Diet and Exercise Part 1

Many chronic diseases result from unhealthful eating and a sedentary lifestyle. Poor nutrition and inadequate exercise substantially increase the risk of such maladies as coronary artery disease, hypertension, stroke, diabetes, obesity, osteoporosis, and certain cancers and account for about 300,000 deaths in the United States each year.1 Dietary factors also contribute to cholelithiasis, hemorrhoids, hernias, constipation, irritable bowel syndrome, and diverticulosis. A rigorous program that combines a low-fat, high-fiber diet with daily exercise can produce dramatic improvement in cardiovascular risk factors in as little as 3 weeks’ time.2

Diet

In the 20th century, the average American diet shifted from one based on fresh, minimally processed vegetable foods to one based on animal products and highly refined, processed foods. As a result, Americans now consume far more calories, fat, cholesterol, refined sugar, animal protein, sodium, and alcohol and far less fiber and starch than is healthful.

In the United States, two out of every three adults are overweight (body mass index [BMI] of 25 to 30) or obese (BMI > 30), compared with fewer than one in four in the early 1960s.3 The consequences include a substantial decrease in life expectancy and an increase in morbidity similar in magnitude to the burden imposed by smoking.

Obesity is a complex, multifactorial disorder, but an element common to all cases is a positive energy balance in which more calories are consumed than expended. Excess calories are stored in body fat; each pound of adipose tissue contains 3,500 calories. Weight loss is accomplished only by achieving a negative energy balance.

Energy

Genetic, metabolic, and behavioral variables make it difficult to predict an individual’s caloric requirements with precision. However, physicians can provide estimates: sedentary adults require about 30 cal/kg/day to maintain body weight; moderately active adults require 35 cal/kg/day; and very active adults require 40 cal/kg/day. On average, therefore, a 70 kg (154 lb) person can expect to maintain body weight by consuming 2,100 to 2,800 calories daily.

Although any source of dietary energy, including carbohydrate, protein, and alcohol, can be converted in the body to fatty acids and cholesterol, the caloric value of foods varies considerably; for example, fat provides 9 cal/g and alcohol provides 7 cal/g, but protein and carbohydrates each provide only 4 cal/g. Patients with excess body fat should be encouraged to shift from high-fat, calorie-dense foods to low-fat, less-caloric foods. Even in a well-balanced diet, it is important to control food-portion size, which has increased dramatically5 in concert with the obesity epidemic in the United States. As an example, to lose 1 lb a week, patients must consume 500 fewer calories than they expend each day; in almost all cases, sustained weight loss requires both an energy-restricted diet and regular vigorous exercise.

Fat and cholesterol

Structure

Most dietary lipids are triglycerides, in which three fatty acids are joined to one glycerol molecule. At the core of every fatty acid is a chain of carbon atoms with a methyl group at one end and a carboxyl group at the other [see Figure 1]. The biologic properties of fatty acids are determined by the presence or absence of double bonds between carbon atoms, the number and location of the double bonds, and the configuration of the molecules.

Most of the fatty acids in foods are composed of an even number of carbon atoms, generally in chains of 12 to 22 atoms. The number of double bonds between carbon atoms determines the saturation of fats. Fatty acids with no double bonds are fully saturated; they have no room for additional hydrogen atoms. Fatty acids with one double bond are monounsaturated, and those with two or more double bonds are polyunsaturated.

Fatty acids contain zero to six double bonds, where additional hydrogen atoms can be attached. The location of the double bonds is of great physiologic importance; an unsaturated fatty acid’s group (i.e., omega-3, omega-6, or omega-9) is determined by the position of the double bond closest to the methyl group. In omega-3 fatty acids, for example, three carbon atoms lie between the methyl end of the chain and the first double bond.

Most of the fatty acids in natural foods are in the curved, or cis, configuration. When hydrogen is added back to unsaturat-ed fats during food manufacturing, however, the molecules assume a straightened, or trans, configuration [see Figure 1].

