Traditionally defined as body weight 30% above the ideal – now defined in terms of body mass index (BMI) – expressed a weight (kg) divided by square of height (m). World Health Organisation guidelines are that a BMI of 18.5-25 is an acceptable weight; 25-30 = overweight; >30 = obese.

Prevalence is high and rising – considered that more than 20% of Americans are obese and this is a 50% increase in the last decade  has significant public health implications in terms of diabetes mellitus, hypertension, heart disease and other diseases.

Obesity is a stigmatised disease  common assumption is that those who are obese are lazy, have no will power and are that way through choice.

Body fat distribution – two major types in adults:
• abdominal distribution (‘apple shaped’) – fat deposited mainly on abdomen
• gynoid distribution (‘pear shaped’) – fat deposited mainly on the hips and thighs

Overweight state occurs when there is a positive energy balance is created when there is an increased energy intake that exceeds output.

Less energy is expended than is consumed.
Genetic – accounts for about 30-40% of variation in body weight. Mutation in ob gene in mice  obesity. This gene codes for leptin  controls body fat by acting as a signal between adipose tissue and central control mechanism.
Environment – socioeconomic (low status is high risk factor); large food intake;
Regulatory – post-pregnancy; brain damage (eg tumour); drugs (steroid hormones); endocrine (eg hyperinsulinaemia); psychologic (eg binge eating disorders)
Behavioural – high fat diets; snacking; consuming energy dense foods; sedentary lifestyle – energy used in physical activity is directly related to body weight.
Drugs – corticosteroids; tricyclic antidepressants; sulphonylureas

Clinical features:
Large adipose mass; consequences (sleep apnoea, orthopaedic complications, skin disorders from sweat)
Diagnosed when BMI >27.8 for men and >27.3 for women

• Increased mortality (risk of premature death increases by 1% for each 0.5kg weight rise)
• Metabolic disorders (insulin resistance, hyperinsulinaemia, hypertension)  diabetes, dyslipidaemia  coronary artery disease, cerebrovascular disease
• higher incidence gall bladder disease, cancer and sleep apnoea in those who are obese
• increased risk for osteoarthritis, varicose veins
• pregnancy complications
• social disapproval, social stigmatisation, depression, reduced employment prospects

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