What you may want to know about the SBMI

SBMI calibration and BMI study data

Why does the SBMI use a scale of 70 points?

For usability reasons. The tens indicate the weight class, whereby all the values beginning with a "3", for instance, indicate a "normal weight" and are shown in the chart as the "green range". The "4" and "2" stand for moderate overweight and underweight, respectively, and are shown as "yellow"; the "5" and "1" stand for a distinct overweight and underweight, respectively, and are shown as "orange". The units represent your position within the weight class. Example: An SBMI of 45/70 is exactly in the middle of the moderate overweight class.

Why not simply take 35 instead of 35/70?

In order to avoid any confusion. An SBMI value of 35 (normal weight) could be mistaken for BMI 35 (obese). You can calculate this SBMI as a single value of 35/70 = 0.5 = 50 % if you wish, which indicates that the value is exactly in the middle of the scale. Values above or below 50 per cent will normally mean a weight higher or lower than average, respectively.

Do the curves of the chart exactly reflect the results of BMI studies?

Yes and no. As far as reliable results of BMI studies were available, I maintained the closest possible match between the curves and the study results. The sparser the data, especially in the case of very old persons, the less reliable and the more disputable the study results become. In these cases I decided to apply common-sense considerations, rather than the data.

For instance, the curves – if shown strictly according to the data – would have to turn upwards in a bizarre way towards the edge of the chart on the right-hand side. However, this would in no way mean a low health risk for high BMI values in the very old. It would just reflect the fact that very old persons are likely to die soon anyway, and that the influence of the BMI is no longer relevant. Thus, I have calibrated the calculator in such a way that it assumes that even very old people could survive ten years longer, so as not to underrate their health risk – neither for overweight nor underweight.

Why are the lower yellow and orange stripes narrower than the upper ones?

This is due to the fact that when losing weight below the normal range, the reserve that is left is increasingly reduced until the point where a life-threatening state is reached: A few pounds can make a huge difference in such a case.

Differences between the SBMI and the WHO weight classification

Doesn't the SBMI overrate the health risk for obese young people?

The health risk of SBMI 60/70 corresponds to a WHO-classified health risk of BMI 40. A twenty-year-old could indeed already reach this risk level with a BMI of 33 or 34. However, we must firstly take into account that where obesity in young people is concerned, there is a very high risk of gaining even more weight. Secondly, the earlier the weight control is applied, the better the chances and the possible benefits will be. Thirdly, study results show that the long-term negative effects of obesity are more pronounced in younger people than in the elderly.

For young, obese persons, preventing a further gain in weight is of even greater importance than losing weight. Therefore, the calculator encourages people to keep their weight stable, even if it is high, by giving them the prospect of a marked decrease of SBMI in the following years: Our twenty-year-old with a BMI of 33 will lose about 8/70 of his SBMI value by the age of thirty.

Doesn't the SBMI underrate the health risk for elderly persons who are overweight?

No, not at all, because the data in this case is very reliable. A sixty-year-old with a BMI of approximately 25 (rated by WHO as overweight) will have a very satisfying SBMI of about 36/70. The health risk that can be attributed to this SBMI value is as good as non-existent, regardless of age, but all the more so at the age of sixty.

Is this due to the fact that slim smokers die early?

No. The data used for the calibration has only been taken for never-smokers without any diseases known at the beginning of the study. Thus, two important disadvantages of being slim – that could have led to a "false advantage" of being fat – have been ruled out.

Doesn't the SBMI overrate the health risk for elderly persons who are underweight?

Should we take the WHO rating for "normal weight" of BMI 19 as gospel for all ages? No, we should not, for three reasons. Firstly, in old age, weight loss is a typical sign of approaching the end of life. Secondly, the BMI study data for elderly persons is too sparse to draw any reliable conclusions about the health risk of underweight. Thirdly, the zero-risk curve is based on sufficiently reliable data and it retains a continuous, upward trend even in old age. My first concern is not to downplay the possible health risks for underweight. Therefore, the lower curves of the SBMI chart follow the green curve at more or less the same distance from youth to old age.

Why is the risk of overweight in old age rarely very high?

The most probable reason is that there is generally a higher risk of bad health, the older we get. The possible causes for bad health increase as we grow older, and thus also their probability. Overweight and obesity can make a difference, but this difference will gradually decline in later years.

In cases where a group of overweight persons has grown very old, despite the fact that overweight is regarded as a risk of dying early, it becomes obvious that there has been no such risk in this specific case. In other words, a selection has taken place where overweight has already done most of its harm – if any – in earlier years. A possible explanation: Good genes can make a person fit to grow very old, even with obesity.

Differences between the sexes

Why can the SBMI be 45/70 for a man but only 43/70 for a woman of the same age with the same BMI?

I see two possible reasons for this. The first one is that the body-fat percentage is normally higher in women than in men. The second reason is due to the pattern of fat distribution within the body.

In most men between twenty-five and forty years of age, fat accounts for approximately about 10 to 18 per cent of the body mass. In most women of the same age, the body fat percentage is 20 to 27 %. In other words, the female body is biologically used to coping with a higher portion of fat deposits, without any negative consequences for the health. When men and women gain the same amount of body fat (in pounds), the relative gain (as a percentage of the initial fat mass) is higher in men than in women. This is associated with a higher health risk and thus with a higher SBMI.

When women gain weight, the body fat is typically deposited under the skin in the upper parts of the body, the buttocks and the thighs. In men, the fat is deposited mainly on the belly as well as inside. This belly fat is more harmful than fat deposits in other parts of the body, as many studies have shown.

Why can the SBMI be 24/70 for a woman but only 16/70 for a man of the same age with the same BMI?

Both of them are underweight, seemingly to the same degree. But whereas this only indicates a moderate health risk for the woman, the risk for the man is high.

I see a simple explanation for this: Most women have a lower BMI than most men do. The statistical difference is somewhere between one and two BMI points, depending on their age. The same difference can also be seen at the point where underweight becomes critical.

And here is a more detailed explanation: Women, in general, have less muscle mass than men, and their fat reserve, as I said above, is normally higher. Slimming presents a low health risk as long as a certain amount of body fat is retained. Most men who slim, start at a lower fat mass and hence reach the critical point sooner than most women would do.

Why is the SBMI for Asians higher than for Non-Asions of the same sex and age with the same BMI?

Because Asians, on average, have a higher body fat percentage than Non-Asians. Therefore, their health risk associated with a given BMI is higher, and so is their SBMI. On the population level, the statistical difference is about 2/70 SBMI points.

Any more questions?

Please send them to: bachmann (at) gesundheit (dot) ch

Christian Bachmann, webmaster

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