Ask almost anyone what the body’s main source of energy is and you’ll get the same answer: carbohydrates. A simple Google search or a query with your favourite AI tool will return similar language: Carbohydrates are both the main and preferred fuel source for the human body. It’s a narrative that has dominated conventional nutrition thinking for decades, echoed by doctors, dieticians and fitness influencers alike.
There is a shred of truth here. For those eating a high-carbohydrate diet, carbohydrates will indeed be the primary fuel source. The body has two distinct energy pathways available to it: one for carbohydrate metabolism and one for fat metabolism. Given that most people in the world today eat a diet rich in carbohydrates it would appear as though carbs are the body’s default fuel source. However, when we dig deeper into the hard sciences, human biochemistry and paleo-anthropology, as well as the alarming rise of chronic illness, a very different picture begins to emerge.
What Did Humans Actually Eat
As I explored in my post about the ancestral dietary framework, our ancestors ate a diet focused primarily in fatty ruminant animal meat. Humans did eat some fruit, nuts, seeds, roots and tubers, but these were secondary to animal based foods, with the amounts eaten varying by seasonal and regional availability. When we hear fruit, we often think of the very sweet, large, colourful tropical fruit that dominates our grocery stores today. However, most of these modern fruits are the product of human agricultural techniques, and the fruits available to our ancestors were far smaller, less sweet, and more fibrous, providing significantly less carbohydrates. Similarly the roots and tubers available to our ancestors looked nothing like the modern carrots, potatoes and sweet potatoes we see today, with most being very fibrous and lacking the starch and sugar in our modern varieties. For the vast majority of our evolutionary history, carbohydrates were scarce and the human body was running on fat, not glucose as its primary fuel.
Evolution is a long-running optimization process, with millions of years of selective pressures shaping exactly how our bodies function. It is reasonable to assume that our bodies are best adapted to the conditions in which we evolved. Therefore, our metabolism, which has been fine-tuned for low-carbohydrate availability is not optimized for the carbohydrate based diet that plagues modern society.
The Glucose Preference Myth
One of the most common arguments in defence of carbohydrates being the body’s main fuel source is simple: when carbohydrates are present, the body uses them to create fuel first, prioritizing their use over fat. This is true, but absolutely does not indicate that they are a superior fuel source.
Your body is optimized to keep blood glucose in a vary narrow range of roughly 4-5 grams or one teaspoon circulating in the entire bloodstream at any given time. Elevated blood glucose beyond this threshold is toxic. Free-floating glucose molecules readily bind to proteins and tissues in a process called glycation, causing structural damage that accumulates quietly over time and is implicated in joint degradation and accelerated aging. Your body responds to elevated blood glucose by increasing insulin, essentially forcing sugar into your cells to get it out of the bloodstream as quickly as possible. Glucose is prioritized as a fuel not because it is superior but because leaving it circulating is damaging.
The body does require some glucose to perform certain functions, but this does not mean that exogenous carbohydrates are required. Glucose is a structurally very simple molecule that the body can produce on its own through a process called gluconeogenesis, using raw materials like the glycerol backbone of dietary fats and excess amino acids from dietary protein. The tightly regulated one teaspoon that the body needs is easily maintained through this process, which is precisely why there is no established requirements for carbohydrates in the human diet.
Consequences of Carbohydrates
Given that the human body tightly regulates blood glucose at such minuscule levels, consuming carbohydrates will almost always push levels beyond what the body actually needs. The consequences of doing this chronically operates on a few different levels.
The most immediate problem is the glycation damage described above. Structural damage caused by excess glucose circulating in the blood accumulates over time and can accelerate aging and chronic disease progression.
A second consequence comes from the cross-inhibition of different metabolic pathways: a phenomenon known as the Randle Cycle. The body has two primary methods of producing ATP (the body’s energy currency): cellular respiration, which breaks down carbohydrates, and beta-oxidation which breaks down fats. The byproducts created by each pathway will inhibit the reactions involved in the other. A mixed-macronutrient diet, one high in both carbohydrates and fat can therefore lead to a less efficient metabolism. The result is reduced mitochondrial function, increased production of reactive oxygen species and greater inflammation [1]. Committing to one primary fuel source resolves this issue, and for the reasons discussed here that fuel source should be fat.
