The biggest reason people fail at diets, according to a dietitian

Business Insider reached out to registered dietitian Whitney Stuart of Whitness Nutrition about the problem.

Stuart told us that the whole concept of a diet is backwards, because in most cases, what society thinks of as a "diet" is "based on the idea of less." That could mean cutting calories or food groups, having to weigh every ingredient, or only eating at specific times — the list goes on.

Most fad diets have strict, specific rules. Ultimately, Stuart said, these diets tend to be unsustainable for long periods of time, let alone the rest of your life.

The idea of deprivation, ingrained in many diets, "gives us control over a situation" in the short term, Stuart said, perhaps leading people to think, "I haven't eaten any cookies, I'm so good."

"This short-term diet doesn't become a habit," she said, "and 10 days — maybe two weeks — later, we see that deprivation rebound when self-control finally dwindles. Because that is limited."

At this point, Stuart says, people often binge. People tell themselves, "I haven't eaten any cookies in two weeks, I deserve one! I earned this cookie!"

Stuart told Business Insider this puts the person back at square one, and sometimes in a negative state of mind because they may now feel guilty for eating too many cookies. They view themselves as "bad" or "out of control," she said.

"You and your goodness (or lack thereof) are not affected by what you eat," Stuart said. "This mindset is so dangerous."

It's true there are some things we should try to avoid, she said. Endless fast food, concentrated sweets, dangerous drugs, and excessive alcohol probably shouldn't be consumed on the regular.

"But we shouldn't promote the deprivation-based diets as the only successful tool that will provide weight loss, because it's not," Stuart said.

How do you stick to a diet?

Stuart recommends making small but sustainable changes, staying away from anything extreme, and building up small changes over time. The kicker is, she said, most people don't want to make the slower lifestyle changes to last.

"Most people would rather have a horrible 10 days of a raw, low-carb, no salt, no sugar, no water diet and return to their old habits than really have to address that $150 Frappuccino bill they're racking up each month," Stuart said.

She recommends easing into things:

Ultimately, smaller changes over a longer period of time can land you on the same diet you wanted in the first place, but in a sustainable way.