AIs: The Nutritional Numbers Formerly Known as ESADDIs

In addition to the RDAs, the Food and Nutrition Board created Estimated Safe and Adequate Daily Dietary Intakes (ESADDI), now renamed Adequate Intake (AI), for eight nutrients considered necessary for good health, even though nobody really knows exactly how much your body needs. Not to worry:

Sooner or later some smart nutrition researcher will come up with a hard number and move the nutrient to the RDA list. Or not. In the meantime, new reports have established AIs for various age groups for the following nutrients:

• Pantothenic acid
• Molybdenum
• Biotin
• Manganese
• Choline
• Fluoride
• Calcium
• Chromium

Different people has different needs

Because different bodies require different amounts of nutrients, RDAs currently address as many as 22 specific categories of human beings: boys and girls, men and women, from infancy through middle age. The RDAs recently were expanded to include recommendations for groups of people ages 50 to 70 and 70 and older. Eventually, recommendations will be made for people older than 85. These expanded groupings are a really good idea. In 1990, the U.S. Census counted 31.1 million Americans who are older than 65. By 2050, the U.S. Government expects more than 60 million to be alive and kickin’.

You wouldn’t want these baby boomers to miss their RDAs, now would you? But who you are affects the recommendations. If age is important, so is gender. For example, because women of childbearing age lose iron when they menstruate, their RDA for iron is higher than the RDA for men. On the other hand, because men who are sexually active lose zinc through their ejaculations, the zinc RDA for men is higher than the zinc RDA for women. Finally, gender affects body composition, which influences RDAs.

Consider protein: The RDA for protein is set in terms of grams of protein per kilogram (2.2 pounds) of body weight. Because the average man weighs more than the average woman, his RDA for protein is higher than hers. The RDA for an adult male, age 19 or older, is 56 grams; for a woman, it’s 46 grams.

Recommendations for carbohydrates, fats, dietary fiber, and alcohol

What nutrients are missing from the RDA list of essentials? Carbohydrates, fiber, fat, and alcohol. The reason is simple: If your diet provides enough protein, vitamins, and minerals, it’s almost certain to provide enough carbohydrates and probably more than enough fat. Although no specific RDAs exist for carbohydrates and fat, guidelines definitely exist for them and for dietary fiber and alcohol.

In 1980, the U.S. Public Health Service and the U.S. Department of Agriculture joined forces to produce the first edition of Dietary Guidelines for Americans. This report has been modified many times. The latest set of recommendations, issued in the spring of 2005, sets parameters for what you can consider reasonable amounts of calories, carbohydrates, dietary fiber, fats, protein, and alcohol. According to these guidelines, as a general rule, you need to

• Balance your calorie intake with energy output in the form of regular exercise.
• Eat enough carbohydrates (primarily the complex ones from fruits, vegetables, and whole grains) to account for 45 to 65 percent of your total daily calories. That’s 900 to 1,300 calories on a 2,000-calorie diet.
• Take in an appropriate amount of dietary fiber, currently described as 14 grams dietary fiber for every 1,000 calories.
• Get no more than 20 to 35 percent of your daily calories from dietary fat. Therefore, if your daily diet includes about 2,000 calories, only 400 to 700 calories should come from fat. Less than 10 percent of your daily calories should come from saturated fatty acids, and your daily diet should have less than 300 mg cholesterol. Eat as little trans fat as possible. The Nutrition Facts label on foods now shows a gram amount for trans fats, but there’s no upper limit because any amount is considered, well, less than okey-dokey.
• If you choose to drink alcoholic beverages, do so in moderation, meaning one drink a day for a woman and two for a man.

RDAs: Guidelines for Good Nutrition

The Recommended Dietary Allowances (RDAs) were created in 1941 by the Food and Nutrition Board, a subsidiary of the National Research Council, which is part of the National Academy of Sciences in Washington, D.C. RDAs originally were designed to make planning several days’ meals in advance easy for you. The D in RDA stands for dietary, not daily, because the RDAs are an average. You may get more of a nutrient one day and less the next, but the idea is to hit an average over several days. For example, the current RDA for vitamin C is 75 mg for a woman and 90 mg for a man (age 18 and older).