The structure of fat and cholesterol is shown. (a) Stearic acid (top) is a saturated fatty acid. Oleic acid (bottom) is a monounsaturated omega-9 fatty acid. (b) Oleic acid (top) displays a cis double bond. Elaidic acid (bottom) displays a trans double bond. (c) Cholesterol has a structure similar to that of fatty acids.

Figure 1 The structure of fat and cholesterol is shown. (a) Stearic acid (top) is a saturated fatty acid. Oleic acid (bottom) is a monounsaturated omega-9 fatty acid. (b) Oleic acid (top) displays a cis double bond. Elaidic acid (bottom) displays a trans double bond. (c) Cholesterol has a structure similar to that of fatty acids.

Cholesterol is a waxy, fatlike molecule that is present in the membranes of all animal cells but is absent from plant cells. Although cholesterol is a sterol rather than a true fat, its metabolism is intimately linked to the dietary intake of fatty acids.

Effects on Blood Lipids and Cardiovascular Risk

Although all fats have the same caloric value (9 cal/g), their effects on human health vary greatly, largely because of their disparate effects on blood cholesterol levels. Saturated fats stimulate hepatic cholesterol production, thus increasing blood cholesterol levels. Of the four saturated fatty acids that predominate in the American diet, myristic acid (14 carbons) has the most potent hypercholesterolemic effect, followed by palmitic acid (16 carbons) and lauric acid (12 carbons). Stearic acid (18 carbons) has little effect on blood cholesterol levels. Evidence strongly suggests that the degree to which saturated fat and cholesterol intake increase the risk of coronary artery disease depends on their effects on blood cholesterol concentration.6

Unsaturated fatty acids are generally derived from vegetable and marine sources; they are often called oils rather than fats because they are liquid at room temperature. When monounsatu-rated or polyunsaturated fatty acids are substituted for saturated fats, blood cholesterol levels fall. Neither type of unsaturated fat, however, has a direct ability to lower low-density lipopro-tein (LDL) cholesterol or raise high-density lipoprotein (HDL) cholesterol levels. Although monounsaturated and polyunsatu-rated fats have a similar, generally neutral, effect on blood cholesterol levels, monounsaturated fats are less susceptible to oxidation and may therefore be less atherogenic. Omega-3 polyun-saturated fatty acids in particular have been shown to have a cardioprotective effect.

Consumption of omega-3 fatty acids is inversely related to the incidence of atherosclerosis and the risk of sudden death7,8 and stroke.9 In high doses, omega-3 fatty acids may reduce blood triglyceride levels, but in dietary amounts, they have little effect on blood lipids. Even in modest amounts, however, omega-3 fatty acids reduce platelet aggregation, impairing thrombogenesis. They may also have antiarrhythmic10 and plaque-stabilizing properties.11 Diets high in a-linolenic acid appear to reduce the risk of coronary artery diseases12-14 and stroke.

Like saturated fats, trans-fatty acids increase blood LDL cholesterol levels; unlike saturated fats, trans-fatty acids reduce HDL cholesterol levels, making trans-fatty acids even more detrimental.15 Diets high in trans-fatty acids have been associated with an increased risk of atherosclerosis and coronary events.

Dietary cholesterol increases blood LDL cholesterol levels but has a less potent hypercholesterolemic effect than saturated fat. Diets high in cholesterol are associated with an increased risk of coronary artery disease independent of their effects on blood cholesterol levels,12 reinforcing the importance of reducing cholesterol intake.

Fat and Health

A high intake of saturated fat from animal sources appears to increase the risk of colon cancer16 and prostate cancer17 but not breast cancer.18 However, some dietary fat is essential. For example, omega-3 and omega-6 fatty acids cannot be synthesized endogenously and therefore must be obtained from food. Dietary fat is required for the absorption of fat-soluble vitamins. Lipids are essential components of cell membranes and steroid hormones; adipose tissue is the body’s major energy depot, and it provides insulation against heat loss. As little as 15 to 25 g of dietary fat a day can provide essential physiologic functions.

