This blog provides free general information for anyone who is seeking to understand more about the different terminology and mathematical calculations that bariatric physicians and surgeons use, not intended as a medical consult. Please seek proper medical advice for individual assessment and management.


Weight is calculated in kilograms (kg)

Height is measured in metres (m)

Waist circumference is calculated in metres (m)

Hip circumference is measured in centimetres (cm)




Body mass index (BMI) is calculated by the formula:

BMI = weight/height square (kg/m2)


Total body weight loss (TBWL) = pre-op weight – post op body weight.

% TBWL is obviously the fraction of body weight expressed in percentage term.


%TWL = (pre-op weight – follow up weight)/(pre-op weight) X 100





Percentage of excess weight loss (%EWL) is calculated using an ideal body weight of BMI 25.

% EWL = (pre-op weight – follow up weight)/(pre-op weight – ideal body weight) X 100






Percentage BMI loss (% BMIL) = (pre-op BMI – post op BMI)/(pre-op weight) X 100


Percentage excess BMI loss (% EBMIL) = (pre-op BMI – follow up BMI)/(pre-op BMI – 25) X 100







A body shape index (ABSI) = (waist circumference) / (2/3 BMI X ½ Height)


Body adiposity index (BAI) = (hip) / (-1.5 height) – 18


Percentage Modified body adiposity index (% MBAI) = 23.6 + ½ BAI + a variable

  • If BMI >50, the variable is 2.2
  • If waist hip circumference is >1.05, the variable is 2.4









Definition of weight loss success


The ideal weight for everyone following the Caucasian standard is to have a BMI 20-25. The world’s population in the last century rarely have morbid obesity issues.

This is obviously not the case any more in the 21st century.


With the introduction of bariatric surgery, there is also a need to define what constitutes successful weight loss surgery outcomes.

  • Around 1982, surgeons began describing weight loss surgery as achieving >50% excess weight loss (EWL) and this has been the standard to which we measure the success of bariatric surgery ever since.
  • More recently the definition of weight loss success after surgery include >20% (total body weight loss) TWL. This is easier to calculate and TWL getting used more often in modern day scientific research.


In contrast the definition for success using medical pharmacology treatments (without bariatric surgery) is >5% TWL.

  • Fortunately reduction in medical co-morbidities can be seen after 5-10% TWL.


Some researchers have noted that:

  • Weight loss surgery provides 4 times the weight loss in the long term
  • Overall the successful long term weight loss (>20% TWL) is expected in 70% of patients



The definition of weight regain after bariatric surgery however is not clear at this stage.

There has been several suggestion, which include:

  • BMI >35 after initial success of BMI <35 after the initial surgery
  • EWL <50% after initial success of >50% EWL after the initial surgery
  • Weight regain >25% EWL from the lowest weight (nadir)
  • Weight regain >10% from the lowest weight (nadir)
  • Not able to maintain >20% TWL at all



Less successful outcome (<20% TWL) is observed in:

  • Gastric band surgery
  • Lower pre-op BMI
  • Patients with pre-op T2DM and HPT
  • Age over 40 years
  • Male gender
  • Substance abuse (alcohol)
  • Medical causes of weight regain





Weight loss results after bariatric surgery


Patients falls into 3 categories after bariatric surgery.

  • Primary responder are patients who achieved >50% EWL.
  • Primary non responder are patients who achieved <50% EWL (previously called failure of weight loss).
  • Secondary non responder are primary responders with successful weight loss in the past but has regained the weight lost
    • Mild regain is 5-10kg from the nadir
    • Severe regain is >10kg from the nadir



Weight regain are often multifactorial and difficult to study.

In a simplistic explanation, they may be grouped as:

  • Technical failure (eg. dilated proximal gastric pouch or loss of restriction and increased food portion size)
  • Hormonal changes (eg, rise in ghrelin, loss of satiety)
  • Nutritional non-compliance (eg. non adherence to dietician dietary plans, sweet eating)
  • Behavioural non-compliance (eg. grazing, emotional eating habits)
  • Inadequate physical activity
  • Psychological factors







Resolution of T2DM after bariatric surgery

It is also noted that  bariatric surgery may halt the progression of T2DM, which is a chronic progressive disease, it is not necessarily about achieving a life-long cure.


Secondly patients with relapse or persistence of T2DM were more better managed and glycaemic control is easier after a sleeve gastrectomy.

  • The restriction in calorie intake allows patients to reduce the medication doses for oral hypoglycaemics or insulin, as proven with better HbA1C blood tests results compare to patients who had medical treatment alone and did not have bariatric surgery.


The success in treating T2DM often depends on other factors, such as:

  • Advanced age of the patients
  • Duration of the T2DM
  • Whether patients are requiring or dependent on insulin
    • In other words the ability of pancreatic Beta cells to recover, to regenerate/release insulin





Patients are divided into 4 categories depending on their response after bariatric surgery.


Complete remission

  • Defined as baseline blood glucose <100 mg/dL (5.6 mmol/L) and a HbA1C <6%, maintained for > 1 year without hypoglycaemic medications

Partial remission

  • Defined as baseline blood glucose 100 – 125 mg/dL (5.6 – 6.9 mmol/L) or a HbA1C 6-6.5% without hypoglycaemic medications or those who have not discontinued hypoglycaemic medications


  • Are patients who satisfied the criteria to define complete remission for more than 1 year but had recurrence of the diabetes, diagnosed as fasting glucose >126 mg/dL (7 mmol/L) or frank diabetes on oral glucose tolerance test >200 mg/dL (>11mmol/L) at 120 minutes


  • Are patients who did not have characteristics necessary to define complete remission for a duration >1 year