One 8-ounce glass of fresh orange juice has 120 mg vitamin C, so a woman can have an 8-ounce glass of orange juice on Monday and Tuesday, skip Wednesday, and still meet the RDA for the three days. A man may have to toss in something else — maybe a stalk of broccoli — to be able to do the same thing. No big deal. The amounts recommended by the RDAs provide a margin of safety for healthy people, but they’re not therapeutic. In other words, RDA servings won’t cure a nutrient deficiency, but they can prevent one from occurring.

How many calories do you really need?

Figuring out ex-act-ly how many calories to consume each day can be a, well, consuming task. Luckily, the Dietary Guidelines for Americans 2005 lays out a list of the average daily calorie allowance for healthy adults with a healthful BMI — 21.5 for women and 22.5 for men — based on the amount of activity a person performs each day. Note that in this context, sedentary means a lifestyle with only the light physical activity associated with daily living; moderately active means a lifestyle that adds physical activity equal to a daily 1.5–3 mile walks at a speed of 3–4 miles per hour; active means adding physical activity equal to walking 3 miles a day at the 3–4 mph clip.

Facing the numbers when they don’t fit your body

Right about here, you probably feel the strong need for a really big chocolate bar (not such a bad idea now that nutritionists have discovered that dark chocolate is rich in disease-fighting antioxidants). But it also makes sense to consider the alternative: realistic rules that enable you to control your weight safely and effectively. Check out the following:

• Rule No. 1: Not everybody starts out with the same set of genes — or fits into the same pair of jeans. Some people are naturally larger and heavier than others. If that’s you, and all your vital stats satisfy your doctor, don’t waste time trying to fit someone else’s idea of perfection. Relax and enjoy your own body.
• Rule No. 2: If you’re overweight and your doctor agrees with your decision to diet, you don’t have to set world records to improve your health. Even a moderate drop in poundage can be highly beneficial. According to The New England Journal of Medicine (www.nejm.org on the Net), losing just 10 to 15 percent of your body weight can lower high blood sugar, high cholesterol, and high blood pressure, reducing your risks of diabetes, heart disease, and stroke.
• Rule No. 3: The only number you really need to remember is 3,500, the number of calories it takes to gain or lose one pound of body fat. In other words, one pound of body fat equals 3,500 calories. So if you simply
• • Cut your calorie consumption from 2,000 calories a day to 1,700 and continue to do the same amount of physical work, you’ll lose one pound of fat in just 12 days.
• • Go the other way, increasing from 1,700 to 2,000 calories a day without increasing the amount of work you do, 12 days later you’ll be one pound heavier.
• Rule No. 4: Moderation is the best path to weight control. Moderate calorie deprivation on a sensible diet produces healthful, moderate weight loss; this diet includes a wide variety of different foods containing sufficient amounts of essential nutrients. Abusing this rule and cutting calories to the bone can turn you literally into skin and bones, depriving you of the nutrients you need to live a normal healthy life.
  
• Rule No. 5: Be more active. Doing exercise allows you to take in more calories and still lose weight. In addition, exercise reduces the risk of many health problems, such as heart disease. Sounds like a recipe for success.

How reliable are the numbers? Considering confounding variables

Weight charts and tables and numbers and stats are so plentiful that you may think they’re totally reliable in predicting who’s healthy and who’s not. So here’s a surprise: They aren’t.

The problem is that real people and their differences keep sneaking into the equation. For example, the value of the Body Mass Index in predicting your risk of illness or death appears to be tied to your age. If you’re in your 30s, a lower BMI is clearly linked to better health.

If you’re in your 70s or older, no convincing evidence points to how much you weigh playing a significant role in determining how healthy you are or how much longer you’ll live. In between, from age 30 to age 74, the relationship between your BMI and your health is, well, in-between — more important early on, less important later in life.

In other words, the simple evidence of your own eyes is true. Although Americans sometimes seem totally obsessed with the need to lose weight, the fact is that many larger people, even people who are clearly obese, do live long, happy, and healthy lives. To figure out why, many nutrition scientists now are focusing not only on weight or weight/height (the BMI) but on the importance of confounding variables, which is sciencespeak for “something else is going on here.”

