There are a whole bunch of studies on yo yo dieting.
What is Yo-Yo dieting?
Yo-yo dieting, also called weight cycling, is a pattern of losing weight and then regaining it. In the scientific literature, they call it "weight cycling" where a person goes on a diet, loses weight, goes off the diet, regains the weight, and continues this in a cycle of losing and regaining, and not successfully maintaining.
To summarize all the studies very simply, research says that weight cycling is unhealthy for you, physically and mentally.
It has been associated with increased risk of most of the major physical health problems - blood pressure, cardiovascular disease and so on, and with psychological health problems.
The concerns about having wreaked havoc with metabolism aka "metabolic damage" while there is some merit to the idea, are largely overblown.
I’ve mostly stopped using terms like "metabolic damage" and "starvation mode" because they are not scientific terms and they make people worry too much about something they dont need to worry about.
Its true though that metabolic rate decreases with dieting and hormones that regulate metabolism (like leptin and thyroid) are negatively impacted.
Hunger hormones also go up with dieting.
This happens to your metabolism when you reduce calories and lose weight.
Especially, if you’ve been on a severely reduced calorie diet for a long time. This causes a "metabolic adaptation" - a scientific name is adaptive thermogenesis.
What this means is, after dieting and losing weight, your metabolism adapts slightly downward which is likely a protective survival mechanism.
It’s usually not that substantial.
For example, you might run a calorie calculator and it says you should be burning 2100 calories per day but temporarily due to metabolic adaption.
But you might be burning only 1900 or 1975 calories per day.
If you dont make the food intake adjustment the result is you lose weight more slowly than you predicted on paper, which is frustrating to people who dont account for this.
Some people worry that because metabolism decreases with dieting it drops so low you won't be able to lose fat at all.
This doesn't happen. Weight loss can be slower due to metabolic adaptation but it won't stop for that reason.
The idea that eating too little makes your metabolism drop so much you stop losing weight (aka going into "starvation mode") is a myth.
With a large calorie deficit you lose more weight, not less.
The bigger problem is not metabolism, its extremely low calorie diets, along with highly restrictive diets that are not sustainable.
People regain weight not because their metabolism was in bad shape but because they can't stick with the crash diet.
Some worry that because they’ve yo yo dieted in the past their metabolism remains "damaged" and they won't be able to lose fat.
This is also a myth.
That being said, there is a little bit of truth in it.
There have been studies, like the biggest loser study, where metabolic adaptation persisted for months after a diet ended.
But these are extreme cases and the exception not the rule.
Could repeated weight cycling be among the extreme cases?
Possibly, but again it won't prevent you from losing fat. It would only slow it down
And what normally happens is metabolism returns to normal sooner or later (usually sooner) after a diet ends and calories go back to maintenance.
If you're in a position now where you have body fat to lose and you're about to embark on an eating plan with a calorie deficit to lose fat, making sustainable lifestyle changes using a sustainable eating plan is the most important thing so you keep off the weight and dont regain it.
Probably the most important thing for sustainability and long term maintenance is to avoid restrictive dieting and restrictive mentality toward food.
Foods dont cause fat gain, excess calories do.
If you fit any food into a calorie deficit you will still lose fat.
You should be aware of calorie dense foods and eat healthy unprocessed foods most of the time - 80% to 90% of the time.
The other 10-20% can be anything you want. This is called flexible dieting and flexible dieting as opposed to rigid, restrictive dieting drastically increases odds of maintenance.
Basically you eat the same way for fat loss as you do for maintenance. The only difference is for fat loss you eat less.
As for metabolism specifically, most of the studies show that when there is metabolic adaptation during calorie deficit and weight loss.
It goes back to normal when you stop restricting calories.
Of course the irony is that if you're not in a deficit you dont lose fat.
But contrary, not being in a deficit all the time is one of the best strategies to maintain metabolism.
Studies show that over the long term, fat loss success rates are actually higher.
One way to to do this is to use refeed days.
In the past a popular method was 3 days in calorie deficit and 1 day at maintenance calories.
A Recent study showed the benefits of 5 days in a deficit and 2 days at maintenance calories.
On the higher calorie days the extra calories cand from healthy carbs which is why this is called carb cycling as well as "refeeding' - same technique.
However, the studies also show that only one or two days has its benefits like helping maintain muscle, increasing energy and making the diet easier to stick to.
But its not really enough to bring metabolism and metabolism regulating hormones back up to normal.
It appears that a longer time at maintenance is needed for that.
This is why its a great strategy to not to stay in a deficit for a long time.
This is called a diet break, and its usually one week or even two weeks at maintenance calories.
This is a good strategy when you have a lot of weight to lose and its going to take more than a few months to get there.
Instead of staying in a restricted diet mode and in a deficit for months and months, you may stay in calorie deficit for a few months, then take two weeks at maintenance calories and that helps keep a healthier metabolism.
Most people haven't adopted this two weeks on deficit, two weeks at maintenance calories method because its somewhat novel and breaks this often might not be necessary.
But what a lot of people have done is started to insert two week diet breaks to break up long periods of deficit.
Its a popular method now to take a diet break after one, two or three months and not stay in a deficit a long time.
Now the hardest part is actually getting yourself to do it.
Many people who have been dieting for weeks feel like increasing calories will stop or slow down their fat loss or even make them gain weight so they won't do it and they just keep restricting.
Sometimes you do gain weight during a diet break, but its water and glycogen, not fat.
And you only increase calories to maintenance level not above which is why you dont gain fat.
The other important strategy is being really serious and consistent about progressive weight training.
People who diet without consistent resistance training tend to lose a greater proportion of their weight loss from lean body mass.
As a result, metabolic rate decreases more.
Maintaining your muscle with weight training is one of the best ways to avoid negative side effects of calorically restricted dieting alone.
Also for what its worth, being consistent with diet is most important for fat loss, not doing extreme amounts of exercise.
However all studies agree strongly that a high level of physical activity is absolutely a key for long term weight maintenance and avoiding yo yo weight cycles.
This includes not just formal exercise like lifting and cardio, but actively lifestyle all around.