Some say that many of the negative effects of dieting, like a decrease in your energy levels, a dip in your metabolic rate, and ravenous hunger, are caused by low leptin levels.
That’s why cheat meals, diet breaks and the refeed day — all of which are supposed to give your leptin levels a boost — are so important.
Hormones will be restored, metabolisms will be boosted, weight loss plateaus will be shattered, and all will be well with the world.
Or at least, that’s what’s supposed to happen.
Hitting the pause button on your diet, be it in the form of a cheat meal, a refeed day, or even a complete diet break lasting 1-2 weeks, does have its benefits.
However, the reason that it does work probably doesn’t have much to do with leptin or your metabolism.
More on that in a moment.
First, what is leptin? What does it do? And how does a diet break or refeed day affect it?
What is Leptin and What Does It Do?
Named after leptos, the Greek term for “thin,” leptin is a hormone released by your fat cells.
The amount of leptin in your blood is linked to the amount of fat stored in your body. The fatter you are, the higher your leptin levels. When you lose fat, leptin levels drop. When you gain fat, they go back up again.
Before the discovery of leptin, body fat was seen as a lifeless source of energy. However, work by researchers at the Rockefeller University in the 1990’s led to a radical change in perspective.
Both of the mice in the picture below have a genetic defect that predisposes them to gain weight. When they were injected with leptin — the hormone this “broken” gene normally produces — the mice lost weight.
After four and a half weeks of daily injections, the mouse on the left, which did not receive leptin, weighed approximately 67 grams. The mouse on the right, who received daily injections of leptin, weighed 35 grams.
As well as being affected by body fat, leptin also falls under the influence of your diet.
Restricting calorie intake for just seven days has been shown to cut leptin levels in half . That’s despite the fact that body fat dropped by less than one percent.
The researchers also found that the desire to eat doubled in response to the diet. And the volunteers reporting the greatest increase in hunger were those with the largest drop in leptin.
In other words, subjects who experienced the greatest increase in hunger were the ones with the biggest drop in circulating leptin.
In fact, leptin appears to respond to any form of calorie deficit. And it doesn’t seem to matter too much whether that deficit is created by diet or exercise [2, 3]. The bigger the deficit, the bigger the drop in leptin.
Comparing the effects of two different exercise sessions, researchers from Wake Forest University found that a bout of exercise burning approximately 900 calories led to a greater drop in leptin than a workout that burned only 200 calories .
In short, leptin signals the brain about how much fat is stored in your body (more fat means higher leptin levels) and how much you’re eating (if you burn more calories than you consume, leptin levels will drop).
Although some have referred to leptin as an “anti-obesity” hormone, it’s probably more accurate to label it as an “anti-starvation” hormone that tells your body what to do when energy is in short supply.
Can I Take Leptin to Lose Weight?
Unfortunately, you can’t just take a leptin pill and hope that your weight problems will be solved forever.
For one, leptin can’t be taken orally because your stomach will break it down. For leptin to be effective, you need to inject it. This is not only very inconvenient but also very expensive.
Some studies show that using injections to raise leptin helps to decrease hunger during dieting  and accelerate weight loss . Other trials show that leptin injections have no benefit compared to a placebo [9, 10].
So what’s going on?
Although it might seem counterintuitive, many obese individuals actually have high levels of leptin. Which, on the face of it, appears to blow all of the theories regarding leptin and weight loss out of the water.
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After all, if obese people have high levels of leptin, why are they obese in the first place?
Part of the problem is caused by a phenomenon called leptin resistance. This appears to be due to decreased leptin transport into the brain or some kind of defect in the way leptin receptors respond to the hormone .
In other words, some people don’t respond to leptin in the way they should.
It appears that leptin isn’t active in the part of the brain that controls eating behavior. When leptin is constantly elevated, which it will be in most overweight or obese people, the transporters that help leptin to cross from the blood into the brain become saturated.
If someone is resistant to leptin, raising leptin levels above the normal physiological range isn’t going to help much.
What’s more, the physiological response to leptin is asymmetrical .
That is, a decrease in leptin has a much bigger impact than an increase. Giving leptin to someone at their usual weight, whether they’re lean or obese, won’t automatically lead to weight being lost.
“The primary functional role of leptin is apparently to defend — not reduce — body fat by increasing food seeking and decreasing energy expenditure when fat stores are insufficient,” writes Dr. Michael Rosenbaum in the Journal of Clinical Investigation .
“Physiological responses to concentrations of leptin below and above this threshold are very asymmetrical: decreased concentrations of leptin trigger full-strength counterregulation to what is ‘perceived’ as a threat to survival; concentrations of leptin above the threshold — signaling ‘sufficient’ or excess fat stores — are not responded to vigorously, or at all.”
Where leptin may help is by normalizing energy expenditure and food intake in people who have lost weight, where leptin has fallen below normal levels.
In other words, leptin “repletion” seems far more useful for people who are trying to keep the weight off, where it’s been shown to reverse many of the physiological and behavioral responses to weight loss .
