As a bariatric surgery patient, you are constantly surrounded by numbers – but what do they all mean, and which ones are important? Is it just a bunch of numbers? Below we review a few common metrics used in bariatric surgery, such as Body Mass Index (BMI), body measurements (often called "inches") and weight, and discuss why they matter.
Key takeaways
- BMI is calculated from height and weight and is used to screen for weight categories that may lead to health problems.
- Track your body measurements as well as your weight: losing centimetres while the scale stays the same often means you are gaining muscle and losing fat.
- Losing weight without losing centimetres can be a red flag for muscle loss.
- Measure consistently: same place, same side, same conditions.
- A waist circumference above 88 cm (women) or 102 cm (men) increases the risk of heart disease, diabetes and stroke.
BMI – what is it? What does it mean?
Body Mass Index (BMI) is a number calculated from an individual's height and weight. BMI provides a reliable indicator of body fatness for most people and is used to screen for weight categories that may lead to health problems and/or conditions.
You can calculate your BMI in several ways. You can use an online BMI calculator, such as the one from the US National Institutes of Health (NIH) (1). The NIH also has a Body Mass Index table you can use to determine your BMI (2). To use the table, find your height (listed in inches) in the left-hand column, move across to your weight (listed in pounds, rounded off) and read the BMI at the top of the column. You can also calculate it yourself: BMI = weight in kilograms divided by height in metres squared (kg/m²).
BMI categories
BMI is often categorised. A normal BMI refers to the lowest risk of chronic health conditions; the higher your BMI, the greater your risk. The opposite is also true: a BMI below normal (underweight) may also increase your risk of chronic health conditions.
| Category | BMI (kg/m²) |
|---|---|
| Underweight | < 18.5 |
| Normal weight | 18.5–24.99 |
| Overweight | ≥ 25–29.99 |
| Obese, class I | ≥ 30–34.99 |
| Obese, class II | ≥ 35–39.99 |
| Obese, class III (morbid obesity) | ≥ 40–49.99 |
| Obese, class IV (super obesity) | ≥ 50 |
Measurements – why are they important?
Many bariatric surgery patients record their body measurements (in the US measured in inches, which is why they are often called "inches") when they start a weight loss programme and continue monthly. This can be an extra source of motivation: some patients report the scale not moving while they are clearly losing centimetres. Tracking your measurements throughout your bariatric journey provides positive reinforcement for the changes you have made.
Muscle is denser than fat, so a kilo of muscle takes up less space than a kilo of fat. Ideally, bariatric surgery patients add muscle mass as fat mass decreases. As a result, the scale might not move while fat mass is decreasing and the patient is getting healthier. This can be frustrating for patients whose only measure of success is the scale. That is why it is important to evaluate other measures too, including tracking your measurements. Centimetres lost are as important a measure of success as kilos lost.
Losing weight without losing centimetres is a red flag for losing muscle mass rather than fat mass. While this looks good on the scale, it does not provide the health benefits most bariatric surgery patients want. Losing muscle may make it harder to keep losing weight and maintain that loss in the long run, because muscle mass helps maintain your metabolism (how you burn calories). Muscle also helps with balance and strength, which becomes extremely important with age to reduce the risk of falls.
Measurements – how, where and accuracy
There is little data on the significance of any one specific measurement technique, but it is important to remain consistent about where you measure. If you take your own measurements, stand in front of a mirror so you can see the full course of the tape measure and make sure it is level. Alternatively, ask your support person or bariatric healthcare team to take your measurements to ensure accuracy and precision. Common measurements:
- Bust (women): place the tape around the largest part of the chest, parallel to the floor.
- Chest (men): place the tape around the largest part of the chest, slightly under the armpits, parallel to the floor.
- Waist (everyone): for the most consistent measurement over time, measure waist circumference (WC) at the top of the iliac crest (the top of the hip bone). Find it by placing your fingers on your lowest rib in the mid-axillary line (an imaginary line down your side from the armpit) and sliding down until you feel the top of the iliac crest. This may require some pressure but should not cause significant discomfort. Keep the tape parallel to the floor, breathe out and measure before breathing in again.
- Hips (everyone): measure at the level of the greater trochanter (the top of the thigh bone). Find it by placing your fingers on the side of the mid-thigh and sliding up along the thigh bone until you feel the prominent bulge. Measure around at this level, keeping the tape parallel to the floor.
- Thigh (everyone): measure the distance between the kneecap and the bend of your upper leg and hip, and measure the thigh halfway. Keep all your weight on the opposite leg (e.g. when measuring the right thigh, stand on your left leg).
- Calves (everyone): measure at the fullest point of the calf (the widest circumference).
- Upper arm (everyone): measure halfway between the top of the shoulder and the elbow (around the biceps).
- Neck (everyone): measure around the largest part of the neck.
Tips for taking measurements
- Stand tall with your muscles relaxed and feet together.
- Apply constant pressure to the tape to prevent sagging, without pinching the skin.
- Use a flexible tape measure (plastic or cloth).
- Measure under the same conditions (e.g. the same clothes or a lightweight gown).
- If you measure yourself, do so in front of a mirror to make sure the tape is positioned correctly.
- The exact place for each measurement varies from person to person – take it in the same place on your body every time.
- Measure only one side of the body (e.g. the right side) and use that side every time.
- Record measurements in centimetres (cm) to better represent changes over time.
- Keep a record of your measurements over time.
Waist measurement
Waist circumference (WC) is the only measurement strongly associated with metabolic syndrome and cardiovascular risk. In women and men, a WC greater than 88 cm (35 in) and 102 cm (40 in) respectively increases the risk of heart disease, diabetes and stroke (3).
Graphing your weight and your success
It is normal, even as a bariatric surgery patient, to experience ups and downs in your weight. What matters is the overall trend. Graphing your weight every time you weigh yourself helps you see whether the trend is up (gaining) or down (losing). This may help you adjust your healthy eating plan and physical activity plan accordingly (after speaking to your bariatric surgeon).*
Do not let all these numbers and recommendations make your head spin. Think of everything as improvement and progress. You may not have the "perfect" or "ideal" BMI yet, but if you are working towards reducing your weight, losing centimetres and improving your health, you are doing amazing things! If you are just getting started, take your starting measurements and weight and calculate your BMI so you can track your progress. Many post-bariatric surgery patients will tell you these numbers are a great motivator and help them stay on the path to success. If you are further out from surgery and never took your initial measurements and weight, start today!
Success is more than just numbers, and it is different for each person. Questions to ask yourself might include:
- Are you off certain medications (e.g. for blood pressure, diabetes or high cholesterol)?
- Are chronic health conditions under control?
- Can you walk up a flight of stairs without feeling out of breath?
These are amazing quality-of-life parameters that may be achieved with bariatric surgery, weight loss and improvements in health.
* A healthy eating plan and physical activity plan should be viewed as an individualised regimen based on each patient's medical history and physical limitations. Patients should follow the instructions of their bariatric surgery team and follow up at the recommended intervals.
Sources
- National Institutes of Health, National Heart, Lung, and Blood Institute. Calculate Your Body Mass Index. https://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm. Accessed 13 October 2020.
- National Institutes of Health, National Heart, Lung, and Blood Institute. Body Mass Index Table. https://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmi_tbl.pdf. Accessed 13 October 2020.
- National Institutes of Health, National Heart, Lung, and Blood Institute. Metabolic Syndrome. https://www.nhlbi.nih.gov/health-topics/metabolic-syndrome. Accessed 13 October 2020.
This article is for general information only and does not replace personal medical advice. Always discuss your situation, blood test results and supplementation with your bariatric team or healthcare provider.