Here are three potential confounding variables in the obesity/health equation:

• Maybe people who are overweight are more prone to illness because they exercise less, in which case stepping up the workouts may reduce the perceived risk of being overweight.
• People who are overweight may be more likely to be sick because they eat lots of foods containing high-calorie ingredients, such as saturated fat, that can trigger adverse health effects; in this case, the remedy may simply be a change in diet.
• Maybe people who are overweight have a genetic predisposition to a serious disease. If that’s true, you’d have to ask whether losing 20 pounds really reduces their risk of disease to the level of a person who is naturally 20 pounds lighter. Perhaps not: In a few studies, people who successfully lost weight actually had a higher rate of death.

Adding to the confusion is the fact that an obsessive attempt to lose weight may itself be hazardous to your health. Every year, Americans spend $30 billion to $50 billion (yes, you read that right) on diet clubs, special foods, and over-the-counter remedies aimed at weight loss. Often the diets, the pills, and the foods don’t work, which can leave dieters feeling worse than they did before they started.

The chance that the diet fails is only half the bad news. Here’s the rest: Some foods that effectively lower calorie intake and some drugs that effectively reduce appetite have potentially serious side effects. For example, some fat substitutes prevent your body from absorbing important nutrients, and some prescription diet drugs, such as the combination once known as Phen-Fen, are linked to serious, even fatal, diseases.

What do they mean when they say that you’re fat?

Obesity is a specific medical condition in which the body accumulates an overabundance of fatty tissue. One way American nutritionists determine who’s obese is by comparing a person’s weight with the figures on the weight/height charts:

• If your weight is 20 to 40 percent higher than the chart recommends, you’re mildly obese.
• If your weight is 40 to 99 percent higher, you’re moderately obese.
• If your weight is more than double the weight on the chart, you’re severely obese.

Fittest and fattest U.S. cities

June is bustin’ out all over. How about you? For several years, the guys at Men’s Fitness magazine have rated the top 25 fattest and fittest cities in the United States. Of course, Men’s Fitness is published in Southern California, where long legs, slim hips, tight abs, and a taste for sprouts are handed out at birth. As a result, the magazine’s pundits may not know that in other places like, oh, New York, Chicago, Milwaukee, youpick-it, Americans come in all shapes and sizes. And they may have missed the fact that running for a bus or climbing subway stairs constitutes a daily workout in metro areas. Or that compared to high-fat, goat-cheese pizza, a West Coast fave, city bagels are health food.

Nonetheless, the magazine’s lists are a warning for the weighty. I’ll tell you right off the bat that in 2005, the 25 fittest cities (starting with the best) were Seattle, Honolulu, Colorado Springs, San Francisco, Denver, Portland (Oregon), Tucson, San Diego, Albuquerque, Boston, Virginia Beach (Virginia), Minneapolis, Fresno, Milwaukee, Omaha, San Jose (California), Jacksonville, Austin, Oakland, Los Angeles, Arlington (Texas), Washington, D.C., Cleveland, and Nashville-Davidson. Yay!

The 25 fattest (starting with the worst) were Houston, Philadelphia, Detroit, Memphis, Chicago, Dallas, New Orleans, New York, Las Vegas, San Antonio, El Paso, Phoenix, Indianapolis, Fort Worth, Mesa (Arizona), Columbus (Ohio), Wichita (Kansas), Miami (Florida), Long Beach (California), Oklahoma City, Tulsa, Atlanta, Charlotte (North Carolina), and Baltimore. Shape up, guys. Men’s Fitness is watching you!

Energy for work

Your second largest chunk of energy is the energy you withdraw to spend on physical work. That’s everything from brushing your teeth in the morning to hoeing a row of petunias in the garden or working out in the gym. Your total energy requirement (the number of calories you need each day) is your REE plus enough calories to cover the amount of work you do.

Does thinking about this use up energy? Yes, but not as much as you’d like to imagine. To solve a crossword puzzle — or write a post of this blog — the average brain uses about 1 calorie every four minutes. That’s only one-third the amount needed to keep a 60-watt bulb burning for the same length of time.

Are you a sensible foodie? If you’re supposed to have no more than 2,000 calories a day, can you pack all the vitamins, minerals, protein, heart-healthy fats, and carbs you need into 1,800 calories? Do that, and the folks who wrote the Dietary Guidelines for Americans 2005 say, reward yourself.

