We've all been there: That moment when you just need to have a burger (or a milkshake, or a bag of potato chips) and nothing else on earth will do.

But why exactly do we have food cravings? And what do they mean?

First, let's clear up a big myth. It's a popular belief that cravings are the result of nutritional shortfalls. Chocolate cravings are often blamed on low levels of magnesium, for example. But most experts say there's just not enough research to support this idea.

"There is very little science-based evidence on food cravings linked to nutritional deficiencies," said Sharon Palmer, RDN, author of The Plant-Powered Life. "And if food cravings were related to something you need, then wouldn't you be craving kale or apples, not ice cream and French fries? Instead, people tend to crave foods that are rich in fats, carbs, and sugar."

(Especially sugar, according to a new study.) Even the popular chocolate theory falls pretty flat when you find out that an ounce of dried pumpkin seeds has more than twice the magnesium of an ounce of chocolate. But you don't see anyone hankering for pumpkin seeds. Plus, one study found that, even on a nutritionally complete diet, people still get cravings. 

MORE: The 13 Most Health-Boosting Food Combos on the Planet

This doesn't mean that food cravings aren't real. It's just that your hankering for pizza is probably linked to emotional needs—seeking a comfort food that releases feel-good chemicals in the brain during a time of stress, for example—not nutritional ones. (Here's 5 weird tricks to make your food more satisfying.) Other studies show that cravings can crop up simply because you're on a restrictive or monotonous diet and want what you can't have.

That said, there are some cravings that really do signal health problems. Here are three to look out for:

Craving: Water

Could be: Diabetes

Excessive thirst is an early symptom of diabetes—but this isn't just the craving for water that hits when you finish a workout. This is far more pronounced thirst that's also typically coupled with excessive urination. If you have diabetes, extra sugar builds up in the blood, and your kidneys have to work extra hard to filter and absorb that sugar. But sometimes they can't keep up, so the extra sweet stuff is diverted into the urine. This means frequent pee breaks, which in turn leave you thirsty for more water. 

Craving: Salt

Could be: Addison's disease

We don't crave salt because we need more of it—in fact, most Americans are getting more than enough salt from their diets. (The only exception? Endurance athletes who can lose too much salt by sweating profusely.) For the rest of us, intense salt cravings could point to Addison's disease, in which the adrenal glands (the ones that sit on top of the kidneys) don't produce enough hormones. And these hormones are important: They include cortisol, which helps the body respond to stress, and aldosterone, which keeps blood pressure balanced. Left untreated, Addison's disease can make your blood pressure drop dangerously low—so see a doctor if you have a new, persistent, excessive craving for salty foods, especially if you're experiencing any of the other Addison's disease symptoms.

Craving: Ice

Could be: Iron deficiency

Craving things with no nutritional value—ice, paper, clay, dirt—is a phenomenon known as pica. (Here are 8 things you definitely didn't know about what your food cravings mean.) And although these cravings aren't totally well understood by scientists, some studies have linked the desires with an insufficient supply of iron. One recent paper in Medical Hypotheses suggests that compulsive ice chewing increases blood flow to the brain, combatting the sluggishness caused by an iron deficiency. 

MORE: The Scientific Reason It's So Hard to Quit Sugar—And the Solution

This article originally appeared on EatClean.com.