Nutrition: The Basics
©2000 by Jerry Sobieraj, MD
Defining healthy eating habits is an important goal for ourselves and our patients. To understand how our human bodies have adapted to utilizing available food resources, it is helpful to look back in time. Humans were sufficiently good hunters by 50,000 years before present to cause extinctions of megafauna around the world. In addition, hominid dispersions over the past 1-2 million years, also related to improved use of resources, which likely included hunting. In addition to hunting, some of our diet came from scavenging, but gathering plant matter would likely have been an important source of calories (large primates, such as apes, eat a great deal of plant matter daily). Though flesh foods ("meat") were the focus of our hominid diet for some time, additional important sources of nutrition would have included gathering fruits, greens, roots, nuts and eggs.
Hominids adapted to surviving even when food supplies were scarce by eating a variety of food sources. Our ancestors would dig for roots, collect nuts and berries when in season, and maybe even try a green or two (we'll leave insects out of this discussion). Yet our current diets may not contain many of these foods. Indeed we do consume them, but often as a small fraction of our total caloric intake. When we aren't getting our calories from meat, we largely get them from fats and starches. So, are we adapted to our present diets (which have more than half of our calories coming from fat and starches)?
Agriculture came into existence about 10,000 years ago in the Fertile Crescent. This area, spanned current day eastern Turkey, northern Iraq and northwestern Iran.. Interestingly enough, agriculture of corn and other crops indigenous to the Americas occurred during a similar time frame. In the Americas, solid evidence of agriculture dates back 6000 years, and this may well be an underestimate. In the fertile crescent, wheat and barley became cultivated. This cultivation spread the use of grains and led ultimately to the processed starches we consume today (i.e. "bread"). The problem with consuming large amounts of grain products, is that during human evolution (after all, our immediate hominid ancestors left Africa about 100,000 years ago), we didn't consume grains at all (or if we did, they were seasonal, and not a major part of the diet). Thus, dietary recommendations need to keep in mind, that we may not be well adapted to utilizing starches, especially as we age (generally after age 40, as that was the approximate life span of hominids during "our" early existence in Africa).
Fats became a larger part of our diet as animal husbandry was added to an agrarian lifestyle. We began to get increasing calories from animal fats, which has become increasingly problematic in "developed" societies. Prior to that, we had little fat in the diet (meat from wild animals is leaner and has less saturated fat in comparisson to the meat we consume today from domesticated animals), and many of our ancestral fats were essential fatty acids derived from nuts.
So as we begin to talk about food and what is healthy to eat, remember that "natural" doesn't mean whole wheat. It really means eating a mixture of berries (i.e. fruit), roots and greens (i.e. vegetables) and finally nuts. Our distant ancestors (100,000 years before present) were likely able to have a piece of meat once in awhile, but this was the exception and not the rule. About 20,000 years ago, humans began using salt to preserve food, and thus meat could have been stored for longer periods of time, and potentially eaten out of season. The primary struggle our ancestors faced nutritionally was getting enough energy (ie calories) every day. Fortunately, (or unfortunately, depending on your perspective), we live in a society of caloric abundance. This significantly affects our food choices, as high caloric density foods must be consumed with care. Also, with the fortification of many of our processed foods, micronutrient and vitamin deficiencies have become rare (but not non-existent). When you try to define how much energy a person should be consuming a day, you begin with their resting metabolic rate.
At the turn of the century, Harris and Benedict empirically derived a formula for estimating metabolic rate. In general, such references to
metabolic rate generally refer to resting metabolic rate (RMR). I will generally adhere to this convention. The formulas of Harris-Benedict are:
Males: RMR = 66.5 + 13.8 x (Wt in kg) + 5 x (Ht in cm) - 6.8 x (age in yrs)
Females: RMR = 66.5 + 9.6 x (Wt in kg) + 1.8 x (Ht in cm) - 4.7 x (age)
As you can see, for both men and women, metabolic rate decreases with age. These formulas can be useful, but a handy rule of thumb is:
Metabolic rate is approximately equal to the weight in lbs. x 10. Thus, one simply adds a zero to the weight in pounds to approximate the RMR.
Knowing the metabolic rates helps us to define how much one may eat to maintain a certain body weight. It is extremely important to realize that for both men and women (as the Harris-Benedict equations show), the major determinant of metabolic rate is body mass (what we call weight when you consider the effects of gravity).
The Harris-Benedict equation will yield results within 2-3 % of those determined by indirect calorimetry (the most commonly used clinical assessment of RMR). Indirect calorimetry can be quite easily determined nowadays with a hooded device (which looks like a
space helmet). The
hood collects all expired gas, and thus can determine metabolic rate by comparing carbon dioxide production with oxygen consumption.
A final point, exercise does not appear to increase resting metabolic rate. Most well designed studies done to measure this have shown no significant effect. Thus, metabolic rate is only increased during the exercise itself, with no good evidence that there is a lasting effect beyond the exercise period.
