One in four Americans has a BMI of 30 or greater and is therefore considered to be obese. This crisis in obesity is the result you would expect when constant access to cheap, high-fat, energy-dense, unhealthful food is combined with lack of exercise. Nowhere is this relationship more clearly illustrated than it is in the case of the Pima Indians.

Several hundred years ago, the ancestral population of Pima Indians split into two tribes. One branch moved to Arizona and adopted the American diet and lifestyle; the typical Pima of Arizona gets as much exercise as the average American and also eats a high-fat, low-fiber diet, similar to most Americans. Unfortunately, the health consequences for them are more severe than they are for most Americans—close to 60% of the Arizona Pima are obese and diabetic. In contrast, the Pima of New Mexico maintained their ancestral farming life and their diet is rich in fruits, vegetables, and fiber. The Pima of New Mexico also engage in physical labor close to 22 hours per week and are 60 pounds lighter than their Arizona cousins. Consequently, diabetes is virtually unheard of.

This example illustrates the impact of lifestyle over genetics since the Pima of Arizona share many genes with their New Mexican cousins but have far less healthful lives due to their diet and lack of exercise. The example of the Pima Indians also shows that genes influence body weight, because the Pima of Arizona have higher rates of obesity and diabetes than other Americans whose lifestyle they share. (Genetics is discussed in Chapter 4.)

Whether obesity is the result of genetics, diet, or lack of exercise, the health risks associated with obesity are the same. As your weight increases, so do your risks of diabetes, hypertension, heart disease, stroke, and joint problems. (Excess weight puts extra pressure on the knees, hips, and back that wears away the cartilage protecting them, resulting in joint pain and stiffness.)

Diabetes Diabetes is a disorder of carbohydrate metabolism characterized by the impaired ability of the body to produce or respond to insulin. Insulin is a hormone secreted by beta cells, which are located within clusters of cells in the pancreas. Insulin's role in the body is to trigger cells to take up glucose so that they can convert the energy-yielding sugar. People with diabetes are unable to metabolize glucose, and as a result, the level of glucose in the blood rises (Figure 2.17).

There are two forms of the disease. Type I, insulin-dependent diabetes mellitus (IDDM, formerly referred to as juvenile-onset diabetes), usually arises in childhood. People with IDDM cannot produce insulin because their immune systems mistakenly destroy their own beta cells. When the body is no longer able to produce insulin, daily injections of the hormone are required. Type I diabetes is not correlated with obesity.

Type II, non-insulin-dependent diabetes mellitus (NIDDM, sometimes called adult-onset diabetes), usually occurs after 40 years of age and is more common in the obese. NIDDM arises from either decreased pancreatic secretion of insulin or reduced responsiveness to secreted insulin in target cells. People with NIDDM are able to control blood-glucose levels through diet and exercise and, if necessary, by insulin injections.

Hypertension Hypertension, or high blood pressure, places increased stress on the circulatory system and causes the heart to work too hard. A hypertensive person is six times more likely to have a heart attack than a person with normal blood pressure.

Blood pressure is the force, originated by the pumping action of the heart, exerted by the blood against the walls of the blood vessels. Blood vessels expand and contract in response to this force. Blood pressure is reported as two numbers: The higher number, the systolic blood pressure, represents the pressure exerted by the blood against the walls of the blood vessels; the lower number is

1. A non-diabetic 2. The pancreas person has high secretes insulin into blood sugar following the bloodstream. a meal.

3. Insulin triggers cells to take up glucose.

4. Excess glucose is stored in liver as glycogen.

1. A non-diabetic 2. The pancreas person has high secretes insulin into blood sugar following the bloodstream. a meal.

3. Insulin triggers cells to take up glucose.

4. Excess glucose is stored in liver as glycogen.

Figure 2.17 Diabetes. The cells of the pancreas secrete insulin, which helps glucose move into body cells. Since diabetics produce less insulin, sugar stays in the blood longer.

the diastolic blood pressure, and is the pressure that exists between contractions of the heart when the heart is relaxing. Normal blood pressure is around 120/80. Blood pressure is considered to be high when it is persistently above 140/90.

As you put on weight, you gain mostly fatty tissue. Just like tissues in other parts of the body, fat tissue relies on oxygen and nutrients in your blood to produce energy. Excess fat tissue increases the demand for oxygen and nutrients, thus increasing the amount of blood required to nourish your body—more blood traveling through the arteries means added pressure on the artery walls. In addition, excess weight often is associated with an increase in heart rate and a reduction in the capacity of your blood vessels to transport blood—these two factors also increase blood pressure.

Heart Attack and Stroke Heart attack and stroke are more likely in obese people because the elevated blood pressure caused by obesity also damages the lining of blood vessels, increasing the likelihood that cholesterol will be deposited there. Cholesterol-lined vessels are said to be atherosclerotic.

Because fats like cholesterol are not soluble in aqueous (water-based) solutions, cholesterol is carried throughout the body attached to proteins in structures called lipoproteins. Low-density lipoproteins (LDL), have a high proportion of cholesterol (they are low in protein). LDLs distribute both cholesterol synthesized by the liver and cholesterol derived from the diet throughout the body. LDLs are also important for carrying cholesterol to cells, where it is used to help make plasma membranes and hormones. High-density lipoproteins (HDL) contain more protein than cholesterol. HDLs scavenge excess cholesterol from the body and return it to the liver, where it is used to make bile. The cholesterol-rich bile is then released into the small intestine and from there much of it exits the body in the feces. The LDL/HDL ratio is an index of the rate at which cholesterol is leaving body cells and returning to the liver.

Your physician can measure your cholesterol level by sampling the amounts of LDL and HDL in your blood. If your total cholesterol level is over 200 or your LDL level is above 100, your physician may recommend that you decrease the amount of cholesterol and saturated fat in your diet. This may

Figure 2.18 Biosynthesis of testosterone and estrogen from cholesterol. The synthesis of both testosterone and estrogen require the use of cholesterol as a starting material.

mean eating more plant-based foods and less meat, since plants do not have cholesterol, and also reducing the amount of saturated fats in your diet. Saturated fat is thought to raise cholesterol levels by stimulating the liver to step up its production of LDLs, and slowing the rate at which LDLs are cleared from the blood.

Before you decide to completely eliminate cholesterol from your diet, however, keep in mind that some cholesterol is necessary—it is present in cell membranes to help maintain their fluidity, and it is the building block for steroid hormones such as estrogen and testosterone (Figure 2.18).

For some people, those with a genetic predisposition to high cholesterol, controlling cholesterol levels through diet is difficult because dietary cholesterol makes up only a fraction of the body's total cholesterol. People with high cholesterol who do not respond to dietary changes may have inherited genes that increase the liver's production of cholesterol. These people may require prescription medications to control their cholesterol levels.

Cholesterol-laden, atherosclerotic vessels increase your risk of heart disease and stroke. Fat deposits narrow your heart's arteries, so less blood can flow to your heart. Diminished blood flow to your heart can cause chest pain, or angina. A complete blockage can lead to a heart attack. Lack of blood flow to the heart during a heart attack can cause the oxygen-starved heart tissue to die, leading to irreversible heart damage.

The same buildup of fatty deposits also occurs in the arteries of the brain. If a blood clot forms in a narrowed artery in the brain, it can completely block blood flow to an area of the brain, resulting in a stroke. If oxygen-starved brain tissue dies, permanent brain damage can result.

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