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Diet & Nutrition
Some basic information on
diet and food choices
 Most people have little knowledge about the nutrient
content of foods and the normal requirements for specific nutrients.
What follows is some general information on these matters. This is
not intended to replace information and advice on individual nutritional
needs, which are best provided by a physician or dietitian.
Included below are some of the standard dietary guidelines for
healthy people. It should be evident that these do not differ very
much from the dietary guidelines given above for patients with porphyria.
People chose foods - not nutrients - when shopping, preparing meals
at home, or ordering meals in restaurants. Their choices are determined
by factors, such as ethnic background, culture, tradition, habits
developed during childhood, income, education, occupation, marital
status and age. Advertising of food products and recommendations
by government agencies, health organizations, and health care providers
also play an important role. Seasonal and regional availability and
cost also influence choices. Agricultural policies, food regulations,
and programs for feeding the poor are also important.
A. Diet Composition
Foods are composed of varying amounts of the following.
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Major
macronutrients (fat, protein and carbohydrate)
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Minerals that are
readily available because they are found in man in large amounts
(sodium, potassium and chloride).
-
Minerals needed in large amounts
and found mostly in particular foods (calcium, phosphorous and
magnesium).
-
Vitamins needed in known amounts.
-
Trace minerals needed in small,
known amounts (iron, zinc, iodine, fluorine).
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Trace minerals needed
in small but less defined or unknown amounts (chromium, manganese,
copper, selenium, molybdenum).
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Minerals of unknown value at least
in humans (nickel, tin, vanadium, silicon, arsenic).
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Water.
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Non-nutrient substances.
Non-nutrient substances include dietary fibers, which may impair
the absorption of some nutrients, but improve colonic function and
are possibly protective for cancer and arteriosclerosis.
Other non-nutrient substances include phytate, oxalate and other
chemicals that can bind calcium, iron, zinc and other minerals and
reduce their absorption. The nutrient compositions of various foods
have been determined by chemical analysis and other methods. These
have been compiled, most notably by the U.S. Department of Agriculture.
These compositions are approximate and may vary considerably.
The "normal American diet" is highly variable among individuals
and can also vary considerably in a given individual over time. Most
Americans consume approximately 10-15 percent of total energy in
the form of protein, 40-70 percent as carbohydrate, and 30-60 percent
as fat.
B. Nutritional Requirements
The Recommended Daily Allowances (RDA
's) provide a means of assessing the adequacy of intakes of nutrients.
RDA's are the levels of intake of essential nutrients considered
to meet the known nutrition needs of practically all healthy persons.
(RDA's are established on the basis of available scientific knowledge
by the Food and Nutrition Board of the National Academy of Sciences.)
An RDA is not as average requirement but rather is considerably above the
average requirement. In statistical terms, an RDA is roughly two standard
deviations above the estimated average requirement for healthy people.
As a result, only a very few healthy people may require more than
the RDA and most people will actually require less than the RDA.
RDA's vary with age and sex, and are tabulated as such. There are
no RDA's for calories, carbohydrate or total fat. RDA's are for healthy
individuals. Nutrient requirements may be altered by disease.
The U.S. RDA for a nutrient is somewhat different. It is the legal
standard established by the Food and Drug Administration and is used
on many food labels. Its aim is to inform shoppers about nutritive
values of foods. It is based on the RDA but is modified to provide
a single value for the entire population in the U.S. four years of
age or older. For most nutrients the U.S. RDA is the same or larger
than the RDA.
C. Dietary guidelines for healthy People
Several widely-known dietary
guidelines are described here. None of these is a complete dietary
plan.
The "Basic Four" Plan. In 1956 the U.S. Department of
Agriculture, in response to a need for simple and specific guidelines
that Americans could use in planning diets that would meet the RDA's
for essential nutrients, established four basic food groups. They
recommended specific numbers of servings from each food group as
part of a well-balanced diet.
Daily Servings
| Age Group |
Milk and milk
products |
Meat |
Fruits and
vegetables |
Breads and
cereals |
| Children less than 9 years |
2-3 |
2 |
4 |
4 |
| Pre-teens |
3 |
2 |
4 |
4 |
| Teenagers |
4 |
2 |
4 |
4 |
| Adults |
2 |
2 |
4 |
4 |
The four food groups differ in nutrient content. Foods within a
given food group also differ and do not all provide equivalent amounts
of the same nutrients. For example, some fruits and vegetables are
particularly good sources of vitamin C, while others are especially
rich in carotene, which is the precursor of vitamin A.
It is important to recognize that when adhering to the "Basic
Four" plan there can be wide variation in nutrient intakes from
day to day. This is particularly true for vitamins and other micronutrients.
This plan recommends servings that should be part of a healthy diet.
The plan itself is not a total diet. Eating only the recommended
number of servings from the four basic food groups may not provide
an adequate diet. Such a diet could be deficient in calories, vitamins
A and E, riboflavin, niacin, folate, magnesium, iron and zinc, for
example. Additional servings of foods within the plan and additional
foods may need to be consumed to provide adequate amounts of all
nutrients. The "Basic Four" plan remains a practical, noncontroversial
and easily understood guideline which can serve as the basis of a
healthful diet.