Several tools have been used for assessing the food types a person is eating. The commonest method is to use a 24 hour recall (i.e. What did you eat yesterday?) or generalities (e.g.
What do you usually eat for breakfast?).
As you can imagine, such questions are fraught with recall bias. However, they may still serve as a way of identifying overall nutritional habits and may be quite useful in identifying problem foods.
In general, a more reliable method is to complete a food log. This log may take many forms but will generally include the times of day when food is eaten, the type of food consumed and some form of quantitation. One's ability to breakdown a meal into its constituent parts, and to quantitate each of them, will depend on skill level and training (some people have extensive knowledge based on prior dieting programs). It is important to keep in mind that even when subjects are specifically instructed in how to maintain an accurate food log, there tends to be a reproducible bias downward in the number of calories consumed. Thus, 80% of patients will underestimate the amount they are eating by 20%. Of those remaining, 10% will be accurate and 10% will overestimate their intake.
This type of data is obtained by having subjects complete food logs and then feed them an amount of food equivalent to what they logged. When subjects are fed in this manner, 10% will maintain their weight (the accurate minority), 10% will gain weight, and the vast majority (80%) will lose weight due to a caloric deficit equaling about 20% of their metabolic rate. Interestingly enough, we are just as inaccurate in our estimation of our exercise (Heymsfeld S, New England Journal of Medicine, 12/31/92). 80% overestimate how much they are doing, when their activities are closely monitored in a controlled setting.
Despite their deficiencies, food logs are not worthless. When you are trying to determine total caloric intake from food logs, you may need to adjust the intake by up to 20%. Also, food logs give you specific information about the types of foods selected and the time of day in which they are eaten. This information may suggest specific dietary recommendations, depending on the overall goals of a person's meal plan.
A research method used to assess nutritional patterns is the food frequency questionnaires. This method asks subjects to rate the frequency (generally number of times per week) at which they consume particular foods (e.g. fish, various types of nuts, bread products, etc.). The lists tend to be quite long and exhaustive, so careful categorization of nutrients consumed can be made. The specificity of the information permits an assessment of total calories, dietary fat, fiber and vitamins and minerals consumed, on average. This is the method used by the often quoted Nurses Health Study, led by Walter Willett, who has pioneered this technique. Food frequency questionnaires give good information when studying a population, but nutritional inferences are prone to error when it is used on an individual basis.
An often neglected aspect of nutritional assessment relates to malnutrition. Generally, cachexia is the hallmark of malnutrition. The most commonly used laboratory tool to help define malnutrition is the serum albumin. The problem with albumin is that its level can be affected by several disease states, and it has a large lymphatic pool, which is not necessarily reflected in the serum level (especially when "third space" fluids are increased). A better measure of visceral protein stores is the transferrin. In fact, when the transferrin level is less than 250 mg/dl, one can no longer use the % Fe saturation to determine if one is iron deficient. There are other serum proteins which can serve as useful markers for malnutrition, but they are generally not readily available (e.g. pre-albumin and retinal binding protein). Previously, a low lymphocyte count was thought to correlate with malnutrition, but that myth has been dispelled. The other simple test one can get is a total cholesterol, which tends to be decreased in moderate to severe malnutrition.
Some dietary modifications become obvious upon review of food logs. To facilitate an understanding of food logs, a basic review of the caloric value of the three major constituents of food may be helpful:
|Food Constituent||Calories/Gram||Examples of these foods|
|Carbohydrate||4||Breads, cereals, pasta|
|Protein||4 to 5||low fat milk products and lean meats|
|Fat||9||Cheese, oils, nuts|
In general, protein needs are on the order of 0.5 g/kg of body weight per day. Some of this protein needs to include essential amino acids. The biologic value of a protein refers to its ability to yield an appropriate mix of the essential amino acids. Egg white (ovalbumin) and milk protein (casein) have biologic values of 1. If you broke down ovalbumin or casein, the mixture of amino acids obtained, would exactly reflect the mixture of amino acids we metabolically consumed. In general, if one has a serving of either of these daily, they will have their fill of essential amino acids (a simple source is a low fat yogurt or hard boiled egg, daily). Vegetarians will generally use a combination of soy and legumes (e.g. kidney beans, pinto beans, lentils, etc.) to give themselves an apt mix of essential amino acids. The ratio of two amino acids may be quite important when considering the impact of protein on serum cholesterol levels. Soy has a high lysine to arginine ratio, while it is low in meat protein. If you add arginine to soy protein, it affects cholesterol similar to meat protein, and vice versa if you add lysine to meat protein.
Fat intake is generally limited to as little as possible. There are essential fatty acids, and these are best represented in walnuts and various seed oils. There is great debate in the nutritional literature to date on whether deficiency or insufficiency of these exists, and what effect if any, it may have on health and disease (e.g. borage and flax seed oil, which contain omega 6 fatty acids are used by "alternative" practitioners to treat dry skin and cracked nails). To date, human data is quite limited as to optimal levels and/or supplementation with essential fatty acids. In general, one may safely limit fat in their diets without undue health consequences, though some seed oil or nuts is useful for ensuring adequate essential fatty acids. If one was concerned about a diet exceedingly low in fat (e.g. a fastidious vegetarian), a recommendation of a handful of walnuts a day would be apt.