All of which brings me on to the subject of the refeed day.
What is a Refeed Day?
On a refeed day, you take yourself out of a deficit, and raise calories to maintenance levels, or even slightly above. Most of the extra calories you eat on a refeed day will come from carbohydrate.
This is supposed to help you lose fat by boosting your metabolism and reducing hunger via an increase in leptin levels.
A refeed day also gives you the perfect excuse to eat some of the foods typically considered off limits, without some of the guilt that normally accompanies “cheating” on your diet.
Leptin is linked to changes in blood sugar. That is, when the amount of sugar in your blood goes up, so does leptin .
That’s why overfeeding with carbohydrate will raise leptin levels, while overfeeding with fat has no significant effect [16, 17]. Leptin is particularly sensitive to carbohydrate-rich foods, especially those with a high glycemic index, which tend to produce the greatest rise in leptin .
However, not all carbohydrates have the same effect. Drinks sweetened with high fructose corn syrup (a blend of fructose and glucose), for example, don’t elevate 24-hour leptin concentrations to the same degree as drinks sweetened with glucose .
How is a Refeed Day Supposed to Work?
So, we know that overfeeding with carbohydrate raises leptin levels. We also know that giving people leptin injections can reverse some of the metabolic adaptations associated with dieting.
That’s where the “have a refeed day to raise leptin and boost your metabolism” idea comes from. You go on a diet, leptin levels drop, you get hungry and your metabolic rate slows.
If a fall in leptin is causing the problem, the argument goes, raising leptin by having a refeed day and eating more carbs should solve it.
However, while it sounds good in theory, there isn’t a great deal of evidence to show that this is how a refeed day actually works.
Leptin levels may rise when you increase your carbohydrate intake. But they’re going to fall again when you return to your normal diet.
And leptin levels drop relatively quickly in response to changes in food intake – I’m talking about a matter of hours rather than days. When you wake up in the morning, for example, your leptin levels will be lower than they were the night before .
While a single refeed day may raise leptin levels, they’re not going to stay elevated long enough to reverse the negative effects of dieting.
What’s more, the fact that leptin levels go up is no guarantee that your metabolism will follow.
In one study, overfeeding with carbohydrate for three days raised leptin levels by around 30% . Daily energy expenditure also rose by roughly 140 calories.
However, the increase in daily energy expenditure didn’t come from an increase in the resting metabolic rate. Rather, it came from an increase in physical activity, as well as the energy cost of processing and digesting all the extra food.
A high-carbohydrate refeed day may well raise leptin levels. But this doesn’t automatically mean that your resting metabolism will go up.
Longer periods of overfeeding have been shown to raise the metabolism . However, the size of the increase isn’t going to offset the additional calories required to generate it in the first place.
That is, daily energy expenditure isn’t going to increase to the extent that you burn off all the extra calories you’ve just eaten.
How Regular Refeed Days Affect Leptin
Even diets that include regular higher calorie days haven’t been shown to halt the decline in leptin levels.
In one study, dieters followed an alternate day fasting protocol, which involves switching between a fasting day and a feeding day .
On the fast day, subjects consumed around 500 calories. On the refeed day, calories were raised to roughly maintenance levels. The dieters went from a fast day, to a feed day, to a fast day and so on, for a total of three months.
Despite having a refeed every other day, leptin levels were around 40% lower at the end of the diet than they were at the start.
In a follow-up trial, researchers at the University of Illinois at Chicago took two groups of overweight and obese participants, and put them on a continuous diet or an alternate day fasting protocol .
On the fast day, they ate just one-quarter of their daily calorie needs. On the refeed day, this was increased to 125% of their daily calorie needs.
For the typical subject burning around 2000 calories per day, this meant eating 500 calories on a fasting day, and 2500 calories on a refeed day. Carb intake on the refeed days came in at around 375 grams per day, or 60% of total calories.
After six months, both groups of dieters had lost roughly the same amount of fat. What’s more, leptin levels also fell across the board, with no significant difference between the groups.
Having a refeed day every other day for six months was no better at preserving leptin than six months of continuous dieting.
When you lose fat, leptin levels are going to drop. Short of injecting yourself with leptin, there isn’t much you can do about it. There’s no strong evidence to show that a single refeed day can prevent the decline in leptin levels associated with dieting.
That being said, it’s possible any benefits that a refeed day has to offer in terms of leptin and your metabolic rate are only going to show up if you’re very lean. And by very lean, I’m talking about single digit body fat percentage for men, and somewhere in the high teens for women.
This study involved subjects who went from being overweight to being slightly less overweight. They weren’t trying to get ripped in preparation for a photoshoot or physique contest.
Diet Breaks and the MATADOR Study
Taking regular diet breaks has been shown, in some studies at least, to accelerate weight loss compared to continuous dieting.
In the MATADOR study (short for Minimizing Adaptive Thermogenesis And Deactivating Obesity Rebound), for example, two weeks of dieting followed by a two-week diet break led to more fat being lost than 16 weeks of continuous dieting .
