If you think it’s hard to lose weight and keep it off, you are not alone — and you are also 100% correct. Long-term weight loss is really difficult to achieve, studies have found.

Estimates vary, but it’s believed that more than 80% of people who lose a substantial amount of weight regain it within five years.

But failure to shed pounds is often not about lacking the willpower to make important lifestyle changes, such as eating healthier, reducing calories and increasing physical activity. The dirty little secret is that our bodies are programmed by evolution to hold on to fat.

“We evolved not to lose weight intentionally,” paleoanthropologist Daniel Lieberman told CNN Chief Medical Correspondent Dr. Sanjay Gupta recently on the podcast Chasing Life. Lieberman, a professor and chair of the department of human evolutionary biology at Harvard University, studies why the human body looks and functions the way it does.

“All animals need some fat, but humans have evolved to have exceptionally high levels of fat, even thin humans,” he said. “And so we are under exceptional sort of biological pressure, always, to put it on and keep it as long as we have it, for when we need it.”

Humans are fundamentally adapted not to be happy or healthy but rather to be reproductively successful, Lieberman said. And for that, we need fat, a lot of fat — which is why Lieberman calls humans “an unusually fat species” compared with other mammals, even other primates.

 
“We have these big brains, which cost a huge amount of energy. … It’s 20% of our metabolism,” he said. “And a baby, when it’s born, half of its energy is paying for its brain. It needs a lot of fat. So … human babies are born very fat because they have to have that energy to make sure that they can keep their brain going.”

Lieberman said fat is storable energy. It helped early humans stay alive, powered their bodies to find food, kept their brains working and made them healthy enough to reproduce.

“It’s like money in the bank account. And so individuals who have appropriate levels of fat did better in our evolutionary history than those who didn’t,” he said. “And so we were selected to make sure that we always could put it on, because there were always times when we had to lose it.”

Lieberman said humans never evolved to lose weight deliberately.

And while our bodies haven’t really evolved from those earlier times, our environment has — and that is, what Lieberman called, a big mismatch. Nowadays, we don’t have to run from wild animals, travel long distances on foot, or hunt and gather our next meal. We can pick up a smartphone to call an Uber or Uber Eats and experience all manner of modern conveniences. As a result, many people now live with weight issues and obesity, and all of the “mismatch diseases” that stem from that.

“So mismatch diseases are defined as conditions or diseases that are more common or more severe when we live in environments for which we’re poorly or inadequately adapted,” Lieberman said, referring to our modern-day “obesogenic environment” that often contributes to weight gain.

“And so, of course, it’s hard. It’s because we evolved not to lose weight intentionally. And so, losing weight requires dieting, requires tricking your body and overcoming those adaptations — which your body’s going to fight you every, every inch of the way.”

Lieberman, who said we need to be “extremely compassionate” toward those who face weight challenges, including ourselves, suggests keeping these five things in mind:

Develop (evolutionary) perspective
Not all humans are meant to be stick figures or willowy waifs — no matter what you see on television, at the movies or on social media.

“Fat is especially important for humans,” Lieberman wrote in an email. “Even thin humans have between 15-25% body fat, which is three to four times more than most mammals.”

You will always have a certain amount of fat, and it is necessary in some ways.

Fat equals evolutionary success
Fat actually helps us survive and thrive.

 
 
Dr. Sanjay Gupta: What weight tells us about our health
“We evolved to store a lot of fat — a source of stored energy — because of our energetically expensive bodies and life history,” Lieberman said. “That fat helps fuel our big brains and our high cost of reproduction all while staying physically active.”

Even so, “we never evolved to store a lot of belly fat, which can lead to health problems,” Lieberman pointed out. “So having a lot of fat around the middle is a sign to do something.”

Small fluctuations are normal
Don’t worry if your weight goes up and down a few pounds over short periods of time.

“Much of that variation is due to water,” Lieberman said. “For most of human history people regularly cycled through times when they took in more energy than they used and stored the surplus as fat and then drew on those fat reserves during lean times when they used more energy than they consumed.”

The deck really is stacked against you
If you find it hard to lose weight, don’t blame yourself.

“Humans evolved to store plenty of fat when possible and then use it when needed,” Lieberman said. “But we never evolved to voluntarily consume less energy than we used — that is, diet.”

 
 
What to consider before starting the new weight loss medications
Lieberman said dieting triggers the body’s starvation responses that cause dieters to crave food and save energy by slowing down their metabolism. “So when people diet, they almost always struggle to overcome ancient, fundamental adaptations to prevent their bodies from losing weight,” he added.

Dieting versus exercise
If you are wondering which is more important for weight loss — exercise or dieting — the answer is both, but for different reasons.

“You can lose more weight by dieting than exercising,” Lieberman said. “But exercise helps prevent gaining or regaining weight, plus it has many, many other benefits for both mental and physical health.”

And as for that mismatch between our Stone Age bodies and our modern, obesogenic environment, Lieberman said we have to “figure out how to engineer our worlds to help us make the choices that we would like to make.”