Obesity is a progressive and relapsing condition characterised by abnormal or excessive accumulation of fat and is becoming increasingly common. Obesity is associated with increased risk of non-communicable disease and is preventable.
In Australia, the prevalence of obesity has increased from 19% in 1995 to 28% in 2014-2015. Now, 71% of males and 56% of females in Australia are classified as either overweight or obese (AIHW Risk Factors to Health Web Report).
Asthma incidence is increased in obesity, with a meta-analysis of 333,102 adults finding that obesity doubles the odds of developing asthma (Beuther et al. 2007). In 2014-2015, 35% of Australian adults with asthma were obese, compared with 27% of adults without asthma (AIHW Web Report). The prevalence of obesity is further increased in people with severe asthma, with an Australian study finding that 45% of those prescribed omalizumab are obese (Gibson et al. 2016).
The prevalence of asthma in Australian adults by body mass index (BMI), 2014-2015. Data source: AIHW Web Report.
The prevalence of obesity in Australian adults with and without asthma, 2014-2015 (first two columns) and *the prevalence of obesity in adults with asthma enrolled in the Australian Xolair registry in 2011. Data sources: AIHW Web Report and Gibson et al. 2016
Obesity is a commonly reported comorbidity of asthma, particularly severe asthma. Asthma and obesity interact, and are associated with poorer asthma control, more frequent exacerbations and a poorer quality of life (Tay et al. 2016). This indicates that obesity contributes to the illness burden of asthma.
The reason obesity prevalence is increased in severe asthma is unknown. It may be due to a combination of inflammatory, mechanical and genetic factors, however more research is needed. Studies adopting a cluster analysis approach have consistently identified an asthma phenotype that exists predominantly in obese women. This cluster has late onset severe asthma, with a high symptom expression, use of high dose inhaled corticosteroids, and an airway inflammatory pattern that is either mixed granulocytic or neutrophilic in nature (Haldar et al. 2008, Moore et al. 2014).
You can calculate BMI using the AIHW BMI calculator found here.
BMI is classified according to the World Health Organization BMI classifications:
| BMI (kg/m2) | Classification | Risk of comorbidities |
| < 18.5 | Underweight | Low (but risk of other clinical problems increased) |
| 18.5-24.9 | Normal weight | Average |
| 25.0-29.9 | Overweight | Increased |
| 30.0-34.9 | Obesity class I | Moderate |
| 35.0-39.9 | Obesity class II | Severe |
| ≥ 40 | Obesity class III | Very severe |
Reproduced from Obesity: Preventing and Managing the Global Epidemic, 2000, WHO
BMI can be less accurate in assessing obesity in people that have variations in fat and/or muscle mass, although BMI is useful for assessing disease risk at a population level.
Body composition varies by ethnicity and attempts have been made to establish whether overweight and obesity classifications should vary in Asian and Pacific populations. Asian people have a higher percentage of body fat than Caucasian people of the same sex, age and BMI, and the prevalence of cardiovascular disease and type 2 diabetes is higher at a similar BMI. However, due to differences within different Asian populations, there is no clear cut-point to define overweight and obesity. As a result, the World Health Organization have recommended retaining the current BMI cut-points universally, however, alternate cut-points have been developed for Asian populations as a point for public health action:
| BMI (kg/m2) | Classification | Risk of comorbidities |
| < 18.5 | Underweight | Low (but risk of other clinical problems increased) |
| 18.5-22.9 | Normal weight | Average |
| 23.0-27.4 | Overweight | Increased |
| 27.5.0-32.4 | Obesity class I | Moderate |
| 32.5-37.4 | Obesity class II | Severe |
| ≥ 37.5 | Obesity class III | Very severe |
Reproduced from: WHO Expert Consultation 2004
Waist circumference is a good predictor of visceral adiposity, with abdominal obesity associated with an increased risk of a number of diseases including cardiovascular disease, diabetes and cancer.
Below are sex-specific recommendations for waist circumference in Caucasians, relating to risk of metabolic complications.
| Risk of metabolic complications | Waist circumference (cm) Men | Waist circumference (cm) Women |
| Increased | ≥ 94 | ≥ 80 |
| Substantially increased | ≥ 102 | ≥ 88 |
The Australian obesity management guidelines recommend measuring both waist circumference and BMI in adults.
Weight loss should be recommended for all people with severe asthma who are obese, as it can provide a wide range of health benefits. People with severe asthma should be advised that weight loss may help to control their asthma. Even a small weight loss can be beneficial, with one study finding a 5-10% weight loss was associated with clinically important improvements in asthma control in 58%, and asthma-related quality of life in 83%, of overweight and obese adults with asthma (Scott et al. 2013).
However, the evidence base regarding the impact of weight loss on asthma and obesity outcomes is weak due to a lack of high quality studies, which are urgently needed to inform asthma management guidelines. There is also insufficient evidence regarding the optimal method to achieve this weight loss in people with asthma. Current recommendations therefore advise to follow the general Australian obesity management guidelines, which can be found here. These guidelines have been described very briefly below.
The weight loss approach used depends on the degree of obesity. An additive approach is recommended, with more invasive approaches reserved for those with the highest BMIs.
Lifestyle change should be recommended to all obese people with severe asthma. This includes reducing energy intake, increasing physical activity and counselling for behaviour change. A dietary intervention producing a 2500 kilojoule/day deficit is recommended and should be tailored to the individual’s dietary preferences (Clinical Practice Guidelines 2013). Recommendations also suggest 300 minutes of moderate-intensity exercise, or 150 minutes of vigorous-intensity exercise, or an equivalent combination, each week (Clinical Practice Guidelines 2013). For more information about physical activity in those with asthma and obesity see Management – Physical Activity & Exercise.
For some patients, more intensive interventions may be required.
A Western dietary pattern is high in fat and low in fruits and vegetables and is common in people with obesity. A Western dietary pattern is also associated with more severe asthma (Barros et al. 2015, Brigham et al. 2015).
A high fat diet is associated with more hyperresponsive airways (Soutar et al. 1997) and airway inflammation (Wood et al. 2011) in asthma. A high fat meal has been shown to suppress bronchodilator recovery in asthma (Wood et al. 2011), which could have a large impact on asthma management when a habitually high-fat diet is consumed.
A low fruit and vegetable diet (≤2 serves of vegetables and ≤1 serve of fruit/day) more than doubles the risk of an asthma exacerbation compared with a high fruit and vegetable diet (≥5 serves of vegetables and ≥2 serves of fruit/day), according to a study of Australian adults with asthma assessed over a 12 week period (Wood et al. 2012).
Dietitians are able to provide specialist dietary counselling and medical nutrition therapy to people who are obese. This can assist both with weight management and improving the quality of the diet to meet national guidelines. Dietitians can also implement a very-low calorie diet (VLCD), such as an Optifast protocol, for those with a BMI>30 if required. A VLCD should be implemented by a dietitian, to ensure the nutritional adequacy of the diet. In Australia, a list of local Accredited Practising Dietitians can be found here.
Referral to a specialist service to receive advice regarding bariatric surgery may be considered in people with a BMI>40, or >35 in the presence of at least two significant obesity-related comorbidities. This approach may be considered for people aged 18-65 years who have attempted, but not succeeded, in achieving clinically meaningful weight loss.
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