[ ♪INTRO ] Losing weight is hard.
Like, really, really, really hard.
The overwhelming majority of people who tryto do it don’t succeed or end up gaining back what they lose, sometimes more.
And that’s not just because pizza is amazing.
It turns out your body actually pushes backwhen you attempt to slim down.
The fat stored in your adipose tissue is asuper energy-rich substance that your body can use in a pinch to fuel your cells.
If you can’t eat for whatever reason, orneed a little extra energy to grow or reproduce, your body can turn to your fat — which iswhy, from a survival perspective, having some fat is actually a good thing! Still, you’d think that losing weight wouldbe pretty straightforward: just eat less than you need, force your body use up some of itsfat, then go back to eating a normal amount when you’re the size you want to be.
But the body doesn’t want to lose its energybuffer — no matter how large or small it is — so when you cut calories, it reactsin ways that ultimately make it harder to lose weight.
A lot of the push back is driven by changesto hormones.
One of the most important is leptin, a hormonesecreted by your fat cells.
The larger your fat cells are, the more leptinthey produce.
So when you lose weight, leptin levels drop.
Parts of your brain like your hypothalamusinterpret less leptin as starvation, and it jumps in and starts telling your body to conserveenergy and to eat more to rebuild those reserves.
Other organs also use hormones to complainto your brain about the decrease in fuel intake.
Your stomach tells your brain it’s not gettingfilled by increasing levels of the hormone ghrelin.
At the same time, your pancreas secretes lessinsulin, which regulates blood sugar, and amylin, which signals fullness.
So when you cut calories, ghrelin levels riseand insulin and amylin levels plummet, signalling your brain to increase appetite — makingyou feel ravenous.
In addition to changing how hungry you feel, a suite of studies have suggested your brain responds to these hormonal changes by makingyou more aware of all the food you’re not eating, and upping the pleasure you feel ifyou do cave in.
Meanwhile, the rest of your body becomes moreenergy-efficient.
For example, your muscles change where theyget their fuel.
When your muscles need energy, they generallyuse a mix of stored fat and circulating glucose.
But when you’re on a calorie-restricteddiet, they rely more heavily on glucose, so they end up pulling more energy from the foodsyou eat instead of those fat stores you’re trying to lose.
They also make other small changes to becomemore efficient — and so do other tissues in your body.
Here’s the really annoying thing: this hormonalstarvation signal doesn’t stop when you stop dieting.
That makes sense for leptin, since it’sbased on the amount of fat you have.
But other hormones which generally respondto food intake can stay on that slower production cycle even when you return to normal eating.
And these hormones can stay altered for years.
So even when you’ve stopped restrictingcalories, your body continues to act like it’s being starved — which is a big partof why people who lose weight often gain it back.
To make matters worse, even regaining theweight doesn’t shift your body out of energy-efficient mode.
In general, the smaller you are, the lessenergy you need to fuel everything.
But it’s not a simple, linear relationship.
How much energy you use per kilo at any givenbody weight varies depending on whether you’ve ever been heavier or skinnier.
And this effect could be clearly seen in a2016 study which followed contestants from a televised weight loss competition for sixyears.
In particular, the researchers looked at theparticipants’ resting metabolic rates: the calories their bodies burned at rest.
It’s basically a measure of the minimumamount of energy needed to keep a person’s cells running.
After the 30 week contest, the 14 participantslost an average of about 58 kilograms, and their resting metabolic rates dropped by about610 calories per day.
In the years that followed, though, they gainedback an average of 41 kilos, and their metabolic rates didn’t go back up accordingly.
They ended up burning 500 calories a day lessthan they should have at their final weights.
Which means to lose weight in the future, they’d have to restrict themselves even more than they did the first time around.
Lots of other studies have come to similarconclusions.
After people lose weight, even if they gainit back, their bodies simply use fewer calories per kilogram than similarly sized people whoseweight hasn’t changed.
And that means they have to eat less to stayat that weight than people who were never heavier, and they gain weight faster if theydo overeat.
It’s not yet clear just how long all theseanti-weight-loss changes last — or if they ever completely go away.
But not everyone experiences the same degreeof resistance from their bodies.
Scientists are still trying to figure outhow our person’s genetics, the foods they eat, and other factors affect how a personresponds to dieting.
But given how fiercely the body can fightslimming down, it’s no wonder so many people struggle with it.
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