Biology 1A - Lecture 27:Nutrition


Uploaded by UCBerkeley on 31.10.2012

Transcript:
>>INSTRUCTOR: Good morning. Little hand is on 8, the big hand is on 2,
so we can start. I would like to make sure in case anybody
was confused. I guess there was an error in the original syllabus, my office hours is
at the original place. 24 LSV, Monday Wednesday, 9 to 10, Tuesday,
2 to 3:00ハp.m. So today we want to discuss material covered in Chapterハ41 in the text
on nutrition. And you know, why bother? What is nutrition?
Nutrition is eating of course. Why do we eat? Well there's a few reasons, first for pleasure,
second forハ to relieve anxiety, but in biological sense, we need energy because any biological
entity is continually fighting against the forces of entropy that tend to make it less
organized and you know, dead, so we need energy to maintain ourselves against the dynamic
process of entropy and if we're going to replace and grow tissues we need the organic molecules
as building blocks for all of the polymers that you've heard of. And we need various
sources of nutrients, I want to talk about that for a little bit and then we'll go on
and talk about some of the processes in digestion. So, in terms of energy, we have the 3 main
food groups, fats, carbohydrate, and protein. Or in my case, butter, brown sugar, and chilies,
I guess. But you notice that you get more calories
from fats and from carbohydrate or protein. And here's a little puzzle question, if there's
9ハcalories in this is what we use about 3,000ハcalories per day, if you're a very
active person, 2 to 3,000, and it takes 1 calorie to warm up 1 milliliter of water,
1 degree centigrade, right? So, what puzzled me at one point was why couldn't
I just drink a liter of water, 10 to the third MLs, at you know, ice water or cold water,
say at 7ハdegrees C, I warm that up to 37, before I pee it out, and that takes 30ハcalories
per mil so that would take 30,000ハcalories. Okay.
Just if I drink a liter of cold water. And then I can eat anything I want.
What's the fallacy? Who can tell me? Quick quick quick. Yes, blue.
>>STUDENT: [Indiscernible] >>INSTRUCTOR: Right.
These are real calories, cal C calories, and these are big K dietary calories there's a
factor of a thousand. So if you really wanted to try this ice water
diet, in order to even work off one decent slice of pizza you would need to 10ハliters
of water and you would soon go into kidney distress and all sorts of things, so don't
try this at home. Happy to clear that up for you.
So, there we go into this question the central nutrients vitamins and minerals, what are
what are these? The inorganic ions are essential to diet we can't carry out nuclear fusion
in our bodies so we need to get these from the food we take in.
Sodium and potassium are required for electrolytes, magnesium is required for lots of enzymatic
activities, calcium is important signaling molecule, etc.
In addition to these observe ones, there's a bunch of trace elements chromium, magnesium,
these in this first set have really been demonstrated to be required for maintaining our cells.
Cobalt is interesting it's the maybe the one case where the essential nutrient, the ion
is actually required to be in a pre synthesized organic form, that's vitamin B 12, which is
a big porphyrin ring. And then there's varying degrees of evidence for other ions being required.
There's lots of controversy about these things. And we'll talk about why it's hard to do these,
you know, why should such an obvious question, question be so difficult to answer? But before
you rush out and start loading up yourself on zinc or iron or selenium you would be aware
that these things are easily toxic you really need to trace amounts only. And high concentrations
they rapidly become toxic to you. Now, going onto the vitamins are small organic
molecules they maybe water soluble or fat soluble there's different with classes of
these things, they're usually serving as enzyme cofactors. And like vitamin C is one. Used
for redox reactions, now if these are organic molecules they're made by living things and
given that we've all evolved from other earlier life forms, our ancestors at some point of
the tree were able to make these compounds. So, now there's been this evolutionary loss
of biosynthetic capacity. Right?
Why would that be? Why would we trade our independence?
