Does anyone know the scientific reason why....

I don't know the biochemistry of it, but from an evolutionary perspective, we're built to survive the palaeolithic era. We're not built to win beauty contests (beauty comes into it because of selection of mates, but it's more a case that what we find beautiful is a fit strong body that's more likely to survive, rather than the human body evolving to match what we find beautiful)

body fat is essential to survival. In the palaeolithic era, food acquisition was hit and miss, and people had to be able to withstand food shortages. Body fat was an essential store for that, i.e. when you don't get enough to eat, body fat will give your body the energy it needs. However that's not your body's only source of energy in a food shortage - muscle is costly to maintain, as in muscle cells use a lot more energy (calories) than fat cells, even when they're doing nothing. So in a food shortage, the body will also burn lean muscle for energy to make the fat stores last longer. When your fat stores run out you die, basically. The less fat you have, the more your body's going to want to protect what fat you have left, because your survival depends on it. So there's a greater risk of losing lean muscle mass. The result of loss of muscle mass is that the metabolism slows, because your body's spending less energy (calories) keeping you alive as there's less muscle to maintain. From a survival point of view this is great, you can live on less food and preserve your fat stores for longer. But from a modern perspective, i.e. trying to lose the last few lbs of fat to look good, it's not what you want at all because you have to eat even less to continue to lose weight, while you still have more fat than you want, but it's what happens, because we're the product of millions of years of natural selection in favour of surviving food shortages. If our palaeolithic ancestors' bodies worked the way we want our bodies to (burning off fat while preserving or even building muscle) they would have died the first time they encountered a food shortage. So our bodies don't work like that either.

This is why it's essential to look after your lean mass... there are things you can do to protect it and ensure that you lose just fat (or mainly fat, because even if you do everything right you'll have some loss of lean mass when you get to the low end of the healthy body fat percentage range)........... from an evolutionary perspective, your body will maintain muscle that's being used regularly for longer (i.e. unused muscle gets burned off early on in a food shortage, but used muscle is protected) because if our ancestors did exercise it was for obtaining food, and if they lost the ability to obtain food then they'd die. So as far as your body's concerned, exercise = necessary for survival = will protect the muscles you use regularly (up to a point, in a severe food shortage you'll lose them anyway) - so exercise helps prevent loss of lean body mass, and exercise that works the muscles hardest, i.e. weight lifting or similar, will be the best for protecting lean muscle mass. Then you need a SMALL deficit so you get fat loss, but slow and conservative fat loss, so you're just losing fat. From an evolutionary point of view, that's imitating a mild food shortage where you're succeeding at hunting. Your body maintains the muscles you use, and slowly uses fat stores to make up the deficit.

anyway, like I said, not a biochemistry explanation (I can't remember biochem much from uni) but the evolutionary explanation explains why our body works the way it does.

Is this a thing? Because I never experienced it when I was losing weight. I lost all my weight pretty steadily.

Scientifically as your weight goes down, you metabolic rate does too, so if you don't adjust for calorie deficit, then loss will be slower. Also at a lighter weight, calories burned through exercise will be less, so that also has to be accounted for too. If one has been doing the same type of exercise long enough (cardio on an elliptical for example) as you weight lowers, your body becomes very efficient at adapting to burning the least amount of calories for that routine. So even just changing to a harder intensity and/or exercise should help. And lastly, from my experience with clients, they tend not to be as adamant about weighing and measuring as they first did when they started. Foods they miss have a tendency to return to their lifestyle. Not that there is anything wrong with that.

A.C.E. Certified Personal/Group FitnessTrainer
IDEA Fitness member
Kickboxing Certified Instructor
Been in fitness for 30 years and have studied kinesiology and nutrition