I'm no expert on this, but I also read about this as I also tried calorie restriction.

You still have to keep your macros (and micros) in balance while on calorie deficit, which is even harder. Your body needs various things, you just need to optimize your food. Also, I think the main contributor for OPs issues was the fat deficit, which is very easy to fall into while you think you eat healthy a lean food. Fat is important for your hormone production.

https://pubmed.ncbi.nlm.nih.gov/26843151/

https://shilpidietclinic.com/low-fat-diet-and-hormonal-imbal...

In my experience(currently about 15kg into a 40kg weight-loss program), eating enough fat can also be very helpful for losing weight. It seems counter-intuitive, but it works for me. Fat contributes a great deal to satiety. My diet setup has been to have breakfast and dinner only, no lunch on most days. This way I can make both meals quite calorific, filling and plenty tasty. Crucial for maintaining adherence to the setup, which is by far the hardest part of weight loss.

When you have to go 7 to 8 hours without eating before dinner you want plenty of slow-burning calories. Long chain fats, protein, slow carbs, with plenty of fiber.

My typical breakfast ends up being one slice of bread with liver pate and cheese, another with peanut butter and either nutella(if I'm doing morning cardio or some other exercise mid-day. Lots of sugar in nutella, which gets used up immediately by the exercise anyway) or various kinds of jam with no added sugar(usually pear and apple, since they're not so tart and are pretty sweet without added sugar), and a protein pudding cup(20g protein). The bread needs to be whole-grain, of course. Ideally 100% whole grain.

This ends up being about 700 calories, which is a pretty substantial breakfast. And most importantly, it includes a lot of protein(from liver, peanut butter, cheese, the bread and the pudding), a good mix of saturated fats with plenty of SCFA and MCT from the cheese and liver, mono- and polyunsaturated fat from the peanut butter, and tons of soluble and insoluble fiber from the bread and peanut butter.

This tends to keep me full until dinner time, at which point I can typically eat up to 1300 kcal depending on how active I've been.

On extremely active days, I might either add another slice of bread to breakfast, or have a protein snack and some fruit after exercise, as well as electrolyte drink with sugar in it during(important both for energy and fluid uptake).

Anyway, I'm rarely hungry except for just before eating, which is the idea. I think this would be much harder on a low-fat diet.

If someone is running everyday like this, do they actually even need to lose weight? Aren't they already very fit?

Also, testosterone also gets impacted by fatigue. Running is more fatiguing than lets say stationary biking or elliptical. So maybe try other forms of cardio to burn calories too instead of only running?

While I didn't run this much, I used to run 50 km / 30 miles a week. My "cardio" was good, but I didn't lose weight because all the running made me 1. extremely hungry 2. too tired to cook a proper meal, 3. have a convenient excuse to eat bad...

"How could I not eat a second donut, I just ran 15k!"

I was not "feeling fit", though, I believe I had low T, and I stayed relatively fat, which is not great for your organs, liver values, heart, etc.

So even if you are able to run 50K a week, you may still be somewhat unhealthy due to poor diet and other factors, and some of them can be improved by losing weight...

For #2, meal prep is a valid strategy; double up on ingredients when you do cook, put the leftovers in the freezer for later.

I was too lazy to cook a proper meal for a good while, but I would make something in weekends. Since all portion sizes in shops are catered for 2-3 people minimum, I'd always have 1-2 portions left over. I had convenient 500ml freezer/microwave containers which was also ideal for portion control. Lost 8 kilos in 8 months without actively dieting during that period.

Mind you it was also a stressful period, with a new job and stuff.

You can eat like shit and do a lot of exercise and be overweight.

I’m 90 kg at 180 cm. I ride my bike 450 km per week. A few weeks ago I did a 340 km, 3000 m elevation ride at 25.6 km/h and yesterday I did a 220 km ride at 27 km/h. Last week I burned 13,468 calories from cycling (this should be fairly accurate as I have a power meter).

I would say I’m quite fit, I can obviously ride my bike further and faster than the vast majority of people but I am definitely overweight and look fat.

I’m no expert, but you should be roughly as tired burning 200 kcal running vs walking vs biking. The difference will be how quickly you’ll burn those calories.

It's not really the type of exercise but the intensity that determines fatigue. If you walk for an hour you will burn 200 kcal and ready to walk one more hour. If you try to burn 200 kcal sprinting, most people would become exhausted before getting close.

> If someone is running everyday like this, do they actually even need to lose weight? Aren't they already very fit?

Not necessarily. There's a reason the saying is "You can't outrun a bad diet."

Before I got my diet dialed in I was cycling every day, upwards of 10 miles a day with a couple 20 mile rides per week, and was still gaining weight because I ate like crap, and more importantly, still ate more than I was burning with all the exercise.

The average American consumes 3,864 calories per day. A moderately active male might have a maintenance of ~2,647 calories, give or take a couple hundred. Just eating 500 calories per day over what you burn will lead to about a 1lb increase in weight per week. A zone 2 run might burn somewhere between 500-600 calories per hour, so its easy to see how quickly over eating can add up and at a point it becomes basically impossible to "run it off."

There's other factors that play into it such as lean mass vs. fat, etc. but in general, you can be very active and still be overweight.

I understand what you are saying but OP is not an average American. They have been running every single day for 10 years, so chances of them already being very conscious about their diet are very high I think.

When people say calorie deficit they usually mean low calories. Not 3000 kcal intake and 3300 burned which would be a healthy approach for a runner

A small calorie deficit alone won’t lead to major problems.

The phrase “calorie restriction” is often used in the context of life extension to refer to periods of very low caloric intake, near fasting. This would cause problems with hormone levels.