Use the “leftover” 200 calories — called discretionary calories — for anything that makes your mouth water. Naturally, some expert spoilsports disagree. They say that giving you an inch (those leftover calories) means you’ll take a mile (three pieces of chocolate cake). Prove them wrong and celebrate your smarts. Yum!

How your hormones affect your energy needs?

If you’re a woman, you know that your appetite rises and falls in tune with your menstrual cycle. In fact, this fluctuation parallels what’s happening to your REE, which goes up just before or at the time of ovulation. Your appetite is highest when menstrual bleeding starts and then falls sharply. Yes, you really are hungrier (and need more energy) just before you get your period.

Being a man (and making lots of testosterone) makes satisfying your nutritional needs on a normal American diet easier. Your male bones are naturally denser, so you’re less dependent on dietary or supplemental calcium to prevent osteoporosis (severe loss of bone tissue) late in life. You don’t lose blood through menstruation, so you need only two-thirds as much iron. Best of all, you can consume about 10 percent more calories than a woman of the same weight without adding pounds.

Teenage boys’ developing wide shoulders and biceps while teenage girls get hips is no accident. Testosterone, the male hormone, promotes the growth of muscle and bone. Estrogen gives you fatty tissue. As a result, the average male body has proportionally more muscle; the average female body, proportionally more fat.

Muscle is active tissue. It expands and contracts. It works. And when a muscle works, it uses more energy than fat (which insulates the body and provides a source of stored energy but does not move an inch on its own). What this muscle versus fat battle means is that the average man’s REE is about 10 percent higher than the average woman’s. In practical terms, that means a 140-pound man can hold his weight steady while eating about 10 percent more than a 140-pound woman who is the same age and performs the same amount of physical work.

No amount of dieting changes this unfair situation. A woman who exercises strenuously may reduce her body fat so dramatically that she no longer menstruates — an occupational hazard for some professional athletes. But she’ll still have proportionately more body fat than an adult man of the same weight. If she eats what he does, and they perform the same amount of physical work, she still requires fewer calories than he to hold her weight steady. And here’s a really rotten possibility. Muscle weighs more than fat.

This interesting fact is one that many people who take up exercise to lose weight discover by accident. One month into the barbells and step-up-step-down routine, their clothes fit better, but the scale points slightly higher because they’ve traded fat for muscle — and you know what that means: Sometimes you can’t win for losing. (Sorry, but I just couldn’t resist.)

Sex, glands, and chocolate cake

A gland is an organ that secretes hormones, which are chemical substances that can change the function — and sometimes the structure — of other body parts. For example, your pancreas secretes insulin, a hormone that enables you to digest and metabolize carbohydrates. At puberty, your sex glands secrete either the female hormones estrogen and progesterone or the male hormone testosterone; these hormones trigger the development of secondary sex characteristics, such as the body and facial hair that make us look like either men or women.

Hormones can also affect your REE, how much energy you use when your body’s at rest. Your pituitary gland, a small structure in the center of your brain, stimulates your thyroid gland (which sits at the front of your throat) to secrete hormones that influence the rate at which your tissues burn nutrients to produce energy.

When your thyroid gland doesn’t secrete enough hormones (a condition known as hypothyroidism), you burn food more slowly and your REE drops. When your thyroid secretes excess amounts of hormones (a condition known as hyperthyroidism), you burn food faster and your REE is higher. When you’re frightened or excited, your adrenal glands (two small glands, one on top of each kidney) release adrenaline, the hormone that serves as your body’s call to battle stations.

Your heartbeat increases. You breathe faster. Your muscles clench. And you burn food faster, converting it as fast as possible to the energy you need for the reaction commonly known as fight or flight. But these effects are temporary. The effects of the sex glands, on the other hand, last as long as you live. Read on.

Resting energy expenditure (REE)

Even when you’re at rest, your body is busy. Your heart beats. Your lungs expand and contract. Your intestines digest food. Your liver processes nutrients. Your glands secrete hormones. Your muscles flex, usually gently. Cells send electrical impulses back and forth among themselves, and your brain continually signals to every part of your body.

