BMI: help or hindrance?
BMI is often used in the fertility and assisted conception world as a marker, reference value or cut off point indicative of your next steps. But what is BMI? And is it more of a hindrance or a help?
What is BMI?
BMI is an abbreviation for Body Mass Index. This means a simple equation is used to give a numerical value of your weight in relation to your height. The concept of a Body Mass Index was thought up in the 1830s by an astronomer from Belgium, who loved statistics and wanted to find out what characteristics defined the ‘average man’. He intended BMI to be used as a measure of overall population characteristics, not a personal measure of health.
However, the calculation was taken up by various governments and health authorities, and has been used to create categories of ‘desirable weight range’ according to height. BMI is undeniably helpful in measuring population norms: it is a quick and easy measure which is reproducible by different researchers all over the world, with minimal equipment and time. That is where BMI is helpful.
BMI Categories:
<18.5 = underweight
18.5 - 24.9 = ideal weight
25 - 29.9 = overweight
>30 = overweight
So what is the hindrance?
One of the problems with BMI is working out ‘normal’ versus ‘healthy’. Even in the 1920s, when population studies were carried out, the average BMI of the American man was 24 or 25 (depending on what kind of average you use). If the average BMI was bordering on being too high even 60 years before the so-called ‘obesity epidemic’ started, could we say that the categories are inaccurate? Additionally, though there is an association between increased BMI and increased morbidity, several studies have found that there was little difference in mortality in a broader range of BMIs than just the ‘healthy weight’. In fact, there has been little association with increased death rate in a range of BMI 24-28.
Body composition
Another hold up with BMI as a measure of a person's health is that it does not take into account the person's body composition. Body composition refers to the percentages of bone, muscle, fat and other tissue that make up your body. As BMI is a measure of the overall body weight, it cannot discern how much of the body is made up of fat, or muscle - which is problematic because a slightly high muscle mass is healthy, whereas a similar fat mass is not. Muscle is very dense tissue and therefore weighs a lot compared to fat, so a very fit muscular person of average height may have an ‘overweight’ BMI despite being in peak physical condition. A way around this is to monitor your waist circumference or hop on a bioelectrical impedance machine (body composition scanner) at your local gym or leisure centre.
What has this got to do with Fertility?
Body fat percentage is an important consideration when aiming to improve fertility, or when considering trying to conceive, both for women and men. In women when body fat percentage is too low, periods and ovulation may stop - which is thought to be the body's way of preventing an unsuccessful pregnancy. We know that in very lean muscular women (who may still have a ‘healthy’ BMI), the lack of body fat and potential relative lack of energy intake often leads to missing or irregular menstrual cycles due to disrupted hormone production and regulation. On the other hand, a particularly high body fat mass can lead to dysregulation of hormonal cycles and impede fertility (more about this in an upcoming blog). Finally, stepping away from the biology and towards the practicality: many NHS trusts have a BMI ‘limit’ on their referral criteria for IVF. This means that in many cases, if your BMI is over 30, you will be told to lose weight before being eligible for assisted conception treatment. This can create stigma and feelings of shame.
Stay tuned for more blog posts about body composition and fertility, in men and women. If you have questions or would benefit from talking about your fertility and nutrition, you can contact us via social media or our website.
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