Photo by ollily.

“All men by nature desire knowledge. ”
- Aristotle

It wasn’t until recently that I really grasped what happens to each macronutrient inside the body. The main reason is that most articles that explain how metabolism work are biased toward a particular diet. Low-carb, low-fat, South Beach, even vegetarians put their own spin on how the body works. I didn’t know what to believe. After some selective Googling and piecing together bits of information from Wikipedia I think I’ve deduced what happens to my dinner.

Monday night we were in a hurry and I ate a Smart Ones Ravioli Florentine as I sat down to watch the new “Little People, Big World” — the one that Matt goes to court for a DUI. It has 250 calories, 5 grams of fat, 11 grams of protein, and 40 grams of carbohydrates — 12 of which were sugar. Some of these will be stored as fat and some will be used by the body but each in a different way.

First, why do we store fat? Fat is stored in defense of famine. The human body has remained relatively unchanged since the Paleolithic era which ended around 40,000 years ago. At the time, food scarcity in winter was inevitable. The human body was designed to store fat to provide energy during those times of famine. This is true of most animals. Specialized cells called adipose cells store and synthesize fat. Adipose tissue is primarily located just under the skin but is also found between muscles and around organs. The location of fat deposits is largely genetic and can differ from person to person.

Back to my dinner. I consumed 40 grams of carbohydrates. Carbohydrates are always associated with energy. This is because, of all nutrients, it converts to glucose easiest. Glucose is the body’s preferred fuel source. If your body is a Prius, glucose is the battery. When I ate the 40 grams of carbohydrates they were converted to glucose in the digestive tract and distributed via the liver to cells throughout the body for use as energy. Once energy demands are met — sitting on the couch didn’t take a lot — the remaining carbohydrates are handled in one of two different ways. Either they are converted to glycogen — primary short term energy source — and stored in the liver or they are converted to fatty acids by the liver and stored in adipose tissue around the body. Simple carbohydrates are converted faster than complex and therefore glucose rises faster in the bloodstream and insulin rises faster in reaction. Unless you are doing exercise after consuming simple sugars there is a good chance the majority will be stored as fat and with the insulin increase you’re priming your body to store even more fat from other nutrients.

The 11 grams of protein is broken down into amino acids in the stomach and small intestine. The liver then distributes the amino acids throughout the body where they are used for cell formation and repair — for example, muscle growth. The liver converts most unused amino acids to glucose which can be uses as energy or stored as fat — just like carbohydrates. Some amino acids remain circulating in the blood stream for future use.

The 5 grams of fat is broken down into fatty acids and glycerol in the stomach and small intestine. Only five percent of the fat becomes glycerol which is then converted to glucose in the liver. The bloodstream and lymphatic system distribute the fatty acids to the cells for a variety of specialized uses. The vast majority of unused fatty acids is stored as fat in adipose tissue.

It seems like my body is really working against me. I’m not feeling too grateful for all those years of famine adaptation. Luckily the fat in adipose tissue can be used as energy as well. If the body suddenly needs energy it gets it from the liver and muscle in the form of glycogen — remember, this is what happens to some of the left over glucose. After these reserves are used up the body releases fatty acids into the bloodstream and the liver does its job to convert them to usable energy.

From this description it’s easy to see why low-carb and low-fat are two popular dieting themes. If you limit the amount of simple carbohydrates or carbohydrates in general then you will force the body to use fat reserves for energy. In contrast, since dietary fat usually ends up in adipose tissue it makes sense to low-fat dieters to limit fats.

I have read many articles that mention how the body metabolizes macronutrients but never really paid much attention. I think it is all very interesting, however, even though I have a suspicion I might be the only one.

I’ve simplified and tried to paraphrase some of the dense bits. I’m not an expert. I went to school for Electrical Engineering and took nothing further than Chem 101. If anyone sees any major errors please leave a comment!