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Physiology
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Lecture 5
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CAUSES OF OBESITY
The most common causes of obesity are overeating and underactivity. There is evidence for differences in metabolism making some individuals more efficient in hanging onto calories, but the most common reasons for differences in the tendency to gain or lose weight relate to age, height, sex and weight. Therefore, obesity results from common genetic predisposition, but is expressed on only when there is excess energy available for storage.

Energy In = Energy Out + Energy Stored

Endocrine Causes of Obesity
There are endocrine genetic forms of obesity but these are rare. The most common types are the HHHO syndromes: i.e. hypogonadism, hypomentia, hypotonia and obesity. Two forms are well-known: Prader-Willi Syndrome and Lawrence-Moon-Bardet-Biedel Syndrome. The latter also includes retinitis pigmentosa.

The most common iatrogenic form of obesity (other than insulin Rx of diabetes) occurs in patients given glucocorticoids for arthritis and other inflammatory conditions. Appetite stimulation and central body fat accumulation results as is seen in Cushing's Syndrome, which is a rare cause of obesity. Certain antidepressants and all oral hypoglycemic agents also stimulate appetite and commonly promote obesity. Progestational steroids including megestrol acetate used in the treatment of breast cancer can stimulate appetite and cause weight gain.

Injury or malformation of the satiety center in the hypothalamus can also lead to hyperphagia and obesity. This is rare and is diagnosed anatomically or by association with other hypothalamic defects.

Basal Energy Expenditure
When prescribing diets for obese patients, it is useful to be able to calculate expected rates of weight loss. The total energy expended per day is made up of several components listed below:

For obese patients who are relatively inactive, the basal metabolic rate is a good estimate of overall energy expenditure. This estimate can be used clinically. While it is an underestimate of total weight loss, since cheating occurs on most diets, it will give a ballpark estimate of expected weight loss.

The most useful estimate of BMR is the BEE or basal energy expenditure developed by Harris and Benedict.

Where, W= weight in kg. , H = height in cm. A = age in years

To lose one pound of weight ( 1/4 lb. of muscle and 3/4 lb. of fat)in a week’s time, an obese patient must take in 500 kcal less per day than maintenance calories estimated on the basis of the BEE. This estimate is useful in clinical practice.

Clinical Cases

Case # 1
A 66 year old caucasian female with a history of hypercholesterolemia. She weighs 153.5 lb. and is 64 inches tall. Past medical history is unremarkable except for paroxysmal atrial tachycardia, mitral valve prolapse, and prior appendectomy and hysterectomy. She is currently taking postmenopausal hormone replacement (Premarin 0.625 mg per day). Her current weight is her maximum weight, having married into an obese family several years ago. Prior to that time her weight was between 116 and 120 lb.

Physical Examination reveals

Exercise
1. Classify the patient's obesity on the basis of body weight and body mass index:

a) to calculate BMI convert inches to meters and lb. to kg. first
b) use the rule of thumb to calculate % ideal body weight

2. The patient had a normal blood pressure of 136/80. Are there any other diseases which may be secondary to obesity, or caused by obesity which might be ameliorated or prevented ?

3. What fat distribution does the patient have based on skinfold fat thickness ?

4. If the ideal percent body fat is about 24% for this patient, what would her target weight be assuming her lean body mass did not change ?

5. If this patient is prescribed a 900 kcal/day diet, how much weight will she lose each week on the diet assuming she sticks with it faithfully?

6. She actually lost seven pounds in the first two weeks on the diet. How do you explain the weight loss above the predicted amount ?

Case # 2
A 37 y.o. graphic designer who has participated in numerous weight loss programs in the past. He owns a treadmill which he keeps in the closet at home. He seeks consultation at this time, because he has developed hypertension and his doctor has told him to lose weight.

On physical examination, he weighs 259.5 lb. at a height of 72 inches. His blood pressure is 146/90.

His calculated BEE is 2380 kcal/day

Exercise
1. What is his BMI ? How would you classify his obesity ?

2. What is likely to improve in him with treatment of his obesity ?

3. Calculate his lean and fat mass based on body weight and percent fat.

4. What is his fat distribution based on skinfolds ?

5. What is his target weight assuming he should be about 18 % fat ?

6. What is his target weight assuming you want him to reach a BMI of 25 ?

7. What is his target weight assuming you want him to be 100 % of his ideal body weight based on the rule of thumb?

8. How do you account for discrepancies in the above estimates?

9. Based on his calculated BEE, how much weight would this man lose on a 1000 kcal/day diet ?

10. If it safe for this man to lose 2 to 3 lb. per week, how many calories would you prescribe for him?

Case # 3 IS YOU !

Exercise
1. Calculate your BEE.

2. Do you need to lose weight based on ideal body weight or BMI criteria ?

3. If you weighed 40 lb. more, what would your BEE be ?

4. At a higher weight, how fast would you lose weight on a 900 kcal/day diet expressed as pounds lost per week?

5. What could you do to increase the rate of weight loss if it is less than 1 lb. per week ?

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