Well, one argument is for example case of vitamin C, which is obtained from eating,
you know, leafy greens all of that good stuff, if you have vitamin supplied to you in your
diet, routinely, right, then there's no pressure, there's no evolutionary pressure to makeハ
(No sound). Maintain the biosynthetic capacity for that
and if you have a mutation that then eliminates your ability to take something that you're
getting anyway, there's no reproductive cost, you don't die. And you get that little minuscule
amount of energy and that you would have otherwise needed to make vitamin C, you can use it for
other purposes. (Lost Gatherplace).
When diet changes, then these deficiencies can be reveled.
Now, demonstrating the need for essential nutrients is not as easy as it might seem.
You think, well standard approach, you just whatever it is you're thinking about might
be important, you just remove that from the diet.
And look to see what happens. Okay.
This is the approach undertaken for the 9 essential amino acids by a guy name William
Cunning Rose, working in the middle of the last century, and by these sort of dietary
exclusion experiments he demonstrated that there are 9 essential amino acids.
Histidine, it's leucine, and cysteine as a sort of group together.
Then alanine + tyrosine, as a group together. Tryptophan, threonine, and valine, I recited
that list just to show you that after 30 years or so succeeded in learning the single letter
amino acid code. And you can do it too.
Actually, I still have troubles with glutamine and aspartic gene, but that's why I left them
off the list, fortunately they're not essential. So, what's the problem here?
Well, the problem arises is that how are you going to carry out these experiments? Most
nutrients are present in most foods, so in order to do this experiment, you need a highly
purified artificial diet. Okay. And then you need to find somebody who will
eat it. Okay.
Orハ so, Rose carried out these experiments with college students, said, healthy males,
that's probably the only people who are in college that the point. And the diet consisted
of purified corn starch, sucrose, butter fat without any protein in it, you know, I don't
know how many of you are familiar with G, the clarified butter.
Just the oil part. So, inorganic salts, the vitamins they had
already known also, that were already there. And finally, a large brown candy made of river
extract flavored with the peppermint oil to supply any unknown vitamins.
And then they he would just add purified doses of various amino acids, feed these to the
students, and see how long it took himハ whether they didハokay or whether they started
nausea, I would start feeling nausea really soon on that diet. But by these experiments
he showed that these amino acids are essential. They'reハ he alsoハ it also became evident
from further work that there are species differences in developmental differences, like, infants
have extra amino acids requirements. And different species of animals also have
different amino acid requirements, which again make sense because if different animals have
different diets, they may have had different evolutionary losses that were sustainable
in terms of they're biosynthetic capacity. When you're thinking about trace elements,
trying to do these what ions are required in our diet, if you go to your bottle of even
the most highly purified sodium chloride or potassium chloride, ultrapure and then you
read the ingredients list on the side there's always traces of impurities you can never
get the salts totally pure. And so, you can't exclude the possibility
that maybe a trace element requirement maybe satisfied by the impurities in your purified
diet. Okay, so those are the sorts of issues that come up.
Another problem is that deciding what's required in our diet and what's not is complicated
by the fact that there are biochemical interconversions of different molecules. For example, the sulfur
containing amino acids cysteine, homocysteine, methionine can be interconverted by our body
and therefore the requirement is just for one or another of the cysteineハ of the sulfur
containing amino acids. Similarly, phenylalanine and tyrosine
differ only in the presence of the hydroxyl group, parahydroxyl group that's present in
tyrosine, so those can be inner converted and more interesting example is one I'm going
talk about next called the inner conversion of the amino acids amino acid tryptophan to
the essential vitamin niacin. Vitamin B 3. Okay.
But basically, just before I leave this, the status of this question, which is sort of
general interest to people, is such that when you look on there's somewhere between 40 and
80 essential nutrients in the human diet. Okay.
And that is a remarkable level of uncertainty in my view.
Okay. So, going onto tryptophanハ any questions
about this? And I guess if you do have questions, you
can just raise your hand and shout and we'll try and deal with them.