Table 1 Recommended Daily Intake of Fat and Other Nutrients*

Nutrient

Recommended Intake

Total fat

20%-35% of total calories

Saturated fat+

< 7% of total calories

Polyunsaturated fat

s 10% of total calories

Monounsaturated fat

s 20% of total calories

Cholesterol

< 300 mg/day 50%-60% of total calories

Carbohydrate++

a 25 g/day

Fiber

15% of total calories

Protein

Total calories§

Balance energy intake and expenditure to maintain desirable body weight and prevent weight gain

+Trans-fatty acids, which raise low-density lipoprotein (LDL) and lower high-density lipoprotein (HDL) cholesterol, should also be kept at low levels.

++Carbohydrates should be derived predominantly from foods rich in complex carbohydrates, including grains, especially whole grains, fruits, and vegetables. Simple sugars should contribute no more than 25% of total calories. §Daily energy expenditure should include at least moderate physical activity (consuming 200 kcal/day).

Dietary Recommendations

The American Heart Association (AHA) dietary guidelines19 for healthy adults suggest that no more than 30% of calories should come from fat, with less than 10% coming from saturated fat and the remainder coming from unsaturated fat in vegetables, fish, legumes, and nuts. The AHA guidelines also specify consumption of less than 300 mg of cholesterol a day. Patients with atherosclerosis or diabetes and persons who are hyperlipidemic or obese should follow more stringent limits, such as a saturated-fat intake of no more than 7% of daily calories, with a corresponding decrease in cholesterol consumption to less than 200 mg a day. In some persons, very low fat diets providing 15% to 22% of calories from fat can reduce blood HDL levels and produce other adverse effects,20,21 but in carefully monitored high-risk persons, diets with about 10% fat and virtually no cholesterol have been beneficial.22 Although reductions in total fat intake can help reduce body fat and serum cholesterol levels, the risk of coronary artery disease may depend more on the type of fat in the diet; saturated fats and trans-fatty acids are the most atherogenic, whereas monounsaturated and omega-3 fatty acids are the most desirable [see Table 1].7,12-15,23,24

Food labels list the fat, saturated fat, and cholesterol contents of packaged foods. They will soon be required to list trans-fatty acids; until then, patients should be advised to check the ingredients list at the bottom of the label for the presence of partially hydrogenated vegetable oils.

Carbohydrates

Carbohydrates are a vital source of energy for metabolic processes. They are also vital constituents of nucleic acids, gly-coproteins, and cell membranes.

Plants are the principal dietary sources of carbohydrates. The only important carbohydrates that originate from animal sources are the lactose in milk and the glycogen in muscle and liver.

Table 2 Types of Dietary Fiber and Representative Food Sources

Fiber Type

Food Sources

Gums*

Oats, beans, legumes, guar

Pectin*

Apples, citrus fruits, soybeans, cauliflower, squash, cabbage, carrots, green beans, potatoes

Mucilage*

Psyllium

Hemicellulose*+

Barley, wheat bran and whole grains, brussels sprouts, beet roots

Lignin+

Green beans, strawberries, peaches, pears, radishes

Cellulose+

Root vegetables, cabbage, wheat and corn, peas, beans, broccoli, peppers, apples

Carbohydrate-rich foods contain varying amounts of simple and complex carbohydrates. Simple carbohydrates include monosaccharides such as glucose, fructose, and galactose and disaccharides such as sucrose (table sugar), maltose, and lactose. Complex carbohydrates include polysaccharides (e.g., starch and glycogen that can be digested into sugars by intestinal enzymes) and fiber (i.e., high-molecular-weight carbohydrates that cannot be split into sugars by human intestinal enzymes). Sugars, starches, and glycogen provide 4 cal/g; because fiber is indigestible, it has no caloric value.

Carbohydrates contribute about 50% of the calories in the average American diet—half from sugar and half from complex carbohydrates. Because sugars are more rapidly absorbed, they have a higher glycemic index than starches. In addition to provoking higher insulin levels, carbohydrates with a high glyce-mic index appear to reduce HDL cholesterol levels and may increase the risk of coronary artery disease.25 Processed foods containing simple sugars are often calorie dense, whereas foods that are rich in complex carbohydrates provide vitamins, trace minerals, and other valuable nutrients. A healthful diet should provide 55% to 65% of calories from complex carbohydrates found in fresh fruits and vegetables, legumes, and whole grains.19,23

Dietary fiber

Dietary fiber is a heterogeneous mix of very long chain branched carbohydrates that resist digestion by human intestinal enzymes because of the ways their monosaccharide components are linked to one another. Fiber is found only in plants, particularly in the bran of whole grains, in the stems and leaves of vegetables, and in fruits, seeds, and nuts. The two general categories of dietary fiber are soluble and insoluble.

