Considering keto or a low-carb diet? Learn how your body can use fat as its primary fuel source. Explore below the science of ketosis, its evolutionary roots, and related health benefits.

What is Ketosis?

Ketosis uses fat for fuel.

More precisely, ketosis is a metabolic state where your body relies on ketones as its primary energy source.

Ketones are molecules produced when the body breaks down fat, which then serves as fuel for your cells.

It’s one of the two main pathways through which our body generates energy. With the other one being the sugar-burning pathway, known as glycolysis.

In contrast to ketosis, glycolysis uses glucose (the simplest form of sugar) as fuel. Glucose can be obtained from simple carbohydrates (e.g., glucose, sucrose, fructose, lactose) or complex carbohydrates like starch.

In Westernised societies, we primarily rely on glucose for energy, which makes switching to ketosis quite challenging. Since our bodies tend to prioritise glucose when sugar and fat are both present.

This is why, to achieve ketosis, it’s generally recommended to limit carbohydrate intake to no more than 20-30g per day.

Teaching your body to tap into ketosis as a metabolic pathway can be an effective way to shed weight and improve overall health.

Constant reliance on carb-burning as the primary metabolic state can lead to weight gain, sugar dependence, and a lack of metabolic flexibility.

Burning Fat vs. Sugar

Carbohydrates are a quick-burn energy source that our bodies are adapted to seek and utilise immediately.

This stems from the historical scarcity of carbohydrates throughout human evolution.

We’re biologically programmed to consume carbs in small amounts and either burn them quickly or store them for later use.

In contrast, fat is a more sustained energy source, providing a steady supply of energy during missed meals, prolonged physical activity, or intermittent fasting.

Fat is also more energy-dense, producing significantly more energy from one molecule of fat (around 146 ATP) compared to one molecule of glucose, which produces only about 36 ATP.

ATP (adenosine triphosphate) is often referred to as the “energy currency” of the body.

It fuels all bodily processes, from cellular functions to muscle movement and brain activity, and is found in all plants and animals.

ATP is why your body needs energy-producing systems like ketosis and glycolysis in place.

However, to turn your body into a fat-burning machine—whether that’s burning stored fat or dietary fat—you need to develop metabolic flexibility.

This involves the challenging process of drastically reducing carbohydrates and sugars for several weeks, whilst increasing your intake of healthy fats.

Our Evolutionary Heritage

It’s likely that our ancestors were in and out of ketosis for a large part of their lives. 

Back in the olden days, and I’m not thinking great-great-grandparents, but over 10,000 years ago.

In the hunter-gatherer times, before agriculture and crop management kicked in, our ancestors were adapted to a feast-or-famine cycle.

Meaning our hunter-gatherer predecessors often experienced shifts between periods of food abundance and food scarcity.

We know this thanks to ethnographic studies, isotope analyses, dental examinations, archaeological remains, and genetic studies.^[1]

This evidence shows that early human diets underwent major shifts around 1.5-2 million years ago, as climate changes reshuffled the landscape.

Forests decreased, grasslands expanded, and fruit became scarce. This increase in scarcity forced our hominid ancestors to adapt during these difficult times.

One branch of our family tree, Paranthropus, adapted by developing robust skulls to chew tough vegetation.

However, our direct ancestors, Homo, evolved with a different strategy—specialised metabolic flexibility that conserves fat in times of food scarcity.^[2]

This is where ketosis comes in: the ability to switch from burning sugar to fat for fuel. This metabolic flexibility would’ve been a real lifesaver during those unpredictable feast-famine cycles our ancestors endured.

But here’s the kicker: modern genetics shows we’re still largely adapted to these ancestral pressures.

See, in today’s world of constant calorie overabundance, this metabolism—fine-tuned for scarcity—is taking a toll.

Many modern diseases linked to overconsumption are actually rooted in the failure to tap into our ancestral metabolic flexibility.

For instance, type 2 diabetes, cardiovascular diseases, and non-alcoholic fatty liver disease are metabolic conditions often caused by a lack of metabolic flexibility.

That is, diseases resulting from the mismatch between our evolutionary metabolism and our modern lifestyle of overabundance.^[3]

Talk about a double-edged sword, huh?

Final Words

Ketosis is one of the two main metabolic pathways our body uses to produce energy in the form of ATP. This pathway relies on ketones, which are produced from fats.

Our other primary metabolic pathway, glycolysis, uses sugar to produce energy, but it is less sustainable during metabolically demanding situations, such as extended physical exercise, missed food breaks, or illnesses.

The ability to switch to ketosis and burn fat when needed is not only sustainable but also indicates that you are metabolically flexible. Metabolic flexibility plays a preventative role in common metabolic diseases, such as type 2 diabetes.

However, the ultimate goal of practising ketosis isn’t to completely cut carbohydrates out of your life forever. It’s about finding that sweet spot where your body can efficiently use both fat and carbohydrates for fuel.

This may require teaching your body to enter ketosis if you’ve been on a carb-based diet for most of your life. While this process can be difficult and may take several weeks to achieve, it could also be your ticket to metabolic flexibility, a leaner body, and improved health.^[4]

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