One thing that always miffs me is the characterization of ketogenesis as a “starvation” diet.
Why do scientists privilege glycolysis as the chemical for our bodies’ metabolic process? Why do they say that fats and ketones and amino acids can be utilized “in the absence of glucose” or during “starvation” or serve as an “alternative energy source”?
I ran across this characterization in a current high school biology course (I work for the school district). I ran across it in a biochemistry class I was watching on The Great Courses. And I run across it on Research papers and blog after blog; using ketones and fats for fuel is not normative but an exception is the common view. But why?
Why are ketones considered an abnormal fuel and glucose considered normal fuel? It’s not a matter of what your body can use for fuel. Energy starts at the citric acid (Krebs) cycle. Acetyl CoA enters the cycle where most of the energy is created through NAD, FAD, and ATP. So where does the Acetyl CoA come from? The answer is it can come from many places:
- Carbohydrates become Acetyl CoA through pyruvate
- Fatty acids become Acetyl CoA through acetoacetate.
- Amino acids become Acetyl CoA through acetoacetate.
- Ketones become Acetyl CoA through acetoacetate.
Why is one of these paths preferred over the others? As one blogger wrote, “Most of what you eat that’s going to be consumed for energy gets converted into glucose before it gets metabolized.” Maybe most of what you eat, not what I eat. Maybe most of what a 21st century AD woman eats, but not what a 21st century BC woman ate.
A bias exists that assumes carbohydrates are our natural, normal, expected, or preferred food source. That is, the scientific explanation is based on a cultural choice. What evidence is there that the citric acid cycle “prefers” Acetyl CoA from pyruvate? What difference does it make? Research has shown that different substrates are differently efficient:
Ketone bodies are suggested to be a more energy efficient substrate than glucose or fatty acids [5,25]. However, energy efficiency may be interpreted in multiple ways. In terms of ATP production efficiency, fatty acids yield ~6.7 ATP per carbon atom, compared to ~5.2 for glucose and ~5.4 for ketone bodies [26,27]. For many researchers, energy efficiency refers to the ATP yield per oxygen atom (i.e., P/O ratio). The P/O ratio for fatty acids is ~2.33, while glucose’s is 2.58 and ketone bodies’ is 2.50 [26,27]. For some researchers, cardiac (or muscle) efficiency is a critical measure, which determines the ratio of mechanical work to oxygen consumed. Determination of these values requires complex experimental designs and data collection techniques, which are commonly performed in isolated, perfused heart preparations. In addition, the intricate interactions of certain biochemical pathways may lead to increased energetic costs and/or losses, particularly for fatty acids . Ketone bodies combined with glucose were shown to elicit a higher cardiac efficiency relative to glucose alone [28,29]. Although ketone bodies increased energy production, particularly in hypertrophied hearts, a significant improvement in cardiac efficiency was not achieved . Whether provisions of ketone bodies, glucose, and/or fatty acids lead to a significant alteration in cardiac or skeletal muscle efficiency in healthy or diseased states remains unclear (Ketogenic Diets and Exercise Performance)
- One can say: Ketone bodies are produced mainly in the mitochondria of liver cells, and synthesis can occur in response to an unavailability of blood glucose, such as during fasting.
- One can say: Ketone bodies are produced mainly in the mitochondria of liver cells, and synthesis occurs in response to the body’s need for energy.
Anthropology has shown through chemical analysis of bones and excavation of hominid sites, that ancient man — the millennia during which our metabolic processes evolved — was primarily a consumer of large herbivores, on the same trophic level as carnivores like wolves (visit the Max Planck Institute for Evolutionary Anthropology). In a reverse of what the scientists are saying, vegetables, fruits, and nuts were utilized when meat was scarce. That means human metabolism was formed primarily on fats, ketones, and amino acids.
One argument I’ve met with is that breaking down carbohydrates for fuel is so much easier for our metabolisms, so doesn’t that mean it’s preferred? Let’s think about that.
What’s the value in utilizing and storing glucose quickly? I mean, when you eat fat and protein, they digest to fatty acids and amino acids and float around in your blood stream for hours before they are used or stored. Is your body ignoring them as it waits for more glucose to be created from the carbohydrates you could be eating? Of course not. They float around because there is no reason to redirect resources to use or store them right away.
The thing is, sugars strongly interfere with your body’s ability to regulate itself, which is why carbohydrates are so much more problematic than other macronutrients you can eat. Your blood can maintain only so much safely. This is more likely the reason we evolved a metabolism to use it right away. Use it or store it…just get it out of the blood. Amino acids and fats, on the other hand, can float around for hours until your body can use them because amino acids and fats don’t cause metabolic problems.
As humans, we are adaptable in many ways. We can use all kinds of foods to fuel and build our bodies. Recent custom has us utilizing a diet high in carbohydrates. A high-carbohydrate diet has been shown to be difficult to maintain historically (before the arrival of cities, modern distribution networks, and specialized farming techniques) and detrimental nutritionally, as the last 50 years of ever-increasing volume of carbohydrates has shown.