Hello to all of you out there in Evans you can come to office hours or phoneハ I could
give my cell phone number I guess or you could just call.
So, here's tryptophan, again you're not required to know the structures memorize the structures,
but interesting you have to start out with bicyclic heteroring, nitrogen carbon. And
in the liver through a series of chemical transformations this can be converted into
nicotinic acid or niacin. Okay.
Which is an essential vitamin. So, in principle if you have enough tryptophan
in your diet you don't require niacin, even though we still call it a vitamin.
So that gets us how these things were discovered is a remarkable tale it's a good example of
all sorts of things involving public policy evolution of culture as well as of plants
and animals. And so, it's niacin, tryptophan, pellagra,
public health and nixtamalization, cautionary tale of intercontinental intrigue.
So, we start with a disease. Pellagra.
It's charactered by they call it the 4 Ds this dramatic dermatitis and this dermatitis
often manifests its as a phototoxin rash so areas of your skin that are exposed to sun
break out in these horrible rashes with a lot of discoloration as well.
Okay. And this was this is called the collar.
So, you have this phototoxic rash and then later, you have dementia, diarrhea, so you
got serious malnutrition and this can lead to death after several years if not cured.
Pretty grim disease. It was one of the first serious syndromes that was defined.
I got to learnハ where am I supposed to put this thing?
Hour about there? Now, you can't read the numbers.
So, pellagra showed up in Spain in the 1700s, in the region called Asturian, it was described
by a name called Gaspar Casal so they call that discoloration the collar of Casal and
the it was also known as Asturian leprosy, so it's present in emergent Spain it was also
seen around the same time in northern Italy, where it actually got its name. Pelle agra
is pelle is skin, agra sour, so this was a disease in Europe and then in the U.S., it
emerged and became epidemic in the U.S. by the 1900s, particularly in the south.
It was associated with the consumption of large amounts of corn, or maize in the diet
and this sort of makes sense because the disease was not seen until the 1700s, and corn was
introduced to Europe after, you know, the Columbian era, after Europeans got to the
new world and started taking plants and things and gold back.
And because corn is such a remarkably high yielding grain crop it became very very popular.
Like corn potatoes in Ireland that was another example of food imported from the new world
that were used to feed large numbers of people, it became important part of the diet, especially
for people who couldn't afford much else, okay, because the high yields made for low
cost. Okay. So, the thinkers of the time, what are the
probabilities? What would you think about this as the sources of this horrible disease?
Maybe there's a toxin in the corn. Right.
So, that if you eat too much of it you get poise. Or maybe there's some sort of infectious
disease because the rash would appear in clumps in people, or living near by or people thought
there was some insect vector that could have come with the corn, we know that when you
bring one thing, a lot of commensal organisms arrive with it.
But then, why did pellagra only show up in Europe and come back to the U.S. if people
had been eating corn heavy diets in MesoAmerica for all of these centuries, had the peopleハ
maybe the Native Americans had evolved some method for dealing with the toxin.
Who knows. So, this got to be a real serious problem
in the United States, in the south by the 1900s there were you know, much much lower
population of maybe a thousand people a year dying, hundreds of thousands of people were
affected with this disease. Especially in jails, orphanages, the disease
of the poor people often. And so, not our ideal living conditions and
can easily imagine that this was some sort of infectious disease.
And since it only affected poor people, why worry about it, right?
But eventually the profile of the disease emerged so that the United States public health
service, this is not a division of bane capital this is actually a government organization
so it's sort of one of the first lessons maybe there is a role for doing things communally
to investigate these things so the United States public health service opened up a pellagra
hospital in spar ton burg South Carolina where there were the a lot of the diseases wereハ
where a lot of the cases where. And they assigned this guy, Joseph Goldberger, and then the
public health service was this sort of semi military organization, so you got to use wear
cool uniforms with buttons and embroidered collars and that's why we call it the surgeon
general of the United States still. So he was working on this and he did experiments.