Soluble fiber delays gastric emptying, which produces a sensation of satiety, and slows the absorption of digestible carbohydrates, which reduces insulin levels. Soluble fiber also lowers blood cholesterol levels, probably by inhibiting bile acid and nutrient absorption in the small intestine and by promoting bile acid sequestration by colonic bacteria.26 Because soluble fiber is metabolized by these bacteria, it has little effect on fecal bulk. In contrast, insoluble fiber increases the water content and bulk of feces and shortens intestinal transit time [see Table 2].

Diets that are high in fiber also tend to be low in fat. Such diets have been associated with a reduced risk of intestinal disorders, including constipation, irritable bowel syndrome, cholelithiasis, hemorrhoids, and diverticulosis. Although earlier data are mixed, new studies suggest that a high intake of fiber can substantially reduce the risk of colorectal cancer.27,28 In addition, a dietary pattern that includes a high intake of fruits, vegetables, legumes, fish, poultry, and whole grains but little red meat, processed meats, sweets, and refined grains appears pro-tective.29,30 A high intake of fiber is associated with a reduced risk of diabetes and, in patients with diabetes,31 improved glycemic control and decreased blood lipids32; it is also associated with a reduced risk of obesity33 and coronary artery dis-ease34,35 and a lower all-cause mortality. A healthful diet should contain at least 25 to 30 g of fiber a day, including substantial amounts of soluble fiber.

Proteins

Unlike reserves of fat (which is stored in large amounts as triglyceride in adipose tissue) and reserves of carbohydrate (which is stored in small amounts as glycogen in liver and muscle), there are no endogenous reserves of amino acids or protein; all the proteins in the body are serving a structural or metabolic function. As a result, bodily function can be impaired if proteins are catabolized because of energy deficiency, wasting diseases, or dietary protein intake that is not sufficient to replace protein losses.

All proteins in human cells are continuously catabolized and resynthesized. In a healthy 70 kg adult, about 280 g of protein is degraded and replaced daily. In addition, about 30 g of protein is lost externally through the urine (urea), feces, and skin.

In healthy adults, daily protein losses can be fully replaced by as little as 0.4 g/kg. Because not all dietary proteins are fully digestible, the recommended dietary allowance (RDA) of protein for healthy adults is 0.8 g/kg. People who exercise strenuously on a regular basis may benefit from extra protein to maintain muscle mass; a daily intake of about 1 g/kg has been recommended for athletes. Women who are pregnant or lactating require up to 30 g/day in addition to their basal requirements. To support growth, children should consume 2 g/kg/day.

A healthful diet should provide 10% to 15% of its calories from protein.19 For healthy, nonpregnant women, an intake of 44 to 50 g/day of protein is required, and for men, an intake of 45 to 63 g/day of protein is needed. Although excessive protein intake has not been proved to be harmful, there are several potential disadvantages to a very high protein intake. The protein in foods derived from animals is often accompanied by large amounts of fat. In the body, excessive protein can be transami-nated to carbohydrate, adding to the energy surplus responsible for obesity. When excess protein is eliminated from the body as urinary nitrogen, it is often accompanied by increased urinary calcium, perhaps increasing the risk of nephrolithiasis and osteoporosis. Because nitrogen is excreted in the urine, an increased protein intake is associated with an increase in renal plasma flow and glomerular filtration rates and, eventually, with increased renal size. In some animal models, increased dietary protein is associated with accelerated renal aging; and in humans with kidney disease, high dietary protein intake is associated with more rapid disease progression.36 On the other hand, high dietary protein intake appears linked to somewhat reduced blood pressure readings,37 possibly because of increased urinary sodium losses, and protein supplements may be beneficial for patients with acute or chronic illnesses.38

The thousands of proteins in the human body are synthesized from just 21 amino acids. Most amino acids can be synthesized endogenously, but nine cannot.

