This post is part of an ongoing series about my learning process as I train to become a personal trainer. Today is the second part of three posts about nutrition. In the last post, I looked at the basic divisions of the three macronutrients. In this post, I go into the roles of the macronutrients in the body.
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But first, a brief note about my thoughts
When I was at university studying theology, one of our lecturers used to have a saying to dampen our enthusiasm when it came to textual criticism. He used to say “if you can make a 2,000-year old system of wisdom look silly then you probably haven’t understood it properly.”
So I make all my comments below in the full understanding that while the science in my textbook isn’t quite 2,000 years old, it is very likely that, where I am disagreeing with it, it may be very likely that I haven’t understood it properly. If you can shed any light, please chip in.
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The roles of carbohydrates
According to my textbook, carbohydrates serve four distinct roles in the body as follows:
- Energy source: carbohydrates serve as an energy source, particularly during exercise. Also, my textbook reads “daily carbohydrate must be adequate to maintain the body’s relatively limited glycogen stores,” and “once the capacity of cells for glycogen storage is reached, excess sugars are converted to and stored as lipid.
- Preservation of proteins: when glycogen reserves are reduced, glucose can be synthesized from protein and from fatty acids through a process called gluconeogenesis (see ”energy release from food” below). If necessary, this means that the body’s muscle proteins can be digested for fuel.
- Primer for lipid metabolism: carbohydrates serve as a primer for lipid metabolism. If carbohydrate metabolism is insufficient, the body will mobilise a greater amount of lipid than it can metabolise. The result is incomplete lipid breakdown and the accumulation of acetone byproducts called ketone bodies, which may lead to acidosis.
- Central nervous system fuel: carbohydrates are used as fuel for the central nervous system (CNS). The brain uses blood glucose almost exclusively as its fuel and has no stored supply of the nutrient of its own.
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My thoughts and references
Overall, I feel that my textbook overemphasises the importance of carbohydrates. I can see how carbohydrates function as an energy source but I just can’t follow the logic all the way through on the points. Have a look at my comments and see whether you agree:
- Energy source:I think that the statement ”daily carbohydrate must be adequate to maintain the body’s relatively limited glycogen stores” is a little sneaky. Technically, if you break it down and say “in order for the body’s relatively limited glycogen stores to be maintained, daily carbohydrate must be adequate,” then I think this is a reasonable deduction from the studies cited. However, as written, it seems to me that a person could easily read this as implying that “daily carbohydrate must be adequate. This is to maintain the body’s relatively limited glycogen stores.” I don’t think this is true or else the Inuit (and many other hunter-gatherer societies would not have survived.
- Preservation of proteins: to me (as a layman), the idea that carbohydrates have a distinct function as a preserver of muscle tissue proteins seems a little stretched. I mean, the studies that are used to demonstrate this are undertaken in periods of starvation. So technically, I would argue that what has been proved here is that food has a muscle protein-sparing effect. If someone can show me a study that shows a sample group losing muscle mass while eating more than their maintenance level in calories of protein and fat while a similar group consuming similar amounts of carbohydrates does not lose muscle mass then I will agree that this is a valid point (and no, I’m not being shirty, I genuinely would be fascinated to read it).
- Primer for lipid metabolism: the main reason for the body needing carbohydrates to assist in the metabolism of lipids is that acidosis occurs otherwise, and acidosis is a bad thing. However, John Berardi has written about acidosis here. He notes that while animal products are high in acid-forming lipids and proteins, so are high grain diets, which are the typical high-carbohydrate diets. Berardi also notes that acidosis is not a problem per se as “all you have to do is consume enough basic foods and supplements to neutralize the acidity.” So this point seems to be a non-issue.
- Central nervous system fuel: again, I feel that the conclusion is bigger than the cited data seem to allow. After all, it is well known that the brain adapts to using lipids in the form acetoacetate if glucose is not available. So the conclusion seems to be pushing the idea that carbohydrates are essential but, clearly, the body is quite happy operating without them.
Jake still can’t work out why we are so obsessed with carbs…
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The roles of lipids (fats)
Lipids are used in the body as follows:
- Providing an energy store: lipids contain more than twice the amount of energy than an equivalent amount of carbohydrate or protein and are stored with very little water, making them a very concentrated fuel. A male’s fat deposits are estimated as containing c. 95,000 calories, compared to his glycogen reserves of just 2,000 calories.
- Cushioning the vital organs: up to 4% of the body’s fat reserves are used to protect the vital organs, including the heart, liver, kidneys, spleen, brain and spinal cord.
- Providing insulation from a cold environment: subcutaneous fat reserves have been shown to keep swimmers warm in cold waters and experienced cold-water swimmers often have higher fat levels than non-swimmers.
- Vitamin carrier: lipids function as a carrier for the fat-soluble vitamins, including A, D, E and K. A reduction in dietary lipids could lead to a deficiency in these vitamins as a result.
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My thoughts and references
Overall, I don’t have much to say about lipids, other than to note that:
- Lipids also provide a “protein-sparing effect” just like carbohydrates but somehow don’t merit a point to that effect; and
- I don’t agree that the data relating to swimmers supports the conclusion that fat is there to provide insulation from a cold environment. However, the data is comprehensible in the context of the Aquatic Ape Hypothesis, which suggests that we were semi-aquatic for a period of our evolutionary development. One of the factors that suggests this is our high levels of bodyfat compared with other primates. Other key factors include our hairlessness and our noses. I’ll do a book review of this hypothesis at some point.
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The roles of protein
The three main sources of body protein are blood plasma, visceral tissue and muscle. The roles of protein are:
- Anabolism: the process of building muscle tissue is called anabolism and amino acids provide the building blocks for the synthesis of tissue. The protein-containing structures in the body are replaced on a regular basis and new structures are constantly being built. This means that there is a definite need for protein in every person’s diet.
- Acid-base regulation: proteins play a role in regulating the acid-base quality of bodily fluids.
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My thoughts and references
Overall, I don’t have much to say about proteins, other than to direct you to John Berardi’s awesome article on the subject. And that’s a really good note to finish on…
