A relationship between obesity and asthma continues to stimulate medical interest (Table 48). Physicians as well as the public have long postulated a link between obesity and asthma. The prevalence of both obesity and asthma are rising in parallel in Western societies. Several human population studies have demonstrated an association between being overweight and the likelihood of carrying an asthma diagnosis. Yet, the precise relationship between the two conditions remains controversial.
Does asthma cause obesity? Does obesity cause asthma? Or, is there a common underlying factor that causes both asthma and obesity? The scientific pulmonary community has started to tackle the questions. One traditional theory maintains that asthma, particularly less than optimally managed asthma, leads to a more sedentary lifestyle. According to the theory, some persons with asthma do not exercise, either because of a mistaken belief that exercise is bad for asthma or because their asthma is not well controlled and flares with exercise. Because of lack of exercise, they become more out of shape, more sedentary, gain more weight, and ultimately may become obese.
An alternative theory suggests that obesity is the primary event. Fat tissue is not inert but is metabolically active. The theory holds that the obese state leads to the generation of inflammatory mediators that, in turn, cause changes in the lungs and airways, and ultimately asthma. Both theories have their merits, but neither one satisfactorily explains why asthma does not affect males and females equally.
A third, more contemporary approach, with intriguing supportive data in laboratory animals, attempts to sort out the influence of chromosomal changes and hormonal factors that might confer predispositions to both asthma and overweight status. In 1994, a protein molecule called leptin was identified in humans. Leptin is primarily produced by fatty tissues. Leptin and a second protein called ghrelin are both suspected of playing important roles in the regulation of food intake, energy balance, and ultimately of weight. Of interest to lung researchers, lung and airway cells appear to carry receptors for leptin on their surfaces, and leptin furthermore stimulates the proliferation of the lung and airway cells, acting as a kind of stimulatory lung growth factor. Some preliminary studies in humans have documented elevated blood leptin levels in certain types of marked obesity, but research that is more recent has failed to confirm a link with the development of asthma.
A key to better understanding the obesity–asthma link may come from the study of girls and boys in puberty and adolescence. More than 1000 babies born between May 1980 and January 1984 were entered at birth in the ambitious and ongoing Tucson Children’s Respiratory Study. Nearly 1300 youngsters enrolled in the study and have been followed and reassessed at regular intervals for over 20 years by Dr. Fernando Martinez and his team.
The study was designed to study respiratory health and illness over time, in a prospective fashion. In particular, the development of asthma was carefully evaluated. In 2001, Dr. Martinez and his coworkers reported that girls who became overweight or obese between the ages of 6 and 11 years of age had an increased risk for developing new asthma symptoms during early adolescence.
They found that girls—but not boys—who became overweight between 6 and 11 years of age were 5–7 times more likely to develop new asthma symptoms at ages 11 and 13, compared to girls who did not become overweight or obese at ages 6 and 11 years. Boys who became significantly overweight between 6 and 11 years of age did not exhibit an increased risk for the development of asthma or asthma-like symptoms. The strongest association between overweight status and asthma risk was seen in females who underwent puberty before the age of 11 years. Being overweight is associated with earlier onset of puberty.
Could there be a common factor that leads to overweight status in girls, followed by early puberty and then the development of asthma? The findings are especially interesting because it has long been observed that new cases of asthma in females are especially common in the adolescent years. Further, the male: female ratio (2:1) of asthma seen in young, school-age children changes over to a female preponderance by adulthood. A role for female hormones has long been suspected, and the data from the Tucson Children’s Respiratory Study supports this theory and suggests avenues for further research.
Apart from the links between obesity and asthma risk, being overweight is medically undesirable. Even my patients with very well-controlled asthma symptoms report that their breathing is much “easier” when they maintain a comfortable weight. They feel more limber, less achy, and describe greater endurance.
It makes sense that if you carry excess weight on your frame, it literally feels as if you are transporting additional pounds as you walk around. Studies in adults have demonstrated that weight loss in overweight persons with asthma improves lung function, reduces exacerbations, decreases the need for oral corticosteroids (bursts), and leads to enhancements in quality of life measures. Everybody with asthma must know their weight, their height, and understand what a healthy weight is for them. The CDC’s Web site points out that “Overweight and obesity are both labels for ranges of weight that are greater than what is generally considered healthy for a given height.
The terms also identify ranges of weight that have been shown to increase the likelihood of certain diseases and other health problems.” How can you tell whether you are overweight or obese? One way is to determine your body mass index (BMI) once you know your height and weight. An adult with a BMI of 25 to 29.9 falls into an overweight category and one whose BMI is equal to or greater than 30 is considered obese. Healthy BMI values range from 18.5 to 24.9, and persons with BMIs less than 18.5 are under-weight. BMI is a measure that also applies to children.
The mathematical formula for calculating BMI is your weight (in kilograms) divided by the square of your height (in centimeters). If you know your height in inches and your weight in pounds, simply divide your weight (in pounds) by the square of your height (in inches) and multiply that result by 703. If you prefer not to do the math, there are many Web sites that will do it for you! Simply go to your preferred search engine, and type a phrase such as “ideal body weight calculator,” or “BMI computation” to find a BMI calculator.
Enter the data it requests, such as your weight (in pounds or kilo-grams), height (in inches or centimeters), and age, and then sit back as it calculates your BMI. The definitions of overweight and obesity for children and teenagers are different from the adult definitions. Age-and sex-specific pediatric BMI charts are used to determine if a child is overweight or obese. You must obtain a table of predicted BMI for children to compare your child’s BMI to the standard, or you may use the CDC’s calculator at http://apps.nccd.cdc.gov/dnpabmi/Calculator.aspx.
A child with a BMI greater or equal to the 85th per-centile and less than the 95th percentile is overweight. One whose BMI is equal to or greater than the 95th per-centile for his or her age, height, and weight is considered obese. A child would be considered severely obese if his or her BMI was greater or equal to the 95th percentile. You can also get a general idea of how your weight com-pares to an ideal by estimating how far off from predicted values your weight is. The formula for calculating an adult’s ideal body weight requires knowledge of your height. Females are allowed 100 pounds for the first 5 feet of height, and 5 more pounds for each additional inch above 5 feet.
A few additional pounds are added or subtracted for persons with large or small frames. There-fore, a 53 woman should ideally weigh about 115 pounds, which corresponds to a BMI of between 20 and 21, well within a healthy range. The formula for men is similar; a man is allowed 106 pounds for the first 5 feet of height, and 6 additional pounds are added for every additional inch over 5 feet. A 6-foot tall man of average build would thus be expected to weigh 178 pounds, yielding a BMI of 25, within the desirable range. As for women, there is a correction for individuals with small and large frames.