foods we eat. All we really need to pay attention to is how much we're eating.

His idea is backed by several big recent studies, one of which finds that modern guidelines on fat consumption aren't backed by scientific evidence.

Nevertheless, the ancient idea that we are what we eat continues to inform our eating and dieting habits. Many of us still believe, at some level, that eating fat will make us fat; that eating cholesterol will give us high cholesterol.

It's one of the biggest motivators of fad diets: That cutting gluten, dodging carbs, or avoiding fatty foods will translate into weight loss. But unless you're eating less overall, that's unlikely. Here's Levinovitz:

"Low fat, low carb, low whatever: for successful dieting, the common denominator is lower consumption across the board."

A recent look at the studies behind our current fat guidelines, for example — which state that we should restrict saturated fat to under 10% of all the calories we eat and that we shouldn't get more than 20% to 35% of our daily calories from fats — find that there wasn't evidence to support those rules in the first place.

After looking at the research on fat consumption that existed at the time, the authors of the new study write that the "dietary advice not merely needs review; it should not have been introduced."

In other words, eating some fat doesn't make us fat. Eating some cholesterol doesn't necessarily give us high cholesterol.

So why do we still think we are what we eat?

We all know that consuming too much of anything will result in health problems. Eating too much sugar gives you cavities. Drinking too much alcohol makes you drowsy and can give you a hangover. But research shows that eating small amounts of any food — be it in the form of rich avocados, cholesterol-laden eggs, or even butter — doesn't result in problems.

Yet we're still compelled to think that eating fat — any of it at all — will make us fat.

Levinovitz cites some psychological research, including a 2009 study by University of Pennsylvania psychologist Paul Rozin and University of Southern Maine psychologist Carol Nemeroff, to help explain why.

For their study, the researchers wrote two descriptions of a tribal society whom they called the Chandorans. Both descriptions were the same except for one aspect: their diet. While one group of Chandorans hunted boar and turtle and ate only boar, the other group hunted boar and turtle and ate only turtle.

Then, the researchers had 167 volunteers rate the Chandoran society in terms of their speed and average lifespan.

The volunteers were consistently more likely to say they thought the turtle-eating people lived longer but were slow, while they tended to say the boar-eating people were heavyset and aggressive.

In other words, Rozin wrote in his paper, the results "are clearly consistent with the hypothesis that…subjects 'believe 'that 'you are what you eat.'"

A simple solution: be mindful of your portion sizes

Rather than focusing on cutting out any specific food group — from fat to carbs — the research suggests we might be better off simply being a little more mindful about how much of everything we eat.

Of course, there are always some foods to keep an eye out for, like those with high concentrations of a few specific ingredients. A 20-ounce bottle of soda, for example, has roughly 65 grams (just about 16 teaspoons) of sugar.

But if there's any "secret" to eating healthy, chances are it's incredibly basic. Michael Pollan said it well a few years ago: "Eat food. Not too much. Mostly plants."