Being under-weight is a general kind of malnutrition called "starving", but being overweight can cause problems as well.
But what is overweight? When we say someone is overweight, we are not talking literally about their weight, but about the distribution of fat in the body.
There is no simple calculation that will tell you for certain whether you are overweight or underweight.
The usual way doctors estimate it is to use the "body mass index" (BMI), which is a calculation which comes up with a number (the index), which corresponds generally with mortality by estimating the fat distribution in the body.
Most research uses the BMI as a base measurement, because it's easiest to measure.
A study of 30,000,000 people concluded that there is a strong correlation between obesity (defined as BMI>30) and early death. For the lowest mortality, you want a BMI of 22, and if you smoke, stop.
Because the BMI estimates the fat distribution in the body using just two parameters - your height and your weight, it can be very inaccurate, as it doesn't take into account whether you are male, female, European, Asian, an adult, a child, sedentary, or very active, etc. A muscular man with little fat can weight the same as a non-muscular man who has some fat on him. The BMI calculation will give you the same figure in both cases, even though one has in reality a higher mortality rate than the other.
A more accurate measurement is the Surface-Based Body Shape Index (SBSI), which uses four parameters - vertical trunk circumference, body surface area, waist circumference, and height. After calculating your SBSI, you will have an index. The closer this index is to .105 (female) or .108 (male), the healthier you are (see S1 Table in the research paper). This can involve changing weight or waist circumference. It's not straight-forward like the BMI, but it's more accurate.
In my case, I am 168cm in height, with a vertical trunk circumference of 168cm. An ideal weight for me is 62kg with a waist circumference of 81cm. This gives me an index of .108. 62kg with a height of 168cm gives a BMI of 22, but the waist circumference is very important as well to make sure the fat distribution is healthy.
Reducing your weight is relatively easy if you are using a future food such as Jimmy Joy. Because the calories are always the same for the meals, it's an easy matter to adjust how much you eat.
Adjusting the waist circumference without adjusting your weight is more difficult. You will need to exercise, but also increase your calorie intake to keep the weight the same.