Dieting without enough protein means you lose muscle alongside fat, your metabolism slows down, hunger gets harder to manage, and the weight you lose is more likely to come back as fat.
I’ve been going to the gym for over ten years, so protein has always been on my radar, but from the muscle-building side, not the dieting side. When I started reading the research on what happens when people cut calories without keeping their protein up, it was genuinely alarming. Most of the people I know who’ve tried to lose weight weren’t thinking about protein at all. They were just eating less.
That’s a problem, and the research is pretty clear on why.
You lose muscle, not just fat
When you cut calories, your body has to find energy from somewhere. Ideally, it burns fat. But without enough protein to signal “keep the muscle, burn the fat,” your body does both. It breaks down muscle tissue for fuel right alongside your fat stores.
A 2010 study on athletes in a 40% calorie deficit found that the group eating standard protein (1.0 g per kg of body weight) lost 1.6 kg of lean body mass in just two weeks. The high-protein group (2.3 g per kg) lost only 0.3 kg. That’s 81% less muscle loss, with similar fat loss in both groups.
You lose weight on the scale but look soft. Lighter but not leaner. It’s what people mean when they say “skinny fat,” and it’s one of the most common outcomes of dieting without paying attention to protein.
A separate study at McMaster University took it even further. Young men eating 2.4 g of protein per kg during a 40% deficit actually gained 1.2 kg of lean mass while losing 4.8 kg of fat. They got leaner and more muscular in a deficit. Protein was the only variable that changed.
Your metabolism slows down
Muscle is metabolically active tissue. It burns calories just by existing. So when you lose muscle, your body needs less energy to function. Your resting metabolic rate drops. You have to eat even less to keep losing weight, which makes you lose more muscle, which slows your metabolism further.
A meta-analysis of 24 randomised controlled trials involving 1,063 people found that high-protein dieters preserved about 142 more calories per day of resting energy expenditure compared to standard-protein dieters. Their metabolisms slowed down less. That difference adds up to roughly 1,000 calories per week, just from keeping more muscle on your frame.
This is what’s behind the weight loss plateau that hits around week four to six. Your metabolism has quietly downshifted to match your shrinking muscle mass. You’re eating 1,400 calories and maintaining your weight because your body has adjusted. The deficit that worked in week one doesn’t work anymore.
You’re hungrier than you need to be
Protein is the most satiating macronutrient by a wide margin. A 2005 study in the American Journal of Clinical Nutrition found that when people increased protein from 15% to 30% of their total calories, without being told to eat less, they spontaneously ate 441 fewer calories per day. They weren’t trying to restrict. They just weren’t as hungry.
The reason is hormonal. Protein triggers the release of GLP-1 and PYY, two hormones that tell your brain you’re full. Research confirms that these satiety hormones are significantly elevated after high-protein meals compared to high-carb or high-fat meals, and the effect persists for hours.
When your diet is low in protein, you’re getting weaker “I’m full” signals throughout the day. Your body is telling you to eat more because it’s not getting the signal to stop. That’s not a discipline problem. It’s a signalling problem.
The weight comes back, and it’s mostly fat
A study published in the International Journal of Obesity tracked people after they’d lost weight and found that those who maintained higher protein intake (18% of calories vs 15%) regained 50% less weight. And the weight they did regain was mostly fat-free mass, not fat.
Flip that around. If you don’t keep your protein up after losing weight, what you regain is disproportionately fat. You end up at the same number on the scale as before, but with less muscle and more fat than when you started. Your body composition is worse than before you dieted.
This is the yo-yo cycle. Each round of undereating protein leaves you with less muscle and a slower metabolism, which makes the next diet harder and the next rebound worse. Biology working against a strategy that was never going to work long-term.
How much protein prevents this?
The numbers depend on how active you are and how aggressive your deficit is.
| Your situation | Protein target | Example (75 kg person) |
|---|---|---|
| General health (no deficit) | 1.0–1.2 g per kg per day | 75–90 g |
| Losing weight | 1.2–1.6 g per kg per day | 90–120 g |
| Active and in a deficit | 1.6–2.2 g per kg per day | 120–165 g |
The most important thing is hitting your daily total. Your body can use well over 30 g of protein in a single sitting, so you don’t need to obsess over spreading it perfectly. But as a practical guideline, aiming for 25–40 g per meal makes life a lot easier. I can’t count the number of times I’ve had barely any protein at breakfast and lunch, then arrived at dinner needing 80+ grams to hit my target. That usually ends in either overeating or just giving up on the goal for the day. Front-loading even a bit of protein at breakfast and lunch means dinner isn’t a rescue mission.
What does 30 g of protein look like?
- A medium chicken breast (~170 g)
- A tin of tuna
- 200 g of Greek yoghurt
- 4 large eggs
- 150 g of cooked lentils
Most people, when they actually check, realise they’re getting plenty of protein at dinner and almost none at breakfast and lunch. I’ve been there. Years of gym training and my breakfasts were still basically carbs and coffee. Fixing that distribution alone made a noticeable difference to how I recovered and how I felt by midday.
The hard part is keeping track
You know the number now. The challenge is hitting it day after day.
Most people have no real idea how much protein they’re eating. They know chicken is protein. They know eggs are protein. But when you ask “did you get 120 g today?” the honest answer is usually “no idea.”
That’s why we built Nemo. He lives in WhatsApp. No app to download, no food database to search through. Just text him what you ate, send a photo of your plate, or fire off a quick voice note, and he’ll give you a protein and calorie breakdown in seconds. He keeps a running daily total so you can see where you stand without doing any maths.
If you want other ways to stay on top of your intake without the usual app hassle, I wrote about how to track calories without an app. Several of those methods work just as well for tracking protein.
Frequently asked questions
Will too much protein damage my kidneys?
There’s no evidence that high protein intake harms healthy kidneys. This concern comes from studies on people with pre-existing kidney disease, where protein restriction is indeed recommended. If your kidneys are healthy, a systematic review of 28 studies found no difference in kidney function between high-protein and normal-protein diets. That said, if you have any kidney concerns, talk to your doctor.
Does it matter whether the protein comes from meat or plants?
Both work. Animal proteins and soy are “complete” proteins, meaning they contain all essential amino acids. Most plant proteins are incomplete on their own, but you can easily combine them (rice and beans, for example) to cover the full spectrum. What matters most is your total daily intake, not whether it comes from a chicken breast or a bowl of lentils.
I’m eating lots of protein but still not losing weight?
Protein isn’t magic. You still need to be in a calorie deficit to lose weight. What protein does is make that deficit more sustainable (less hunger), more effective (you keep more muscle), and more likely to last (less metabolic slowdown). But 200 g of protein on top of 3,000 calories of everything else won’t produce weight loss on its own.
If you take one thing from this article, let it be this: the difference between a diet that works and one that leaves you worse off often comes down to protein. Get enough of it, spread it across your meals, and you keep your muscle, your metabolism, and your sanity. Skip it, and you’re fighting your own biology the entire way.
