Obesity

Definition/diagnostic criteria

Adults

Overweight and obesity are defined as abnormal or excessive fat accumulation that presents a risk to health. Body mass index (BMI), a person’s weight in kilograms divided by the square of his height in metres (kg/m²), is used to categorise adult weight status.¹

• Overweight is a BMI greater than or equal to 25;
• Obesity is a BMI greater than or equal to 30.

BMI provides the most useful population-level measure. From a clinical perspective, factors such as age, sex, ethnicity and muscle mass can influence the relationship between BMI and body fat. BMI does not distinguish between excess fat, muscle, or bone mass, nor does it provide any indication of the distribution of fat among individuals. In particular, it does not accurately reflect visceral fat accumulation, considered instrumental in leading to most of the metabolic and clinical consequences of obesity.

Metabolic risk assessment can be more accurately assessed from waist circumference. Healthy waist size will depend on gender and ethnicity.² Recommended waist measurements are less than:

• 80cm (31.5in) for all women;
• 94cm (37in) for most men;
• 90cm (35in) for South Asian men.

BMI has limitations in assessing risk to health in muscular athletes who may have a higher BMI but retain metabolic health while continuing to exercise. Metabolic syndrome risk rises if former exercise routines are abandoned and muscle bulk then converts to visceral adipose tissue.³

There is a growing recognition of a ‘metabolically healthy’ obesity state, in which some individuals are free from the metabolic complications of obesity, most likely because of less visceral fat and preserved insulin sensitivity.⁴

Children

A child’s BMI centile is a measure of how far their BMI is above or below the average BMI for their age and sex in a reference population, to take account of growth patterns by age and gender. The UK National Child Measurement Programme (NCMP) uses the British 1990 growth reference (UK90) to define the BMI classifications. Children with a BMI above the 98th centile are considered clinically obese. For population monitoring those above the 95th centile are classed as obese.^5,6

Epidemiology

Worldwide, obesity rates have grown to epidemic proportions in adults and children, with over 4 million people globally dying each year as a result of being overweight or obese in 2017. From 1975 to 2016, the prevalence of overweight or obesity in children and adolescents aged 5–19 years increased more than four-fold from 4% to 18% globally.

The Health Survey for England 2021 estimates that 25.9% of adults in England are obese and a further 37.9% are overweight but not obese.⁷ Men are more likely than women to be overweight or obese (68.6% of men, 59.0% of women). People aged 45-74 are most likely to be overweight or obese.

Higher obesity rates are linked with deprivation (children in the most deprived areas are approximately twice as likely to be obese, and rates of severe obesity among children are around four times higher in the most deprived areas), disability and lower educational status, and ethnicity.⁸

Genetic basis of obesity

Twin/family studies indicate that around 77% of variation in body weight is genetically influenced, with evidence showing that genetic makeup can predict how different individuals will respond to the socioeconomic environment.⁹ Genetic susceptibility from over 200 separate gene loci influence diverse factors such as food preferences, perception of hunger or satiety, susceptibility to advertising and engagement in physical activity or sedentary behaviour. Meaningful interventions should involve food, environmental and social policies, address aggressive marketing tactics (particularly those targeting children) and empower individuals to take personal action.

Diagnosis

Making a judgemental ‘first glance’ assessment of weight status is inaccurate and clinically inappropriate. Recording up to date weight status, along with other metabolic risk indicators – blood pressure, family history, smoking status and physical activity patterns – is routine clinical practice.

Confidence among health professionals in raising the topic of weight assessment can be low due to fear of causing offense or risking complaint.

Simple openers to raise the issue sensitively and avoid implying judgment include:

• asking permission – ‘Would it be ok if I ask you about your weight?’ or
• asking about feelings – ‘How do you feel about your weight?’

The response may be

• unexpected, (eg, ‘I feel great, I’ve just lost 2 kilos’ – indicating that the patient is already engaged) or
• revealing (‘It’s a nightmare, I’m at my wits end’ – the offer of help may be welcomed), or
• constructive in understanding the patient’s priorities (‘I don’t want to discuss this now as I am more concerned about XXX’ – leave the conversational door open for the patient to come another time if they wish).