The National Research Council Guidelines (NRC), which is the action
arm of the National Academy of Sciences, issued two somewhat conflicting
sets of dietary recommendations in 1980 and 1982. The two different
advisory panels that prepared these guidelines were concerned about
the broad issues of diet and health, including the prevention of
cancer and arteriosclerosis.
A report in 1980 entitled "Toward Healthful Diets" concluded
that no specific dietary recommendations were appropriate for the
entire U.S. population. Rather, the report recommended the following
advice on how to achieve a balanced selection of foods and a moderate
and adequate consumption of nutrients. This advice is consistent
with the Basic Four Plan.
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Select a nutritionally adequate diet
from foods available, by consuming each day appropriate servings
of dairy products, meats or legumes, vegetables and fruits, and
cereals and breads.
-
Select as wide a variety of foods in each of
the major food groups as is practicable in order to ensure a
high probability of consuming adequate quantities of all essential
nutrients.
-
Adjust dietary energy intake and energy expenditure so
as to maintain appropriate weight for height; if overweight,
achieve appropriate weight reduction by decreasing total food and
fat intake and by increasing physical activity.
-
If the requirement
for energy is low (e.g. reducing diet), reduce consumption of
foods such as alcohol, sugars, fats, and oils, which provide energy
but few other essential nutrients.
-
Use salt in moderation; adequate
but safe intakes are considered to range between 3 and 8 grams
daily.
A second report in 1982 entitled "Diet, Nutrition and Cancer" focused
on the putative relationship between diet and cancer. Specific recommendations
about certain foods were made.
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Reduce intake of both saturated and
unsaturated fats, from approximately 40% to approximately 30%
of total calories.
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Include fruits, vegetables and whole-grain cereal
products in the daily diet, especially citrus fruits and carotene-rich
and cabbage family vegetables. Avoid high-dose supplements
of individual nutrients.
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Minimize consumption of cured, pickled,
and smoked foods.
 The latter recommendations have been controversial. The report
itself stated "The data are not sufficient to determine the
contribution of diet to the overall cancer risk or the percent reduction
in risk that might be achieved by dietary modification." The
emphasis of cured and smoked meats was not appropriate, in the opinion
of many. Few meat products in the U.S. are preserved by heavy smoking,
salting or nitrate treatment. Liquid smoke flavorings are extensively
used; these contain little if any benzo(a)pyrene, the principal carcinogen
in smoke. Benzo(a)pyrene is one of the chemicals known as "polycyclic
aromatic hydrocarbons" found in cigarette smoke and in charcoal
broiled meats. These chemicals can stimulate the heme biosynthetic
pathway in the liver and might be harmful in porphyria. Meats are
preserved primarily by refrigeration rather than salting, and the
addition of nitrate is closely regulated. Curing solutions for bacon
contain ascorbate (vitamin C) or erythorbate to prevent formation
of nitrosamines. The capacity of bacon to produce nitrosamines during
frying is routinely monitored by the FDA. Therefore, there is no
evidence that cured or smoke meats routinely sold in U.S. supermarkets
are hazardous. Similar recommendations with regard to consumption
of fats, fruits and vegetables have been made by other organizations,
including the American Heart Association, the American Cancer Society,
and the Surgeon General. Some others feel that such recommendations
should apply only to individuals or groups known to be at risk for
diseases such as arteriosclerosis, hypertension, or cancer, rather
than to the general population.
D. Strategies for changing diet
Some approaches useful in enhancing
the motivation of individuals to change their diets are listed
below. Approaches described here for encouraging dietary changes
are very general and do not include more specific approaches that
are useful, for example, for weight control. Similar strategies for
altering health behavior are used to encourage individuals to make
other changes that are favorable to their health (stopping smoking,
for example).
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The positive health effects of the recommended dietary
change and the negative effects of not making the change should
be discussed by the patient, the physician and the dietitian. Motivation
is increased if the focus is on health consequences in the near
future rather than the distant future.
-
Motivation is enhanced by
using individual examples of the health benefits of changing
diet rather than only providing general or statistical information.
-
Advice should be concrete and specific, and include food lists,
meal plans, etc. Such written material helps an individual develop
a specific plan for making a behavior change. It is generally
best if specific face-to-face advice is given by a physician
and a dietitian.
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Goals should be realistic given the circumstances
of an individual.
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An individual should not be overly discouraged
if initially there are problems in accomplishing dietary goals.
It is sometimes advisable to start with small changes that are
easily achieved and then build progressively on initial success
in order to achieve larger changes.
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Self-monitoring before and
after making a dietary change helps to identify incentives, disincentives
and situations that interfere with accomplishing a change.
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Individuals
that are serious about making a long term dietary change need
to free their homes of foods not on their diet, stock up on recommended
foods and learn to avoid situations which lead to eating foods
other than those on the recommended diet.
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Enlisting the support
of family Sponsors, roommates or coworkers can provide encouragement,
reminders and praise when goals are achieved.
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