The energy that your resting body uses to do all this stuff is called (surprise! surprise!) resting energy expenditure, abbreviated REE. The REE, also known as the basal metabolism, accounts for a whopping 60 to 70 percent of all the energy you need each day.

To find your resting energy expenditure (REE), you must first figure out your weight in kilograms (kg). One kilogram equals 2.2 pounds. So to get your weight in kilograms, divide the number in pounds by 2.2. For example, if you weigh 150 pounds, that’s equal to 68.2 kg (150 ÷ 2.2). Plug that into the appropriate equation in Table below — and bingo! You have your REE.

How Many Calories Do You Need?

Think of your energy requirements as a bank account. You make deposits when you consume calories. You make withdrawals when your body spends energy on work. Nutritionists divide the amount of energy you withdraw each day into two parts:

• _ The energy you need when your body is at rest
• _ The energy you need to do your daily “work” To keep your energy account in balance, you need to take in enough each day to cover your withdrawals.

As a general rule, infants and adolescents burn more energy per pound than adults do, because they’re continually making large amounts of new tissue.

Similarly, an average man burns more energy than an average woman because his body is larger and has more muscle, thus leading to the totally unfair but totally true proposition that a man who weighs, say, 150 pounds can consume about 10 percent more calories than a woman who weighs 150 pounds and still not gain weight.

Every calorie counts

People who say that “calories don’t count” or that “some calories count less than others” are usually trying to convince you to follow a diet that concentrates on one kind of food to the exclusion of most others. One common example that seems to arise like a phoenix in every generation of dieters is the high-protein diet.

The high-protein diet says to cut back or even entirely eliminate carbohydrate foods on the assumption that because your muscle tissue is mostly protein, the protein foods you eat will go straight from your stomach to your muscles, while everything else turns to fat. In other words, this diet says that you can stuff yourself with protein foods until your eyes bug out, because no matter how many calories you get, they’ll all be protein calories and they’ll all end up in your muscles, not on your hips.

Boy, wouldn’t it be nice if that were true? The problem is, it isn’t. Here’s the absolute truth: All calories, regardless of where they come from, give you energy. If you take in more energy (calories) than you spend each day, you’ll gain weight. If you take in less than you use up, you’ll lose weight. This nutrition rule is an equal opportunity, one-size-fits-all proposition that applies to everyone.

Measuring the number of calories

Nutrition scientists measure the number of calories in food by actually burning the food in a bomb calorimeter, which is a box with two chambers, one inside the other. The researchers weigh a sample of the food, put the sample on a dish, and put the dish into the inner chamber of the calorimeter. They fill the inner chamber with oxygen and then seal it so the oxygen can’t escape.
The outer chamber is filled with a measured amount of cold water, and the oxygen in the first chamber (inside the chamber with the water) is ignited with an electric spark. When the food burns, an observer records the rise in the temperature of the water in the outer chamber. If the temperature of the water goes up 1 degree per kilogram, the food has 1 calorie; 2 degrees, 2 calories; and 235 degrees, 235 calories — or one 8-ounce chocolate malt!

Empty calories

All food provides calories. All calories provide energy. But not all calories come with a full complement of extra benefits such as amino acids, fatty acids, fiber, vitamins, and minerals. Some foods are said to give you empty calories. This term has nothing to do with the calorie’s energy potential or with calories having a hole in the middle. It describes a calorie with no extra benefits.

The best-known empty-calorie foods are table sugar and ethanol (the kind of alcohol found in beer, wine, and spirits). On their own, sugar and ethanol give you energy — but no nutrients People who abuse alcohol aren’t always thin, but the fact that they often substitute alcohol for food can lead to nutritional deficiencies, most commonly a deficiency of thiamin (vitamin B1), resulting in loss of appetite, an upset stomach, depression, and an inability to Of course, it’s only fair to point out that sugar and alcohol are ingredients often found in foods that do provide other nutrients. For example, sugar is found in bread, and alcohol is found in beer — two very different foods that both have calcium, phosphorus, iron, potassium, sodium, and B vitamins.

In the United States, some people are malnourished because they can’t afford enough food to get the nutrients they need. The school lunch program started by President Franklin Delano Roosevelt in 1935 and expanded by almost every president, Republican and Democrat, since then has been a largely successful attempt to prevent malnutrition among poor schoolchildren.