Diagnostic assessment should include metabolic impact of obesity on target organs as well as psychosocial assessment. Adipose tissue, far from being inert, causes inflammatory and immune responses on remote organ function through dysfunctional adipocytes that secrete inflammatory adipokines, cytokines and chemokines.¹⁰

Clinical sequelae include:

• Cardiovascular disease, heart failure.
• Cerebrovascular disease / stroke.
• Type 2 diabetes.
• Cancer (endometrium, breast, colon, oesophageal, kidney, gallbladder, pancreatic, and liver cancer).
• Non-alcoholic fatty liver disease.
• Polycystic ovarian syndrome, subfertility and obstetric complications.¹¹
• Chronic kidney disease.
• Obstructive sleep apnoea.
• Depression and low self esteem.
• Disordered eating and binge eating disorder.
• Dermatological disorders including acanthosis nigricans, poor wound healing and ulceration.

Baseline investigations include blood pressure measurement, FBC, U&Es, liver function test, Hba1c, lipid profile. Waist circumference can give a more accurate estimation of cardiovascular risk than BMI.¹²  The Diabetes UK website provides useful information on interpreting waist circumference.

Treatment

Unfortunately, the ‘calorie in – calorie out’ concept of weight regulation oversimplifies the physiologic process of appetite and weight regulation.¹³ Add in the complex genetic, environmental, socioeconomic and cultural factors that influence food choices, hormonal and metabolic activity and availability/accessibility of healthy choices and it is unsurprising that diet and exercise programmes commonly have modest impact and can feel hard to maintain.

Weight ‘set point’ theory is a concept that relates to homeostasis. The theory posits that the human body has a predetermined weight or fat mass set-point range. Various compensatory physiological mechanisms maintain that set point and resist deviation from it. This can explain the common tendency of ‘yo-yo’ weight regain after weight loss.¹⁴

Aiming for weight loss of between 5-10% over three to six months or one to two pounds (0.5-1 kg) per week is a realistic target, with evidence supporting regular monitoring and support with goal setting.¹⁵

Particular interest as well as much conflicting advice surrounds which particular dietary approaches are most effective or have most impact on glycaemic control in people with Type 2 diabetes. This relates to the high prevalence of Type 2 diabetes in people who are overweight or obese, and the fundamental role of dietary control as a first line approach to it. Most ‘diets’ advise restriction of one or other macronutrients – low fat, low carbohydrate, or other variations.

In practice people with obesity are likely to try multiple diets over time. They can be reassured that adoption of a healthy lifestyle (regular physical activity of 30 minutes most days with a healthy dietary regime that the individual/ family enjoys and can maintain) can produce worthwhile health benefits even where weight reduction is hard to achieve or to maintain.¹⁶

The DIRECT study showed that significant weight loss (15kg) is effective in reversing Type 2 diabetes. 15kg weight loss can be achieved through a supported weight management programme in a primary care setting.¹⁷

A meta-analysis of lower carbohydrate diets for adults with Type 2 diabetes suggests beneficial effects for some outcomes (HbA1c, fasting plasma glucose, serum TAG) in the shorter term (up to 6 months). The longer-term impact is less clear due to specific dietary regimes proving hard to maintain.¹⁸

There is strong evidence that combined interventions of a low-calorie diet, increased physical activity, and behaviour therapy provide the most successful therapy for weight loss and weight maintenance. But exercise programmes alone (without dietary restriction) result in subconscious dietary increase that compensates for energy used. Thus those new January gym memberships do not necessarily result in weight loss outcomes even if cardiovascular outcomes benefit!

