It very much depends on starting weight. Is it possible? Theoretically, yes. Is it safe? No, unlikely for most people.
20lbs or 9kg is a month is extreme weight loss. To lose 1kg of fat, you need a deficit of 7700kcals. For 9 kg that’s 69,300 kcals deficit needed over the month. Lets say thats over 28 days / 4 weeks: that’d be an average deficit of 2,475 kcals every day.
An average 30 year old man, who weighs say 80kg and average height.
Basal Metabolic Rate (BMR) - 1750 kcal a day
Total Daily Energy Expenditure (BMR x activity factor, lets say 1.2 assuming they are sedentary) - 2,100 kcal a day.
So they need 2100kcal a day just to stay as they are. If they did nothing else, the maximum possible deficit they could have is to eat nothing - but they’d only achieve 2100kcal deficit a day, and they’d not last very long.
For that person to lose 9kg in a month, they would realistically need to massively increase their exercise. Even if they increased their daily energy expenditure to 3,323 kcal a day (1.9 x BMR for a very active person), they’d then need to calorie restrict their diet to only 848 kcal a day to achieve the deficit. Even then a 848kcal intake is dangerously low intake and would require very careful management. And somehow that person would also need to have enough intake to feel able to do all that extra exercise.
It would put the body under extreme stress and be very difficult to maintain.
Now if the person was very severely obese - lets say 200kg - the BMR would be 2949kcal (it takes energy just to maintain the fat), and the TDEE would be about 3540kcal if sedentary (even doing nothing, they use more energy). In that situation, if they suddenly became very active, they could boost their TDEE to say 4000-4500 kcal, and would be able to eat 1690-2190kcal a day, and still have a 2475kcal a day deficit. They could lose the 9kg weight in a month. However it is not very realistic for a severely obese person to go from sedentary to extreme activity in such a short period, and maintain it, and while calorie restricting too.
So it could be done by an obese person, but it’s not realistic then and certainly not for anyone who is only overweight.
A big mistake people make when they try to lose weight is to try and fix the problem as fast as possible. But fast dieting leads to over exertion, over strenuous food restriction, failure and then despair. Even 1kg a week lose is a very high rate and difficult to sustain. Instead of trying to lose as much as possible in a month, it’s better to aim for a sustainable amount every week for a long time - such as 0.5kg. 6 months of sustained slow lose is generally much more successful and achievable than crash/fast/extreme diets that just fail over and over for 6 months. And generally when people do slow diets, they change habits and have a better chance of maintaining it.
If someone is actually considering a diet then the real key is: don’t make it a speed race, make it a marathon. Slightly reduce intake, slightly increase activity every day, and aim for a slow but steady loss. A 550kcal deficit every day is much much easier than trying to rush with 1100kcal deficit, and allows a sustainable lifestyle. For example the 80kg 30 year old person could do moderate exercise (e.g. 10,000 steps a day just walking), they could realistically consume 1900-2000kcal a day, maintain a 550kcal deficit each day and lose 0.5kg every week. In a month they’d lose 2kg, and in 4.5 months they’d lose 9kg.
It very much depends on starting weight. Is it possible? Theoretically, yes. Is it safe? No, unlikely for most people.
20lbs or 9kg is a month is extreme weight loss. To lose 1kg of fat, you need a deficit of 7700kcals. For 9 kg that’s 69,300 kcals deficit needed over the month. Lets say thats over 28 days / 4 weeks: that’d be an average deficit of 2,475 kcals every day.
An average 30 year old man, who weighs say 80kg and average height.
So they need 2100kcal a day just to stay as they are. If they did nothing else, the maximum possible deficit they could have is to eat nothing - but they’d only achieve 2100kcal deficit a day, and they’d not last very long.
For that person to lose 9kg in a month, they would realistically need to massively increase their exercise. Even if they increased their daily energy expenditure to 3,323 kcal a day (1.9 x BMR for a very active person), they’d then need to calorie restrict their diet to only 848 kcal a day to achieve the deficit. Even then a 848kcal intake is dangerously low intake and would require very careful management. And somehow that person would also need to have enough intake to feel able to do all that extra exercise.
It would put the body under extreme stress and be very difficult to maintain.
Now if the person was very severely obese - lets say 200kg - the BMR would be 2949kcal (it takes energy just to maintain the fat), and the TDEE would be about 3540kcal if sedentary (even doing nothing, they use more energy). In that situation, if they suddenly became very active, they could boost their TDEE to say 4000-4500 kcal, and would be able to eat 1690-2190kcal a day, and still have a 2475kcal a day deficit. They could lose the 9kg weight in a month. However it is not very realistic for a severely obese person to go from sedentary to extreme activity in such a short period, and maintain it, and while calorie restricting too.
So it could be done by an obese person, but it’s not realistic then and certainly not for anyone who is only overweight.
A big mistake people make when they try to lose weight is to try and fix the problem as fast as possible. But fast dieting leads to over exertion, over strenuous food restriction, failure and then despair. Even 1kg a week lose is a very high rate and difficult to sustain. Instead of trying to lose as much as possible in a month, it’s better to aim for a sustainable amount every week for a long time - such as 0.5kg. 6 months of sustained slow lose is generally much more successful and achievable than crash/fast/extreme diets that just fail over and over for 6 months. And generally when people do slow diets, they change habits and have a better chance of maintaining it.
If someone is actually considering a diet then the real key is: don’t make it a speed race, make it a marathon. Slightly reduce intake, slightly increase activity every day, and aim for a slow but steady loss. A 550kcal deficit every day is much much easier than trying to rush with 1100kcal deficit, and allows a sustainable lifestyle. For example the 80kg 30 year old person could do moderate exercise (e.g. 10,000 steps a day just walking), they could realistically consume 1900-2000kcal a day, maintain a 550kcal deficit each day and lose 0.5kg every week. In a month they’d lose 2kg, and in 4.5 months they’d lose 9kg.