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Everybody is built differently. Different ratios of fat to muscle, height and limb length. All of these things combined mean that there is no single ideal weight that can be applied to everyone.
When a person is said to be under or over weight, what is usually meant is that the person is far from the average for a healthy person of that height.
What does it mean to be "over" or "under" weight? Is one weight better than another? What is the ideal weight for a specific person? We'll discuss these in this chapter.
The simplest way that is used to find an "ideal" weight for someone is to use the BMI (Body Mass Index), which is a ratio of the height of a person to their weight.
To measure your BMI, take your weight in kilograms, then take your height in metres and multiply it by itself, then divide the weight by the height squared. BMI = kg/m2.
A study of 30,000,000 people concluded that there is a strong correlation between BMI and mortality, with the healthiest people averaging at around a BMI of 22-23.
The paper shows that when your BMI is 35 or above, it starts seriously affecting your mortality, with a hazard ratio of 1.34. This means that it is 34% more likely for a person to die in any year if they have a high BMI than if their weight is average for their height.
The BMI is a general indicator of whether a person is under or over weight, but it is not ideal. For example, a person that does weight-lifting a few times a week might weigh the same as a person that sits and watches TV all the time, and both might have the same height, so are they both overweight? According to the BMI, they might both be overweight, but obviously one is more healthy than the other.
BMI does not take into account whether you are male, female, European, Asian, adult, child, sedentary, active. There is no single index that does.
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.
Losing weight or gaining muscle
Sometimes research papers can seem misleading. A paper in 2017 studied 11.5 million people and showed that if weight is lost quickly over a short tme, then it can drastically affect mortality. For example, a person losing 15% of their weight over 5 years (for example, a weight loss from 80.5kg to 70kg) is twice as likely to die early as a person who keeps the same weight.
However, a key question to ask in this case is why was the weight lost? There is a very big difference between losing weight intentionally because you are overweight and want to reach a more ideal weight, and losing weight because you are sick and the weight-loss is a symptom. Obviously, a sickness that causes a 15% weight loss is also going to hugely affect your life-span.
A paper in 2015 addressed this by studing intentional weight loss, and found that on average, a person that loses weight intentionally lowers their mortality risk by 15%.
The effect on mortality associated with weight loss or gain is temporary. A long-term study showed that the effect of the weight loss/gain leveled off over time until after 15 years, it was difficult to see any difference at all between a person that had reached a certain weight through gain or loss, and a person who had been that weight all along. In other words, if you work intentionally to reach your ideal BMI or SBSI, there will be a benefit in the short term, and in the long term it will be as if you were at your perfect weight all along.
Being over or underweight affects your life expectancy. The ideal is for you to reach a BMI of 22, and an SBSI of 0.108 (male) or 0.105 (female).
If you are overweight, losing weight intentionally (to reach your ideal weight) gives you an immediate boost in life expectancy.