Weight loss medications

Orlistat

Orlistat binds dietary fat which suppresses stomach and pancreatic lipases, helping to restrict fat absorption from consumed food. It improves lipid metabolism in obese individuals, thus lowering caloric intake. In practice, orlistat can be associated with troublesome side effects such as oily diarrhoea when taken with fatty foods. It can however illuminate if a person’s diet contains hidden fats.¹⁹ Orlistat is prescribed at 120mg tds taken shortly before or during meals, and should be accompanied by lifestyle change advice. The expectation is of around 5% initial weight reduction over 12 weeks or else discontinuation is advised. Avoid taking if meals are omitted and if an occasional predictably high-fat meal is to be consumed such as for a celebration. Follow prescribing guidelines in the BNF. Avoid in cholestasis or malabsorption syndrome and use with caution for people with CKD.

GLP-1 receptor agonists

Liraglutide is a daily subcutaneous weight loss injection that helps to regulate food intake by increasing fullness and reducing appetite. It is licensed under the name Saxenda for use alongside physical activity and dietary advice where BMI is >30 or >27 with co-morbidities.²⁰ BNF adult prescribing advice is initially 0.6mg once daily, then increased in steps of 0.6mg, with the dose to be increased at intervals of at least 1 week up to a maximum maintenance dose of 3 mg once daily (or the maximum tolerated dose). Consider discontinuation if escalation to the next dose is not tolerated for 2 consecutive weeks. Discontinue if at least 5% of initial body weight has not been lost after 12 weeks at maximum dose.

Note the same medication (under the name Victoza) is also prescribed to treat Type 2 diabetes but using a different dosage regime.

Semaglutide is given as a weekly injection for weight loss and for Type 2 diabetes. It is licensed for weight management under the name Wegovy in conjunction with dietary measures and increased physical activity in individuals with a BMI of 30 or more, or in individuals with a BMI of 27 or more in the presence of at least one weight-related co-morbidity. BNF prescribing guide is 0.25mg once weekly for at least 4 weeks, then increased if tolerated to 0.5 mg once weekly for at least 4 weeks, then increased if tolerated to 1mg once weekly for at least 4 weeks, then increased if tolerated to 1.7mg once weekly for at least 4 weeks, followed by maintenance 2.4mg once weekly. Review duration of use if weight reduction does not reach 5% by six months. Specific prescribing details are available from the Department of Health and Social Care.²¹

Consider specific circumstances where weight loss and improved nutrition could be particularly clinically beneficial such as to improve outcomes prior to planned orthopaedic surgery or perhaps where weight reduction might negate the need for surgery at all.

Bariatric or metabolic surgery

Consider metabolic surgery for someone with a BMI of 40 or more, or a BMI between 35 and 40 and an obesity-related condition that might improve with weight loss (such as Type 2 diabetes or high blood pressure) when other approaches have been unsuccessful. Consider metabolic surgery as a first-line option if BMI is >50.  Pre-surgical psychological counselling is critical to ensure suitability for surgery. Motivation to adopt healthy lifestyle and comply with regular lifelong follow-up is an essential criterion.²²

Common types of surgery include:

• Gastric band – a band is placed around the top of the stomach, which reduces appetite and capacity.
• Gastric bypass – the top part of the stomach is joined to the small intestine, reducing appetite and limiting absorption of calories from food.
• Sleeve gastrectomy – some of the stomach is removed, limiting capacity and promoting early satiety.

Follow up after metabolic surgery

Regular follow up is essential for both successful weight loss control and to monitor for nutritional deficiency. Primary care-focused details of recommended supplements, blood test monitoring, potential nutritional deficiencies and side effects are available from the British Obesity and and Metabolic Surgery Society.²³

Prognosis

Obesity, particularly when it develops in childhood, is a chronic relapsing disease process. Active long-term management strategies as described can give effective control but relapse is common, even after surgical procedures where pre- and post-surgical support is inadequate. Adoption of a healthy active lifestyle with a low-calorie diet can reduce risk of metabolic sequelae of obesity even where weight reduction is unsuccessful.

Dr Rachel Pryke is a retired GP, former Clinical Champion for Nutrition and Obesity for the RCGP, World Obesity Federation Clinical Committee member and held NICE Fellowship 2015-18. She has created extensive obesity training and educational resources