But many Americans who can afford enough food nevertheless are malnourished because they simply don’t know how to choose a diet that gives them nutrients as well as calories. For these people, eating too many foods with empty calories can cause significant health problems, such as having weak bones; being underweight (yes, being too thin can be a problem); getting bleeding gums, skin rashes, and other nasties; and developing mental disorders, including depression and preventable retardation.

Counting the Calories in Food

When you read that a serving of food — say, one banana — has 105 calories, that means metabolizing the banana produces 105 calories of heat that your body can use for work.
You may wonder which kinds of food have the most calories. Here’s how the calories measure up in 1 gram of the following foods:

• Protein: 4 calories
• Carbohydrates: 4 calories
• Alcohol: 7 calories
• Fat: 9 calories

In other words, ounce for ounce, proteins and carbohydrates give you fewer than half as many calories as fat. That’s why — again, ounce for ounce —high-fat foods, such as cream cheese, are high in calories, while low-fat foods, such as bagels (minus the cream cheese, of course), are not. Sometimes foods that seem to be equally low-calorie really aren’t. You have to watch all the angles, paying attention to fat in addition to protein and carbohydrates.

Here’s a good example: A chicken breast and a hamburger are both high-protein foods. Both should have the same number of calories per ounce. But if you serve the chicken without its skin, it contains very little fat, while the hamburger is (sorry about this) full of it. A 3-ounce serving of skinless chicken provides 140 calories, while a 3-ounce burger yields 230 to 245 calories, depending on the cut of the meat.

About Calories

Automobiles burn gasoline to get the energy they need to move. Your body burns (metabolizes) food to produce energy in the form of heat. This heat warms your body and (as energy) powers every move you make. Nutritionists measure the amount of heat produced by metabolizing food in units called kilocalories. A kilocalorie is the amount of energy it takes to raise the temperature of 1 kilogram of water 1 degree on a Centigrade (Celsius) thermometer at sea level.

In common use, nutritionists substitute the word calorie for kilocalorie. This information isn’t scientifically accurate: Strictly speaking, a calorie is really 1⁄1000 of a kilocalorie. But the word calorie is easier to say and easier to remember, so that’s the term you see whenever you read about the energy in food. And few nutrition-related words have caused as much confusion and concern as the lowly calorie. Read on to find out what calories mean to you and your nutrition.

The large intestine

After every useful, digestible ingredient other than water has been wrung out of your food, the rest — indigestible waste such as fiber — moves into the top of your large intestine, the area known as your colon. The colon’s primary job is to absorb water from this mixture and then to squeeze the remaining matter into the compact bundle known as feces.

Feces (whose brown color comes from leftover bile pigments) are made of indigestible material from food, plus cells that have sloughed off the intestinal lining and bacteria — quite a lot of bacteria. In fact, about 30 percent of the entire weight of the feces is bacteria. No, these bacteria aren’t a sign you’re sick. On the contrary, they prove that you’re healthy and well. These bacteria are good guys, microorganisms that live in permanent colonies in your colon, where they:

• Manufacture vitamin B12, which is absorbed through the colon wall
• Produce vitamin K, also absorbed through the colon wall
• Break down amino acids and produce nitrogen (which gives feces a characteristic odor)
• Feast on indigestible complex carbohydrates (fiber), excreting the gas that sometimes makes you physically uncomfortable — or a social pariah When the bacteria have finished, the feces — perhaps the small remains of yesterday’s copious feast — pass down through your rectum and out through your anus. But not necessarily right away: Digestion of any one meal may take longer than a day to complete.

After that, digestion’s done!

Peephole: The first man to watch a living human gut at work

William Beaumont, M.D., was a surgeon in the United States Army in the early 19th century. His name survives in the annals of medicine because of an excellent adventure that began on June 6, 1822. Alexis St. Martin, an 18-year old French Canadian fur trader, was wounded by a musket ball that discharged accidentally, tearing through his back and out his stomach, leaving a wound that healed but didn’t close. St. Martin’s injury seems not to have affected what must have been a truly sunny disposition:

Two years later, when all efforts to close the hole in his gut had failed, he granted Beaumont permission to use the wound as the world’s first window on a working human digestive system. (To keep food and liquid from spilling out of the small opening, Beaumont kept it covered with a cotton bandage.)