Table 3 The Vitamins

Vitamin

Functions

Deficiency Effects

Toxic Effects

Sources

RDA for Adults

A (retinol, retinoic acid)

Vision, epithelial integrity; possible protection against epithelial cancers and atherosclerosis

Night blindness; increased susceptibility to infection

Teratogenicity, hepatotox-icity, cerebral edema, desquamation; yellowish skin discoloration by carotenoids

Liver, dairy products, eggs; dark-green and yellow-orange vegetables (carotenoids)

Men, 5,000 IU or 1,000 RE; women, 4,000 IU or 800 RE

B1 (thiamine)

Metabolism of carbohydrates, alcohol, and branched-chain amino acids

Beriberi, Wernicke-Korsakoff syndrome

None

Grains, legumes, nuts, poultry, meat

Men 19-50 yr: 1.5 mg; men > 50 yr: 1.2 mg; women 19-50 yr: 1.1 mg; women > 50 yr: 1.0 mg

B 2 (riboflavin)

Cellular oxidation-reduction reactions

Stomatitis, dermatitis, anemia

None

Grains, dairy products, meat, eggs, dark-green vegetables

Men 19-50 yr: 1.7 mg; men > 50 yr: 1.4 mg; women 19-50 yr: 1.3 mg; women > 50 yr: 1.2 mg

B3 (niacin, nico-tinic acid)

Oxidative metabolism; reduces LDL cholesterol; increases HDL cholesterol

Pellagra

Flushing, headaches, pruritus, hyperglyce-mia, hyperuricemia, hepatotoxicity

Meat, poultry, fish, grains, peanuts; synthesized from tryptophan in foods

Men 19-50 yr: 19 mg; men > 50 yr: 15 mg; women 19-50 yr: 15 mg; women > 50 yr: 13 mg

B 6 (pyridoxine)

Amino acid metabolism and heme synthesis; neuronal excitability; reduces blood homocysteine levels

Anemia, cheilosis, dermatitis

Neurotoxicity

Meat, poultry, fish, grains, soybeans, bananas, nuts

2 mg

B12 (cobalamin)

DNA synthesis (with folate); myelin synthesis (without folate); reduces blood homocysteine levels

Megaloblastic anemia, neuropathies

None

Meat (especially liver), poultry, fish, dairy products

2-4 lg

Folic acid

DNA synthesis (with B12); reduces blood homocys-teine levels

Megaloblastic anemia, birth defects

None

Vegetables, legumes, grains, fruit, poultry, meat

400 l g

Biotin

Metabolic processes

Rare

None

Many foods

30-100 l g

Pantothenic acid

Metabolic processes

Rare

None

Many foods

4-7 mg

C (ascorbic acid)

Collagen synthesis; possible protection against certain neoplasms

Scurvy

Nephrolithiasis, diarrhea

Fruits, green vegetables, potatoes, cereals

Men, 90 mg; women, 75 mg

D (calciferol)

Intestinal calcium absorption

Osteomalacia and rickets

Hypercalcemia

Fortified dairy products, fatty fish, egg yolks, liver

< 50 yr, 200 IU; 50-70 yr, 400 IU; > 70 yr, 600 IU

E (a-tocopherol)

Reduces peroxidation of fatty acids: possible protection against atherosclerosis

Rare

Antagonism of vitamin K, possible headaches

Vegetable oils, wheat germ, nuts, broccoli

15 mg

K

Synthesis of clotting factors VII, IX, X, and possibly V

Hemorrhagic diathesis

None

Leafy green vegetables (K1 ), intestinal bacteria (K2)

Men, 120 l g; women, 90 lg

HDL—high-density lipoprotein

IU—international units

LDL—low-density lipoprotein

RDA—recommended dietary allowance

RE—retinol equivalents

Not all dietary proteins contain all nine essential amino acids; in particular, vegetable proteins may be incomplete. However, by eating a varied diet with foods that contain a mix of proteins, even strict vegetarians can obtain all the amino acids they need.

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