This can occur primarily in two groups of people: the obese and overweight and the untrained (no previous weight lifting experience). Intermediate lifters can also accomplish it. It is also healthy if you are aiming primarily for weight loss to engage in lifting as it’ll preserve lean body mass or increase it.
For Obese/Overweight Individuals
If someone is 20% body fat or higher than they have an easier time losing fat and gaining muscle in a deficit compared to someone who is already lean or fairly lean (10% bodyfat or lower). Its a myth you can “convert” fat to muscle and muscle weighs more than fat. Fat cells are fat cells, muscle cells are muscle cells and 1 lbs of feathers equals 1 lbs of sand so 1 lbs of fat is 1 lbs of muscle. Muscle is just denser than fat. That aside there are numerous studies showing someone who is fat losing fat and gaining muscle. Especially in untrained heavier individuals. Maintaining extremely low body fat levels as a natural trainee is difficult and it’s not very healthy for a man to stay at 4% or lower bodyfat for long periods of time.
For Untrained (also returning lifters)
This basically is common sense to most people. Someone whose never lifted weights before will increase muscle mass and can decrease fat at the same time. And also someone whose taken a break from weights for a while will too.
For Someone With 2+ Years of Training Who Is Already Fairly Lean
This is very difficult to do naturally. Most cases it happens when someone was off due to an injury or retraining. Gaining strength but not necessarily muscle mass can occur as well without significant fat loss.
Some Things to Keep In Mind:
Most natural trainees should not aim for sub-10% because without the use of anabolic steroids there is basically no way to be huge, lean and maintain it year round. There’s no way to be 250+ lbs guy and 4% bodyfat naturally. Not even all the chicken breast in the world will accomplish it. 12-15% is a realistic goal in my opinion. And you’ll look 99% better than most men with your shirt off (albeit pending on how much steroids or how good your genetics are you may look small in clothes).
Also how much muscle mass you have will affect how you look. It’s the difference between being a bit chubby looking vs an off season bodybuilder look or being a skinny vegan vs being ripped to shreds
More muscle mass the better you look even at a high bodyfat percentage.
The caloric deficit should be 300-500 less calories than maintenance unless you are high in bodyfat then you can get away with up to 800-1000 less calories. But fasting and strength training MAY prevent muscle lost and actually increase it. Anabolic steroids and SARMs will definitely preserve or increase it. Protein intake should be 0.68-0.9 grams per lbs in bodyweight. Higher than that has no real benefit but to make supplement companies money.
Measuring bodyfat is difficult to do without proper equipment and the equipment you have at home such as a tape measure, bodyfat calipers or looking at photos online can all underestimate or overestimate bodyfat percentage.
If your waist circumference is decreasing (or staying the same) and your weight is going down and your strength and circumference of your biceps, thighs etc. are increasing then you can fairly safely assuming what you are doing is working. Your body is wired to do things on a weekly rather than daily basis. So your weight can go up and down quite a bit over a week. And if you lose 10 lbs for example you have to recalculate your caloric needs once more. The first weight you lose is mostly water.
And that’s basically how it is for losing fat and gaining muscle naturally.