So, he had the novel idea that there might be some dietary cause.
And he did experiments on prisoners. Another issue that comes up.
How do you find people to do these sorts of experiments and what are the ethics and more
or less of those issues? So he started with 11 prisoners who were promised
freedom in exchange for participation in this experiment.
And heハ they had been eating a mixed diet and he put them on a corn only diet, and after
a couple of weeks, they started developing symptoms of pellagra and it was so horrible
that at least a couple of the guys, said just put me back in jail, take me off of the experiment,
I can't do it and what happened then was he restored a more varied diet with mixed greens
and things from the prison guard her in and all of these prisoners all recovered so his
conclusion was, it's still could have been the all sortsハ it wasn't clear cut but his
conclusion which was heavily resisted by the medical establishment is that there was a
dietary component to this disease that you're missing something when you ate only corn.
But this was sort of contrary to the idea that it was an infectious disease so it's
hard for him to get these ideas across. But eventually he did and the story then turns
to the University of Wisconsin that was late. That's Madison Wisconsin the main campus,
so this guy, Conrad eventually gem. This guy's parents came from Norway, so you
know, maybe immigrants are good for something after all.
So he discovered thatハ he eventually identified a liver filtrate, a clear filtrate kind up
liver, filter it, make it clear and he could use that clear filtrate to reproduce the affects,
the curative effects that Goldberger had seen with the corn only diet.
Okay. So, and then you can start purifying factors
and eventually he shows that nick tin [Indiscernible] or niacin as it's more commonly known was
the active ingredient. Okay. And they started out calling this vitamin
G in honor of Goldberger. But then realized it was a previously described molecule so
they stayed with the vitamin B descriptor. But, okay. So now we've solved pellagra and
it's no longer an issue in the United States. Although it can appear in people with mutations
that prevent the up take of tryptophan, alcoholism, also interferes with amino acids amino acid
up take so you can get pellagra in sort of weird cases like that.
And of course, in developing countries where the diets are poor. But now why did pellagra
first show up in Europe? It turns out this gets us to the final part
of this, nixtamalization, and I hope, or I expect that at least for most of you you'll
have learned at least one new word today for your vocabulary. If that makes it worth getting
up at 7:30 we'll see. So nixtamalization, is from the Aztec language, naxtli is ashes
and tamalli is the unformed corn dough. This is used to traditional used corn in the Aztecs
and Native Americans of using ashes from fire, which are a source of lye, high pH, and they
use the ashes mixed with the corn in cook it and it turns out that this traditional
method of preparation for food releases niacin from and increases its bioavailability. Okay.
So, corn is still a poor source of tryptophan and niacin, but you can get enough if you
do theseハ if you treat it properly. And then when people in Europe started relying
more heavily on corn in the diet, out of, you know, poverty, and they failed to introduce
that traditional way of preparing it they just ground it up they started eating it like
polenta in Italy and then you see these pellagra appearing.
Okay. Questions about this?
So now let's think about what happens when we eat.
Basically, a series of processes. At least for mammals, unless you're a liquid feeder,
you have some sort of mechanical digestion, filter feeders would also by pass this, but
mechanical digestion you grind things up and then the key steps of chemical digestion enzymatic
hydrolysis, absorption is the taking up of these, usually, small molecule building blocks
and there are undigested material is pooped out.
So, we want to talk hereハ hoping I have time, mainly about some of the interest things
that go on during digestion and absorption, the book does a good treatment of everything.