The researchers took two groups of obese men, and put them on a diet for 16 weeks.
- The first group dieted for 16 straight weeks, cutting calories by 30-35% below what they needed to maintain their weight.
- The second group did exactly the same thing, but took a two-week break from their diet every couple of weeks.
In other words, dieters in the second group only restricted their calorie intake for two weeks out of every four. They switched between two weeks of reduced calorie eating and two weeks of eating enough food to keep them from gaining or losing weight.
The group taking regular diet breaks ended up losing 50 percent more weight than the continuous dieters. That’s a big difference.
Six months after the end of the dieting phase, both groups gained back some of the fat they’d lost. In fact, the continuous dieters ended up almost right back at square one, while the intermittent dieters did a much better job of maintaining their weight loss.
Why Did the Diet Break Group Lose More Fat?
The researchers think that taking regular diet breaks helped to “protect” the metabolism, which in turn made it easier to lose fat.
However, we can’t say for sure that the extra fat loss in the diet break group had anything to do with changes in metabolism.
In fact, the difference in resting metabolic rate between the two groups of dieters wasn’t sufficient to explain the superior rate of fat loss seen with intermittent calorie restriction.
Instead, an increase in dietary compliance may have had a lot more to do with it.
Dieting for 16 weeks straight is hard work. Knowing there’s going to be a regular break from the physical and psychological grind of continuous dieting, so you don’t have to wait too long before you get to enjoy some of the foods you’ve been missing out on, does make it a lot easier to stick to your diet in the long run.
What’s more, the findings haven’t been replicated in other studies.
In The ICECAP Trial, short for Intermittent versus Continuous Energy restriction Compared in an Athlete Population, researchers looked at two different fat loss programs .
- Twelve weeks of continuous dieting
- Four bouts of dieting lasting three weeks, with each one separated by a week-long diet break
At the end of the study, there was no significant difference between the groups in terms of fat loss, fat-free mass, strength, endurance or resting energy expenditure.
The diet break didn’t seem to do much for hormone levels either. There were no significant differences between groups for leptin, testosterone, insulin like growth factor-1, reverse T3 (a form of thyroid hormone) or active ghrelin.
Levels of a hormone called peptide YY, linked to food intake and appetite control, was higher in the diet break group. They also experienced lower levels of hunger and desire to eat, along with greater food satisfaction, compared to the continuous dieters.
However, it’s debatable how relevant these changes actually were, as they didn’t have any beneficial effects on body composition.
Do Refeeds Help Preserve Muscle Mass?
Weekly refeeds are also thought to help you hold on to more of your muscle while you drop fat.
How well do they work?
One of the few studies to put the idea to the test comes from researchers based at the University of South Florida .
They took two groups of trained men and women, and put them on a diet for seven weeks. Both groups were told to cut calories by the same amount. The only difference was in how that reduction in calorie intake was achieved.
Group one, the continuous dieters, stuck to a daily 25% calorie deficit for the entire seven-week study.
The refeed group, on the other hand, reduced their calorie intake by 35% for five days (typically Monday to Friday), then bumped up their calorie intake for two consecutive days (typically Saturday and Sunday). The extra calories on both refeed days came from carbohydrate.
In other words, the reduction in calories — 25% below maintenance — was the same in both groups. It was just the distribution of those calories throughout the week that differed.
So, what happened?
There was no significant difference in terms of fat loss between the two groups. However, the researchers did find a difference in terms of dry fat-free mass, a reasonable proxy for muscle mass.
On average, the refeed group appeared to keep more of their muscle than the continuous dieters.
In fact, three subjects in the refeed group ended up gaining around four pounds (1.8 kilograms) of fat-free mass, an effect that wasn’t seen in the continuous dieters.
While this sounds like a big win for refeeds, there are several limitations with the study that limit the conclusions we can draw.
For one, there was a relatively small number of subjects. The researchers started out with 58 subjects, but were left with only 27 by the time the study had finished – 13 in the refeed group and 14 in the continuous group. A sample size that’s too small reduces the statistical power of the study, which in turn makes the conclusions less reliable.
What’s more, ultrasound scans and bio-electrical impedance analysis were used to estimate changes in fat mass, fat-free mass and total body water. These aren’t the most accurate ways to measure someone’s body composition, especially when you’re trying to detect such small changes in fat-free mass.
For most people at least, the effect that refeed days and diet breaks have on your metabolism and hormone levels isn’t as great as was once believed.
None of this means you shouldn’t use refeeds, or that they’re a waste of time. As long as the rest of your diet is set up properly, they’re not going to hurt, and can be useful in terms of appetite control, sleep and general well-being.
However, any benefits associated with hitting the pause button on your diet have more to do with improved dietary compliance than with changes to leptin levels and your metabolic rate.
What’s more, the evidence supporting the use of weekend refeeds as a way to retain more muscle while you lose fat isn’t particularly compelling.
One study by itself doesn’t tell you much. It’s only when you get several studies showing the same thing, ideally from different scientists acting independently of each other, that it’s worth sitting up and paying attention.
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