The calories in our diet which do not come from protein or fat are due to carbohydrates (CHO). Attitudes do exist about the
evils of refined sugar, and that unrefined sugar and honey are "better". I can assure you that this attitude is clearly in the realm of a belief system with no good data to support it. The advantage of complex carbohydrates is that they are often associated with fiber, and the benefits of fiber with respect to decreasing the incidence of diverticular disease and diabetes are well established. Much of the carbohydrate we eat is in the form of starch. In order to stratify the affect of carbohydrate bearing foods on our blood sugar, the glycemic index (GI) was developed. The GI compares the effect of a particular food on blood sugar in comparison to glucose or white bread. A glycemic index of 100 is arbitrarily chosen as the reference level (i.e. the effect on blood sugar when one eats 100 grams of glucose or white bread). The stratification of starches using the glycemic index may be useful when managing diabetes risk (insulin resistance)
The simplest dietary recommendation, which has a wealth of data to support it (largely epidemiologic), is to eat at least five fruit and vegetable servings per day. It is important to remember that the data showing decreased risk of cancer is largely based on information showing increased consumption of fruits and vegetables in those with lowest risk/incidence. The popular focus on anti-oxidants is based on the yet to be proven hypothesis that it is this aspect of fruits and vegetables which are responsible for their benefits. The anti-oxidant data is positive in animal studies, but there is no human study to date which shows a clear benefit when
anti-oxidants are consumed. Supportive clinical data of the anti-oxidant hypothesis comes from studies which looked at surrogate markers (e.g. DNA adducts, or other markers of cellular damage). Of course, those supporting the clinical use of anti-oxidants claim the published, prospective clinical trials used the "wrong" anti-oxidants in the "wrong" doses, or that a combination of antioxidants is most important. This latter hypothesis may be true, but at this time doesn't meet the evidence standard we consider acceptable in traditional medicine (i.e. you must show a clinical benefit in humans, and not just in animals or test tubes). The five fruit and vegetable serving rule is arbitrary, and likely low when one considers an
optimal diet. These foods have traditionally be a large part of the human diet over the past 100,000 years. We likely have adapted to their consumption, and the fact that I can think of no disorder associated with excess fruit and/or vegetable intake, must mean something. There was a report in the New England Journal of Medicine (NEJM Volume 342:1149-1155 April 20, 2000), showing no benefit of increasing fruit and vegetable consumption on colonic polyp formation from 4 servings a day to 6 servings a day. Colon cancer is clearly a disease which should be affected by fruit and vegetable consumption. One has to wonder if the increase in the above study was adequate to have an effect, as it may take 10 or more servings a day to see a benefit in this type of study. Also, fruit juices can be a useful way to get calcium (e.g. OJ fortified with calcium) and fruit servings at the same time.
With respect to vitamins and minerals, there are clearly deficiency states related to these. However, whether an insufficiency can be defined or whether pharmacologic dosing is helpful is largely an unproved hypothesis. Most deficiency states occur in the setting of malnutrition or odd diets (e.g. a 1995 case report in the NEJM of scurvy in an alcoholic who almost exclusively ate pasta with butter). A basic balanced diet should provide adequate levels of vitamins and micronutrients to prevent deficiency states, especially if people are consuming at least five fruit and vegetable servings per day.
meatlessdinners a week (generally on those days, pasta or rice with vegetables serve as the basis for the meal).
garnishingtheir carbohydrate with fat (e.g. crackers which contain 40% fat aren't just a
starch). In patients at risk for insulin resistance or diabetes, starches may need to be limited. Starches with a low glycemic index are preferred (e.g. legumes, pumpernickel bread, and occasionally rice/pasta). Breakfast cereals are best avoided, as many of the popular varieties have glycemic indices over 100.
Keep in mind that the advertisements on food labels often contain amounts of fat on a unit weight basis (not caloric, when a nutritionist says something is
30% fat, they mean 30% of the calories (not weight) come from fat). Some, claims such as "50% less fat" are meaningless, as standard of comparison may not be relevant. The new nutritional labels will note the percent of calories coming from fat, and even note saturated fat, though I think the latter is not helpful. When the cover label of a product suggests low fat, reading the nutrition label may dispel any illusion that the food actually is
low fat (A defined level that would universally qualify as
low fat has never been defined by the food industry).
Breakfast is the most important meal of the day. This was popularized by the breakfast cereal industry, and highlights the effectiveness of marketing.
You need to eat three meals a day. As noted above, our bodies have adapted to periods of caloric restriction. As people get older, one
meala day, along with a couple light
snacksmay be more than enough calories.
Eating just before you go to bed leads to weight gain. This is not supported by clinical data when one has controlled for caloric intake per 24 hours. This pattern of eating, especially of fatty foods may increase the risk of GERD, but not weight gain.