One of the key points to consider when losing weight is that your goal usually isn't simply to shed kilos, but to shed FAT without losing too much existing muscle (lean mass). When I got my DEXA scan done, the technician mentioned that typically dieting reduces weight in the ratio 1:1 in terms of lean mass vs fat. For example, on a typical weight loss diet I could expect to reduce my weight by 20 kg (from 100 to 80) and that would consist of equal parts fat and muscle (lean mass includes bones, blood etc., but that won't change significantly in a short period of time).
So, under a 'typical' diet regime (caloric restriction) if you started at 25% body fat (25 kg of your 100 kg was fat), you'd expect to end up at 80 kg with 15 kg fat remaining (half the 20 kg weight loss being fat and the rest lean mass). So your new body fat percentage would be 18.75%. This would be a considerable improvement compared to 25% body fat, and accordingly your BMI would have improved.
However, a 'smarter' way to diet is to combine caloric restriction (to reduce fat stores) with additional activity that retains (or builds) lean mass (muscle) ie. weight training. This is known as body recomposition (ie. changing the ration of lean mass to fat). By modifying your calorie restricted diet so that you are still getting adequate amounts of protein you will either retain lean mass, or possibly even add some muscle while shedding fat. For building muscle a typical amount of protein intake would be 1.6 - 2.0 g protein per kg of body weight.
This is one reason that a 'keto' diet can be useful, as you can reduce caloric intake by slashing the amount of carbs being consumed without reducing protein intake.
Anyhow, I've been tracking my daily morning weight as well as the body fat (measured via bioelectrical impedance analysis) since late October, when my weight was 102.4 kg (I'd been tracking my weight but not my body fat reading prior to that). Although both the weight and body fat readings can vary considerably from day to day (depending on hydration, sleep, bowel movements, recent meals and exercise etc.) taking a reading first thing in the morning minimizes the intrinsic variability. Since I already had the data available in my diet spreadsheet, I've used the weight and body fat readings to calculate daily lean mass and fat values, and plotted them:
Combining dieting with regular weight training (2-3 45 minute sessions per week) and increased activity (walking 10K+ steps/day) I've managed to reduce my weight by 10kg with minimal loss of lean mass. The trend lines show that my lean mass has remained almost constant while my fat stores have been depleted. The ratio of fat loss to lean mass loss has been a pleasing 7.75:1
I'll continue with my current (VLCHP - very low carbohyrate, high protein) version of 'keto' dieting and weight training until I achieve my target weight of around 80 kg, by which time my body fat should have reduced to around 10%, which is my initial goal.
After that I'll increase by carb intake slightly (adding more potatoes, pasta, rice, fruit and vegetables) so I'm no longer in caloric deficit, and (hopefully) slowly add bulk (muscle mass) and reduce body fat slightly over the following 12 months. At some point everything will reach equilibrium, with my body weight, body fat percentage, and lean mass remaining fairly constant (as long as I continue with the same caloric and macro nutrient intake levels and amount of weight training).
If I wanted to seriously 'bulk up' I'd eat more than maintenance calories and do additional weight training in order to add muscle, then 'cycle' with periods of reduced caloric intake to 'cut' (ie. remove fat while retaining as much muscle as possible). But I'm too lazy (and old) to put in the hours and effort in the gym that would be required to put on lots more muscle, so I'll see what happens after 1-2 years of 'modest' amounts of weight training.
BTW, when I finished high school I weighed about 78 kg and was a bit 'chubby' so my lean mass back then must have only been around 64-66 kg (15%-18% body fat I'm guessing), so I've actually put on a considerable amount of lean mass over the past forty years, not just fat!
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