Background Information

Aetiology

Overweight and obesity are caused by a chronic imbalance between energy intake and energy expenditure, resulting in excess body fat accumulation.

Aetiology is multi-factorial: ^[Ref1][Ref2]

• Genetic predisposition (usually polygenic and interacts with environmental factors)
• Environmental contributors
  • Excessive caloric intake
  • Consumption of ultra-processed foods and sugar-sweetened beverages
  • Low physical activity
  • Low socioeconomic status
  • Chronic stress
  • Insufficient sleep

Medical causes of overweight and obesity: ^[Ref1][Ref2]

• Depression
• Hypothyroidism
• Cushing’s syndrome
• Binge eating disorder

Medications associated with weight gain: ^[Ref1][Ref2]

• Certain diabetes medications (insulin, sulfonylurea, pioglitazone) (other medications are either weight-neutral or promote weight loss)
• Antipsychotics (esp. olanzapine)
• Systemic corticosteroids
• Antidepressants (e.g. amitriptyline, mirtazapine)
• Antiepileptics (e.g. gabapentin, carbamazepine)
• Beta blockers
• Protease inhibitors

Complications

Weight-related comorbidities include: ^[Ref1][Ref2]

• Metabolic syndrome (including hypertension, diabetes, dyslipidaemia)
• OSA
• Osteoarthritis
• Hepatic steatosis, NAFLD
• GORD

Identification and Assessment

Methods of Measurement

NICE recommends BMI as a practical measure of overweight and obesity.

• BMI: weight (kg) / (height in m)²

NICE also recommends measuring waist-to-height ratio in those with BMI <35 kg/m²

• Waist-to-height ratio is a practical estimate of central adiposity, which can be used to assess and predict health risks

Interpretation (Adults)

BMI

Classification	BMI range (kg/m2)
Healthy weight	18.5 – 24.9
Overweight	25.0 – 29.9
Obesity class 1	30.0 – 34.9
Obesity class 2	35.0 – 39.9
Obesity class 3	≥40.0

NICE recommends lower BMI thresholds in people with South Asian, Chinese, other Asian, Middle Eastern, Black African, and  African-Caribbean backgrounds as they are prone to central adiposity and their cardiometabolic risk occurs at lower BMI:

• Overweight: BMI 23 – 27.4 kg/m²
• Obesity: BMI ≥27.5 kg/m²
• Obesity class 2 and 3: minus 2.5 kg/m² from the above thresholds

Central Adiposity (Waist-to-Height Ratio)

Central adiposity should be classified based on waist-to-height ratio

 

The following waist-to-height classification can be used for people with BMI <35 kg/m² of both sexes and all ethnicities, including adults with high muscle mass.

Classification	Waist-to-height ratio range
Healthy	0.4 – 0.49
Increased	0.5 – 0.59
High	≥0.6

Interpretation (Children and Young People)

BMI

Centiles and standard deviations should be used to classify BMI.

Classification	BMI (centile and SD)
Overweight	BMI 91st centile + 1.34 SD
Clinical obesity	BMI 98th centile + 2.05 SD
Severe obesity	BMI 99.6th centile + 2.68 SD

Central Adiposity (Waist-to-Height Ratio)

Central adiposity should be classified based on waist-to-height ratio

 

The classification is the same as adults:

Classification	Waist-to-height ratio range
Healthy	0.4 – 0.49
Increased	0.5 – 0.59
High	≥0.6

Management

Conservative / General Management

Key point is employing a multicomponent management strategy with long-term support, including:

• Increase physical activity changes, and
• Reduced-calorie diet

Pharmacological Management

All medicines for weight management should be used alongside a reduced-calorie diet and increased physical activity.

Drug	Class	Indications	Important side effects	Pregnancy and contraception
Orlistat	Lipase inhibitor	BMI ≥30 kg/m2, or BMI ≥28 kg/m2 + associated risk factors	Flatulence Oily fatty stools Fat-soluble vitamin deficiency Gallstones and kidney stones Rare but risk of serious hepatitis	Do not use
Tirzepatide	GLP-1 and GIP agonist	BMI ≥35 kg/m2, and At least 1 weight-related comorbidity	GI upset is common but not serious (e.g. nausea, vomiting, diarrhoea, constipation, bloating) Pancreatitis Bowel obstruction Thyroid C-cell tumour (thus contraindicated in personal / family history of medullary thyroid carcinoma or MEN2)
Semaglutide	GLP-1 agonist	BMI ≥35 kg/m2, and At least 1 weight-related comorbidity
Liraglutide		BMI ≥35 kg/m2, and Non-diabetic hyperglycaemia

 

For tirzepatide, semaglutide and liraglutide, NICE recommends using lower BMI thresholds (usually reduced by 2.5 kg/m²) for people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean ethnic backgrounds.

Children and Young People

Medications are NOT generally recommended for those <12 y/o

• Only consider in exceptional circumstances, if severe comorbidities are present

Surgical Management

Referral Criteria

Refer to specialist overweight and obesity management service to assess for bariatric surgery eligibility if:

• BMI ≥40 kg/m² or BMI 35-39.9 kg/m² + significant health conditions that can be improved if they lose weight, and
• Patient agrees to necessary long-term follow-up after surgery

Early assessment for bariatric surgery should be considered in:

• BMI 30-34.9 kg/m², and
• Recent-onset type 2 diabetes (diagnosed <10 years), and
• Receiving, or will receive, assessment in a specialist overweight and obesity management service

Consider referral for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African–Caribbean background using a lower BMI threshold (reduced by 2.5 kg/m²)

Surgical Options

NICE did not make any specific recommendations on which surgical interventions to use, but it is important to know about the commonly used bariatric surgeries.

Procedure	Type	Mechanism	Disadvantages
Sleeve gastrectomy	Restrictive	Removes ~80% of the stomach (greater curvature)	Irreversible May worsen GORD
Adjustable gastric banding		Inflatable band around the upper stomach, adjustable via subcutaneous port	Less weight loss than other methods Risk of band slippage
Roux-en-Y gastric bypass	Restrictive + malabsorptive	A small stomach pouch is created and connected to the intestine to bypass the duodenum and proximal jejunum	Nutrient deficiency (those absorbed in the duodenum and proximal jejunum) (e.g. iron, folate) Vitamin B12 deficiency (intrinsic factor secreted by the stomach) Dumping syndrome
Biliopancreatic diversion with duodenal switch		Sleeve gastrectomy + bypass of most of the small intestine	Nutrient deficiency Vitamin B12 deficiency (intrinsic factor secreted by the stomach) Protein-calorie malnutrition

References