Consuming excessive exogenous carbohydrates will also imbalance the ratio of two critical metabolic hormones: insulin and glucagon. Insulin is anabolic and pro-inflammatory. It is released from the pancreas in response to rising blood sugar, signalling the body to take glucose out of the bloodstream and either send it to the cell to use for energy, to the muscle to be stored as glycogen, or to the liver to make and store fat. Glucagon is its counterpart and is catabolic and anti-inflammatory. It is released when blood sugar falls, stimulating the breakdown of stored fat and the production of new glucose via gluconeogenesis. When these hormones are well balanced, the body maintains a healthy equilibrium between storing and burning energy, keeping inflammation in check and fat mass regulated. A high carbohydrate diet keeps insulin elevated and glucagon suppressed, tipping this balance in a direction that promotes fat storage, suppresses fat burning, and drives a low-grade inflammatory state that underlies many chronic health issues.
Why Low Carb Works
The benefits of a low-carbohydrate diet follow directly from everything above: less glycation damage, a balanced insulin to glucagon ratio, and less Randle cycle activation, freeing the fat metabolic pathway to operate without inhibition. Over time the body becomes increasingly efficient at using fat as its primary fuel, a state known as fat adaptation. Those who have made this transition, including myself, consistently report more stable energy, reduced hunger and improved mental clarity.
When Carbohydrate intake is reduced sufficiently the body enters a state of nutritional ketosis. This is where the liver begins producing ketones, an alternative fuel source particularly effective at fueling the brain. Nutritional ketosis seems to confer additional benefits including reduced oxidative stress, improved mitochondrial efficiency, and increased mitochondrial biogenesis (the creation of new mitochondria) [2]. These mitochondrial health benefits have made ketogenic metabolic therapy a promising area of research, with researchers like Chris Palmer proposing it as a potential therapeutic tool for mental illness [3], and Thomas Seyfried suggesting it as an adjunctive cancer treatment [4].
Is a Low-Carb Diet Right For You
The biochemical case for reducing carbohydrates applies broadly, and in my opinion most people will benefit from reducing carbohydrate intake and eating in a macronutrient distribution more closely aligned with our ancestors. However, some people may feel its benefits more significantly than others. If any of the following resonate with you, a low-carb diet may be worth serious consideration.
Struggle to Lose Weight
If you have been struggling to lose excess body fat despite reasonable efforts, the anabolic, pro-inflammatory effects of carbohydrates and insulin, as well as Randle-cycle driven slowing of mitochondrial function are likely working against you.
Inflammatory Issues
If you experience chronic pain, autoimmune, or other inflammatory conditions, reducing dietary carbohydrates is one of the most effective levers for down-regulating the inflammatory response.
Collagen issues
If joint pain, poor skin, or other connective tissue disorders are at play, the cumulative effects of glycation damaging your collagen might be a significant contributing factor.
Mental health and neurological issues
If you deal with mood instability, brain fog, low energy, or have been diagnosed with a mental health condition, consider trying a ketogenic approach. The emerging research of ketogenic metabolism and brain health is very promising and makes this an avenue worth exploring.
Cancer
If you are navigating a cancer diagnosis or concerned about potential cancer risk, the relationship between glucose availability, metabolic health, and cancer progression is an area of growing research with some compelling early results.
General Wellness
If you want to feel better, think more clearly, and age slower, the reduction in oxidative stress, glycation damage, and inflammation that occurs in a properly formulated low-carbohydrate diet is worth exploring. You might be surprised about how good ordinary days begin to feel.
References
- Randle PJ, Garland PB, Hales CN, Newsholme EA. The glucose fatty-acid cycle: its role in insulin sensitivity and the metabolic disturbances of diabetes mellitus. The Lancet. 1963.
- Greco T, Glenn TC, Hovda DA, Prins ML. Ketogenic diet decreases oxidative stress and improves mitochondrial respiratory complex activity. Journal of Cerebral Blood Flow & Metabolism. 2016.
- Norwitz NG, Sethi S, Palmer CM. Ketogenic diet as a metabolic treatment for mental illness. Current Opinion in Endocrinology, Diabetes and Obesity. 2020.
- Salido-Bueno et al. Effects of ketogenic diets on cancer-related variables: a systematic review and meta-analysis of randomised controlled trials. Nutrition Bulletin. 2024.