Either way both methods are not sustainable which is the point I'm trying to make. 1250kcal is way too low and as taligent said, keto is used by bodybuilders to lean out a few weeks before competition, it is not something they do for long periods of time. I think the key to prolonged success is to make permanent lifestyle changes that you can commit to for the rest of your life.

I actually did the 1250kcal method for a full year when I was 10 years younger, and it worked like a charm. It required a LOT of discipline and extremely careful calorie counting, but it definitely worked.

Being in my late 20s now, 1250kcal is just not sustainable, but going with keto is working great and is likely to remain so for a long time. It helps that all I ever drink are water, tea, black coffee, and milk. (That's been the case my entire life, so that part isn't unusual.)

I eat eggs/sausage/bacon for breakfast (one of the three), then I eat an earlyish dinner of steak or bratwurst. Since I'm on Adderall XR, lunch never happens. On days I feel like carbs, I go out to Ivanna Cone (amazing local gourmet ice cream) for half a scoop. That consistently puts me at less than 30g of non-fiber carbs per day, with a fairly steady decline in overall weight. I'm never hungry outside of meal time, and I don't have problems saying no to carbs, aside from when I'm eating out with friends.

Once I've lost the weight I want to lose (another 20 pounds or so) I'll be able to figure out what level of carbs I can have without gaining on a daily basis, and live with that.

I have to eat carefully, but not eating carefully is what got me fat in the first place, so that's just what I'll have to deal with.

There are more and less extreme forms of every diet. What Keto is for weight loss, Low Carb is for maintenance.

If you are 5kg overweight, a "sustainable lifestyle change" will be sufficient. And there are those people for who even the severe risks of a gastric bypass outweigh the damage their weight does. Quite obviously, in between those extremes, there will be people were an unsustainable diet will be healthier than staying at their weight or losing it only slowly. And why not choose a diet that has been shown to minimize the loss of lean body mass.

1250kcal is unhealthily low for an adult male. My weight loss intake is between 2000 and 2500. BMR in a healthy adult male is 2500ish alone, tack on any sort of activity and it's easy to get your TDEE well past 3000. A deficit of ~1000 a day should be enough to sustain about 2lbs of loss per week.

Getting into a keto state is by the far the fastest and most reliable way to lose weight. Probably also hard to maintain as it can be hard to get that full feeling like you do with carbs.

It is the same diet which professional bodybuilders use. And they know EVERYTHING about how to lose weight since every % of body fat they lose can make a massive difference in muscle definition.

Schwarzenegger's book on bodybuilding (which I read in the late 90's) had a fairly long section on ketosis. He actually bought urine testing strips to make sure he didn't go into ketosis. When cutting, he reduced his carbohydrate intake, but he immediately increased it again at the first sign of ketosis for reasons he explained in some detail. AFIK, most successful bodybuilders eat tons of carbs while bulking and then try to go as low as possible without going keto while cutting.

Bodybuilders generally have not bothered with keto, until recently.

They just do the standard bulk (overeat in the off season, gain muscle and fat) in the off season and cut (heavy calorie restriction to cut the fat) to look good on stage.