So let's think about thisハ oh, no. We'll think about this from an evolutionary
standpoint first. And you can imagine where the basic animals
at theハ with a simpler body plan things like hydra, the Cnidarian or flat worms, platy
helmets they have aハ they're characterized by the absence of a through gut as I'm saying
to you. So they have a vastハ what's it called, a
gastrovascular cavity with one opening into theハ there's a single opening into the body
cavity, we showed before there's an inner and outer epithelium, so food has to come
in, there's this rotifer like thing, it goes gets the central cavity by this tentacles
and then the cells, lining the epithelium, the endothelium, secrete digestive enzymes
that break down this food as best they can. And then at some point when this fills up
with potritis(?) you have to reverse the flow and put the excreted material back out through
the same opening. Okay. So this works, these things have been around
a lot longer than we have. So, it's not that bad. Jellyfish are doing
just fine in the ocean. But, still, it's hard to avoid the conclusion
that if you have a through gut, you can have continuous processing of food, so you take
it in, grind it up, digest it, absorb it, excrete, and that allows you to have specialization
within the gastrointestinal tract where different cells and tissues and different parts of the
GI tract are specialized to carry out different functions that are optimal for the processing
the dietary needs of very different kinds of animals.
So, once the through gut evolved it appears to have been very strongly maintained.
This are very few animals that have lost it. Can you imagine one that might?
You have some sort of parasite that gets a tremendously rich diet by living in the intestine
and then it might lose a gut. A lot of the digestive things because it can take nutrients
into the body wall directly as long as it maintains a fairly small body size.
So inverters we'll be dealing with the mammalian system, so there's this basic process in which
food is taking in at the mouth, digestion begins here with secretions from salivary
glands, and coating it with saliva and mucus to make it easier to go down the esophagus
by peristaltic motions. And then in the stomach there are more secretions,
some called gastric juice. Very acidic environment, food passes from there into the small intestine
where more secretions from liver and pancreas work and most of the absorption of the broken
down molecules is taken up, or taken up within the small intestine and then waist goes to
the large intestine and on out. What's missing from thisハ one of the many things missing
from the this guy gram are the various organs the salivary glands the liver and pancreas
that are not properly part of the GI tract but they have ducts into it to add they're
secretions. Another important thing missing is the valves
at various points along here, they're muscular valves to make sure that food many most cases,
does not go backwards in this tract, it's a unidirection of flow except in cases that
are familiar to all of us. So, let's look at a few questions. First how
is food broken down into the molecular components that are suitable for synthesizing new molecules
in the diner? Well the key things are enzymatic hydrolysis, and in addition there's hydrochloric
acid in the gastric juice of the stomach. So here you can look at theseハ this table
from the book shows pretty nicely how secretions first in the oral cavity, first start breaking
down polysaccharide starch and things like that only. Okay. And then when you get to
the strum, you secrete acid, also pepsin is the first enzyme that's working and that's
working on protein digestion. Around pepsin has thisハ has to be a particular
molecule because the pH in the stomach is round two, right, and that's going up that
low pHハ part of the digestion process is denaturing most of the proteins that you've
taken in. To make themハ turn them inside out and make
their peptide bonds more accessible for other proteases to digest. So this one moleculeハ
this one protease pepsin has to be able to survive and function in this low pH environment
that denatures most other proteins. So, then when you get out of the stomach into
the small intestine, you have a variety of hydrolytic enzymes that are secreted from
the pancreas. They carry on this digestion process, but
why don't they get denatured? Because there's also secretions here that neutralize the acid
coming out from the stomach. Okay.
And thenハ yeah. And then, the other enzymes that are associated
with the epithelium of the small intestine that do the final steps. They take a disaccharide
cleave it into monosaccharide, dipeptides and convert those into the constituent amino
acids. Okay. So enzymes are key. Now, with all of these
proteolytic enzymes and acid, and obvious question is why don't we digest ourselves?
Okay. And the first answer is to a certain extent
we do. It's a really rough environment down there.
Cells are dying constantly. Both from the effects of the hydrochloric acid, the enzymes
the abrasion of the food in the gut and so there's a constant on going replacement of
the digestive epithelial, so it's a source of interesting kind of stem cells these proliferated
cells that we'll talk about later. Other ways that we're protected from autodigestion
are this mucus that's secreted through out all of our internal membranes, right? It's
an important process and another very interesting evolutionary connection if you go back to
the earliest animals, or we're not going to make it through this lecture, but let's just
have a good time. So there's this animals and now I'm not even
going remember what they're called. Ascidians.
