Nutrition MiraCosta Biology


Uploaded by MeerdesIrrtums on 22.03.2012

Transcript:
If you ask a typical college student, “why do you eat?” you’d probably get a variety
of responses. “I eat because I’m hungry” or “I eat because I’m bored” or “I
eat because I’m mad at my boyfriend or girlfriend”—but none of these answers is sufficiently biological
for someone taking this class, and by now you should know the proper way to answer this
question.
You eat to obtain three things. First, you eat to obtain energy. Sugars, polysaccharides,
proteins, fats, oils—these all get broken down to make ATP energy.
How much energy do we need? Well energy will here be measured in units
called kilocalories—this is the same as the familiar “calories” on food packaging.
In physical sciences terms this is the amount of energy needed to raise one kilogram of
water by one degree Celsius. Part of your daily energy requirement is needed just to
keep your body alive and warm with all of your physiological systems functioning at
minimal level. If you were in a coma with no brain activity—or very little brain activity—this
would be about your rate of energy consumption. We call this your basal metabolism and it
varies to person to person based on age, gender, but mostly body size. For example comatose
adult female of 115 lb. would probably need about 1400 Cal per day. A bigger person would
need more calories, a smaller one less.
Apart from these minimal energy requirements, any activity beyond just “being alive”—including
such activities as being awake, digesting coffee and pastries while you’re studying
biology and ice skating at the same time—add to your energetic requirements. The more demanding
the activity the greater the energetic needs. A moderately active male 160 lb. college student
will probably have a need for something like 2400-3200 calories per day. A triathlete who
is training for a big race by running/swimming/biking daily may need upwards of 5000 calories.
The second thing we absolutely must get from the food we ingest is the raw materials for
our biosynthetic work such as making all of the parts of the cells for maintenance and
growth. And the third thing that overlaps somewhat with the second, is that we rely
on our diets to provide specific substances that our bodies need but cannot make. For
example, nine out of the 20 amino acids that are required for protein synthesis are identified
as essential amino acids because they cannot be made from scratch by the biosynthetic pathways
in our human physiology. So in order to make our proteins we need to “import” these
amino acids from the food that we take in. To give you an example, we can’t make the
amino acid lysine, but we need lysine to make our protein. Maybe you ate some peanut butter
today. The protein content of peanut butter (as well as lots of other legumes—that is,
vegetables in the bean and pea family) are lysine-rich. In breaking apart the peanut
proteins you undoubtedly got and absorbed some lysine that is presently being incorporated
into the proteins of your body.
Let’s think for a bit on this human inability to make amino acids like lysine—and remember
there are eight other amino acids that we can’t make, as well as many other essential
nutrients. Our inability to make these compounds might seem like it could be a huge problem,
but it really isn’t. When you think about the context of natural selection that has
given shape to human physiology, the diversity of foods that our ancestors have eaten just
in order to meet their caloric needs would have provided these nutrients. We don’t
need to synthesize lysine because we have a long history of getting all the lysine that
we need from our food. The enzymatic machinery required to produce lysine is about as useless
to us as eyes are for the blind cave fish, and mutation has taken the ability away.
We probably did have—at some point in our evolutionary histories—the ability to synthesize
lysine. Plants, fungi, as well as most kinds of single-celled life forms have this ability.
As it turns out, all animals lack the ability to synthesize at least some out of the twenty
amino acids, and all vertebrates share the same set of nine essential amino acids—lysine
being one of them.
You might be wondering about herbivores and their ability to meet their amino acid requirements
from their very low-protein diets—remember that vegetable matter tends to be rather low
in protein content. Herbivores are—almost without exception—either ruminants or hindgut
fermenters (like the koala we talked about earlier). Bacteria in their specialized gut
compartments consume the vegetable matter and make their own proteins out of the complete
set of amino acids that they’re able to synthesize—unlike us, many bacteria have
the ability to make all twenty amino acids. Digesting the bacterial protein herbivores
get a mixture containing all of the essential amino acids. Omnivores like humans and carnivores
like bears don’t have specialized fermentive chambers in their guts, and we don’t get
too much nutrition from digested bacteria. Herbivores do and they rely on bacteria as
a very important protein supplement to the veggies that they consume.
Our human digestive anatomies are not well-suited to take advantage of the protein content of
hind-gut bacteria, so a person following a strictly vegan diet would need to include
some protein-rich food items, such as nuts, into his/her diet.
In addition to essential amino acids, we also cannot make mineral nutrients or vitamins,
which are organic cofactors that are required for certain necessary biochemical reactions.
Here is a table of minerals and vitamins needed by humans. A diet that is deficient in any
of these substances will result in adverse health effects, as you’ll be seeing in the
nutrition laboratory.
People who decide to adopt a strictly vegan diet must make a very special effort to complete
their nutritional requirements—certain nutrients are very difficult to acquire from purely
vegetable matter. We already know that vegetable material tends to be low in protein, but a
person has the option of selecting items that are relatively rich in protein, like tofu
or peanut butter. However, at least one vitamin—B12—is practically impossible to get from a purely
plant-based diet, and a B12 deficiency causes a condition known as pernicious anemia, which
is one of the most serious and most common of nutritional deficiencies experienced by
vegans. Most vegans either know this and make that special effort to meet this need, or
they find out about it when they are hit by declining health.
When you think about how bad it is to be malnourished—the effects range from general weakness to blindness
to defective blood clotting mechanisms—and the huge list of essential nutrients, it may
initially seem kind of surprising that our ancestors were able to stay alive and healthy
at all. Somehow they were able to meet all of those dietary requirements, and they did
so without daily multi-vitamin supplements.
In the modern human diet, a great deal of what we eat is based on refined starches,
sugars and oils. White starches come from a more complete source of nutrition—whole
grains—from which only the starch is isolated. First the bran and germ are removed, and then
the more nutritional outer layers of the grain are polished away until what you have left
is a lump that has a very neutral flavor—this is your grain of white rice or the part of
the wheat grain that gets ground into flour. It’s the absence of flavor that makes refined
starches so desirable. White starch is a great source for calories, but in making it we’re
throwing away the outer layers of the grain that have a higher content of protein and
essential nutrients. Sugars and oils are similarly high-calorie, low-essential nutrient extractions
coming from more complete food sources. This practice of refining the nutrition out of
foods is a relatively recent thing and it’s unique to our species. For a human living
a few thousand years ago under primitive conditions, taking in 2200 calories worth of food would
have meant taking in a LOT more nutritional diversity, as compared with 2200 calories
worth of Cheetos, donuts, and beer. In this regard the idea of avoiding white foods (white
flour, white sugar, white rice) and meeting your daily caloric needs with whole foods
is really good nutritional advice.
We’ve talked about pernicious anemia, but nutritional deficiency associated with veganism
isn’t a predominant health concern for the human population in the U.S. With an abundant
availability and consumption of high-calorie/low-nutrition foods, it’s not surprising that we see a
far greater incidence of unhealthy eating habits in which a person consumes too many
calories while at the same time not meeting all of the nutritional requirements—a combination
of overnourishment and malnutrition. And yes, they can occur at the same time. We’ll talk
about this more in the next video.