Position Paper for Diet Typing Systems
In its simplest terms, metabolic biochemistry is just “fuel in, energy out”. The energy derived from the foods you eat is used to drive all the processes going on in your body. These include the building of proteins (muscles, tissues and organs), DNA (your genetic material), and fat.
Fuel for the human body comes in only two primary forms: carbohydrates (sugars), and fat. The body rarely uses protein as a fuel source, and when it does it is almost always under conditions of starvation. To use protein as fuel, the protein must first be disassembled into its individual amino acids, and only then into sugars and fats. This process of converting protein into fat and sugars for metabolic fuel is complex, slow and inefficient. Because of this undeniable biochemical fact, there really is no such thing as a “Protein Type” of metabolism. Only when sugar and then fat stores are depleted does the body begin digesting non‐essential proteins and then finally essential proteins, which ultimately leads to organ damage and death. The body does derive some fats and carbohydrates from the proteins we eat, but the protein that remains is then processed into amino acids, not metabolic fuel. Thankfully, the body is efficient and uses the best available fuel first before it has to tap into essential reserves.
Carbohydrates
Carbohydrates, or “carbs”, are simply sugar molecules linked to one another in varying arrangements. For example, starch, the most important carbohydrate in the human diet, is nothing more than numerous glucose molecules linked together in a long strand. Potatoes are an excellent example.
Another example of a carbohydrate is glycogen. Glycogen is how humans store excess glucose (a single sugar molecule) for later use. Unlike starch, which is a long chain of individual glucose molecules, glycogen is a highly branched structure that allows the body to rapidly cleave off individual sugar molecules to be burned for energy.
Carbohydrates can be further broken down into 2 categories: simple and complex. We’ve all heard of the term “complex carbohydrates”, which is a fancy way of saying multiple sugar molecules linked together in a complicated way. Contrarily, a simple carbohydrate is merely a few (usually 1 to 3) sugar molecules linked together.
Why make the distinction between simple and complex carbs? For starters, simple carbohydrates are rapidly absorbed by the gut and enter the bloodstream very quickly. Candy bars are a great example. If you need a quick boost of energy unwrap a Snickers!
The problem is that since simple carbs enter the bloodstream so rapidly they get metabolized quickly. This causes you to lose that energy boost fast, which is why you often feel “de‐energized” an hour or so after eating “junk food”. In contrast, complex carbs get degraded by the gut much less rapidly, and therefore slowly trickle into the bloodstream. This gives you a more sustained, but less pronounced energy boost. Whole grains are a great example of complex carbs.
Carbohydrates are almost always the first energy source that is utilized during exercise. This forms the basis behind “carbo loading”, or eating a meal rich in carbohydrates the night before, or morning of, a planned work out. During exercise, the body will then utilize the individual sugar molecules in the carbohydrates to provide energy for your muscles and brain. Once you run out of sugar (or the form that humans store it in, glycogen) your body turns to using fat.
Fats
All human beings have a certain percentage of body weight that is fat. From an evolutionary stand point this is advantageous. During times of drought or famine there were not enough crops to provide adequate carbohydrates, and thus humans survived by “burning” their fat stores. In biochemical terms, fat is nothing more than long chains of carbon atoms linked together. Suffice it to say that it is the carbon in the fat that gets utilized to form energy that your muscles and other body tissues use.
Why not burn fat first? Because fat is not as efficient an energy provider as sugar. This is the reason that endurance athletes, a few hours into a work out, hit the proverbial “wall”. The wall represents the point where they have burned up all the carbohydrate in their body, and are now running on fat reserves. The decreased amount of energy gained per unit of fat, when compared to what you get with carbs, results in a relative feeling of fatigue.
These principles can also be used as a weight loss system. Using the basics of carbohydrate and fat metabolism it makes sense that people have difficulty losing weight when they exercise vigorously for only half an hour. This is because the quick vigorous exercise burns mostly carbohydrate stores in the liver (ie: glycogen); the body never touches its fat reserves.
In contrast, running a marathon (or a nice long walk or jog in the park) causes the body to tap into its fat reserves. This is also the idea behind exercising early in the morning before having breakfast. In the morning your body has been burning carbohydrates to keep all your organs functioning; therefore, in the morning your body has less carbohydrate available to burn because it was slowly getting eaten away during sleep. If you exercise at this point you’ll have to tap into your fat stores earlier than you normally would.
Proteins
The third and final fuel is protein. The body rarely uses protein as its sole fuel source, and when it does it is usually under conditions of starvation. Interestingly, when no carbohydrate is present in the diet, the body will use the amino acid backbones of protein to form glucose (a carbohydrate) in order to supply the brain with adequate energy.
It was once thought that protein provided the energy that athletes used during exercise. This was the basis behind the “steak‐and‐eggs” breakfast prior to an athletic event. This has been disproven by biochemists. It is a simple fact that the body prefers to burn carbohydrates, then fat, and finally protein if all else fails.
Overview
The three main fuel sources in humans are carbohydrates, fats, and proteins. They are used preferentially under different conditions. In general, the body burns carbohydrates, then fats, and then proteins, in that order.
It is important to realize that energy metabolism is not an “all‐or‐none” phenomenon. The body is constantly fine tuning the exact blend of carbohydrate, fat, and protein metabolism to ensure the appropriate supply of energy to the body’s tissues.
References and Resources
- Champe PC. Lippincott’s Illustrated Reviews: Biochemistry. Second Edition. Lippincott‐Ravens Publishers, 1992.
- Nelson DL, Cox MM. Lehninger Principles of Biochemistry. Fifth Edition. New York: Worth Publishers, 2008.
- Summerbell CD, Cameron C, Glasziou PP. WITHDRAWN: Advice on low‐fat diets for obesity. Cochrane Database Syst Rev. 2008 Jul 16;(3):CD003640.
- Elliott SA, Truby H, Lee A. Associations of body mass index and waist circumference with: energy intake and percentage energy from macronutrients, in a cohort of Australian Children. Nutr J. 2011 May 26;10(1):58. [Epub ahead of print]
Carbohydrates produce 4kcal/g, proteins produce 4kcal/gram and lipids (fat) produce 9kcal/gram. The major form of digestive fat that circulates through the plasma is called a triglyceride, which consists of three fatty acid hydrocarbon chains bonded to a glycerol backbone, are packaged into lipoproteins transporters e.g. chylomicrons, very low density liporoteins, LDLs, HDLs etc. that travel through the blood plasma before being cleaved by lipoprotein lipases i.e. triglyceride cleaving enzymes bound to glycoproteins along the luminal surface of vascular endothelial cells. The carbons in the liberated fatty acids are at a lower oxidation state than carbons in carbohydrates or proteins. Lower oxidation states release more free energy when bonds are broken during oxidation through the citric acid cycle to ultimately produce High energy intermediates (e.g. ATP) as well as CO2 and H20 byproducts. This is why fat is stored and why excess carbohydrates are converted to fats. This is also why excess proteins are degraded through the urea cycle to ultimately be excreted when the body has sufficient levels of amino acid substrates for biosynthesis.