They're veryハ they're basal Cordaites. The adult doesn't even look like a Cordaites,
it's hard to imagine it's relation to anything that could have given rise to us, but in the
vertebrateハ in the sorry, deuterostome branch, we'll talk about this later, there's an early
branch whose modern descendants look like this, how do these thing eat? Well they have
ciliated cells all along here. And the cilia are beading, unidirectionally
so you get a water through this oral opening and then out through a siphon. And these cells,
these epithelial cells in addition to cilia they're also secreting mucus and so that mucus
is continually, thanks to the ciliary beading throwing down into this central cavity, carrying
any bits of food, little sea creatures that get stuck in here they get down here and get
digested. Okay. And when thinkハ I think about this every
time, especially if I have a cold, and that's just what I'm doing right.
We're clearing things from our nasal cavities from our lungs, by the secretion of mucus
and then down into the digestive tract. And we also use it in the digestive tract
to grease the skid so the food can get down better. And I think this is seen in pretty
much all higher animals and so this is really an evolutionary harking back.
Other ways that vertebrates control autodigestion is by controlling the timing and spatial or
occurrings of the process itself, we don't eat constantly. So we don't need to digest
constantly, right. We don't need to have constant stomach acid,
constant secretion of all of the proteases and other digestive enzymes, okay. So theseハ
there's a section in the book on the control regulation, you know it's the mechanical stimulus
first of your gut getting stretched that leads you to start secreting ACL and pepsin in the
stomach and then as it passes into the next stage that triggers the secretions from the
pancreas, etc. Ah, Jesus Christ.
Okay. I apologize, this isハ I have to get this dealt with.
Ah. Okay. Let'sハ the notes are on theハ but
we need the iClicker question. What?
Do it next time? >>STUDENT: No.
>>INSTRUCTOR: Okay. I'll be doing thisハ so, I can't even remember
what I wanted to say next is
the problem. Wealth go straight to theハ we'll go straight
to the iClicker question, maybe. I think I've told you enough information for
that. It's that I was so proud of this question.
Because I thought it was sort ofハ you know, yeah that was a 1:00ハo'clock at night, at
1:00ハo'clock at night it seemed like a good question.
I'll be real careful, okay. Just a second.
Just a second. Okay. Don't panic.
The highly acidic environment of the stomach is advantageous, both for killing most microbes
or potential parasites that might be ingested and also for denaturing dietary proteins there
are exceptions of course, if you eat a tapeworm egg that acid helps it hatch, right and that's
how you get tapeworms. Also there's a special bacteria helicobacter
pylori that survives in the stomach, grows in the stomach and actually is what give you
ulcers, so you treat ulcers with antibiotics you don't need to cut out the stomach.
Okay. So, acid.
But then in the duodenum, which is also the entrance to the small intestine, pancreatic
secretions, why might this be important? Okay.
So, iClicker gives youハ oh, I need to turn on iClicker. Oh mama.
[Laughter]. That was the wrong thing.
Damn it. Okay.
Start. Oh!
Why do we neutralizeハ why do we neutralizeハ why do we neutralize the acid in the small
intestine? Byハ secrete massive amounts of bicarbonate
into the beginning of the small intestine. This is A. because the generation of CO2 helps
propel food down the intestine. B. it prevents denaturation of the pancreatic
digestive enzymes. C. the neutralization is required to hydrolyze
triglycerides. D. is result of CO2 prevents ignition of bacterial
methane in the gut. E. both A and D are correct. Okay.
So now you've already had a minute. Everybody's done.
2 seconds more. 452 and we are done. Okay. I'm going to give up there we'll finish
this next time. What's the answer?
Well, we just finished talking about B right? And you don't have hydrolyze triglycerides
at neutral pH you can do with very high pH but not neutral pH.