Beaumont’s method was simplicity itself. At noon on August 1, 1825, he tied small pieces of food (cooked meat, raw meat, cabbage, bread) to a silk string, removed the bandage, and inserted the food into the hole in St. Martin’s stomach. An hour later, he pulled the food out. The cabbage and bread were half digested; the meat, untouched. After another hour, he pulled the string out again.

This time, only the raw meat remained untouched, and St. Martin, who now had a headache and a queasy stomach, called it quits for the day. But in more than 230 later trials, Beaumont — with the help of his remarkably compliant patient — discovered that although carbohydrates (cabbage and bread) were digested rather quickly, it took up to eight hours for the stomach juices to break down proteins and fats (the beef).

Beaumont attributed this to the fact that the cabbage had been cut into small pieces and the bread was porous. Modern nutritionists know that carbohydrates are simply digested faster than proteins and that digesting fats (including those in beef) takes longest of all. By withdrawing gastric fluid from St. Martin’s stomach, keeping it at 100° F (the temperature recorded on a thermometer stuck into the stomach), and adding a piece of meat, Beaumont was able to clock exactly how long the meat took to fall apart: 10 hours.

Beaumont and St. Martin separated in 1833 when the patient, now a sergeant in the United States Army, was posted elsewhere, leaving the doctor to write “Experiments and Observations on the Gastric Juice and the Physiology of Digestion.” The treatise is now considered a landmark in the understanding of the human digestive system.

The small intestine

Open your hand and put it flat against your belly button, with your thumb pointing up to your waist and your pinkie pointing down. Your hand is now covering most of the relatively small space into which your 20-foot-long small (20 feet? small?) intestine is neatly coiled. When the soupy, partially-digested chyme spills from your stomach into this part of the digestive tube, a whole new set of gastric juices are released. These include:

• Pancreatic and intestinal enzymes that finish the digestion of proteins into amino acids
• Bile, a greenish liquid (made in the liver and stored in the gallbladder) that enables fats to mix with water
• Alkaline pancreatic juices that make the chyme less acidic so that amylases (the enzymes that break down carbohydrates) can go back to work separating complex carbohydrates into simple sugars
• Intestinal alcohol dehydrogenase, which digests alcohol not previously absorbed into your bloodstream While these chemicals are working, contractions of the small intestine continue to move the food mass down through the tube so that your body can absorb sugars, amino acids, fatty acids, vitamins, and minerals into cells in the intestinal wall.

The lining of the small intestine is a series of folds covered with projections that have been described as “finger-like” or “small nipples.” The technical name for these small fingers/nipples is villi. Each villus is covered with smaller projections called microvilli, and every villus and microvillus is programmed to accept a specific nutrient — and no other. Nutrients are absorbed not in their order of arrival in the intestine but according to how fast they’re broken down into their basic parts:

• Carbohydrates — which separate quickly into single sugar units — are absorbed first.
• Proteins (as amino acids) go next.
• Fats — which take longest to break apart into their constituent fatty acids — are last. That’s why a high-fat meal keeps you feeling fuller longer than a meal such as chow mein or plain tossed salad, which are mostly low-fat carbohydrates.
• Vitamins that dissolve in water are absorbed earlier than vitamins that dissolve in fat.

After you’ve digested your food and absorbed its nutrients through your small intestine:

• Amino acids, sugars, vitamin C, the B vitamins, iron, calcium, and magnesium are carried through the bloodstream to your liver, where they are processed and sent out to the rest of the body.
• Fatty acids, cholesterol, and vitamins A, D, E, and K go into the lymphatic system and then into the blood. They, too, end up in the liver, are processed, and are shipped out to other body cells. Inside the cells, nutrients are metabolized, or burned for heat and energy or used to build new tissues. The metabolic process that gives you energy is called catabolism (from katabole, the Greek word for casting down). The metabolic process that uses nutrients to build new tissues is called anabolism (from anabole, the Greek word for raising up).

How the body uses nutrients for energy and new tissues is, alas, a subject for another chapter. In fact, this subject is enough to fill seven different chapters, each devoted to a specific kind of nutrient.