Introduction
In the past decade, carbohydrates have become the dietary villain of popular culture. Low-carb, ketogenic, and carnivore diets have all built followings by positioning carbs as the cause of obesity, metabolic disease, and poor health. The biochemical reality is considerably more nuanced — and for active individuals, the case for carbohydrates is compelling.
What Carbohydrates Are and How the Body Uses Them
Carbohydrates are organic compounds made of carbon, hydrogen, and oxygen. Dietary carbohydrates are broken down in the digestive system into glucose, which enters the bloodstream and is transported to cells for energy production.
Glucose is the preferred fuel for the human brain (which consumes approximately 120g per day), the nervous system, and working muscles during moderate-to-high intensity exercise. It is also the only fuel the red blood cells can use.
When carbohydrate intake exceeds immediate energy needs, glucose is stored as glycogen — primarily in the liver (approximately 100g capacity) and skeletal muscle (approximately 400–600g capacity, depending on muscle mass). Muscle glycogen is the primary energy substrate for any exercise above approximately 65% of VO2 max.
When glycogen stores are depleted — as occurs during prolonged moderate-to-high intensity exercise — performance declines sharply. Athletes call this "hitting the wall." It is not a metaphor; it is a physiological reality driven by fuel depletion.
Simple vs. Complex Carbohydrates
Not all carbohydrates behave the same way in the body.
Simple carbohydrates (monosaccharides and disaccharides) are digested and absorbed rapidly, causing a faster rise in blood glucose. Examples include glucose, fructose, sucrose, honey, fruit, white bread, and sports drinks.
Complex carbohydrates (polysaccharides) are longer chains that require more enzymatic processing to break down, resulting in a slower, more sustained rise in blood glucose. Examples include oats, sweet potatoes, brown rice, whole grain bread, legumes, and vegetables.
The glycaemic index (GI) formalises this distinction — high-GI foods cause rapid glucose rises; low-GI foods cause gradual ones. For general health and body composition, lower-GI complex carbohydrates should form the majority of carbohydrate intake: they provide sustained energy, support gut health through fibre, and avoid the insulin spikes and energy crashes associated with high-GI foods.
However, high-GI carbohydrates are actually preferable in specific contexts: immediately before or during sustained exercise (rapid energy availability), and in the 30–60 minutes after training (accelerated glycogen resynthesis).
The Importance of Carbohydrate Timing Around Training
Carbohydrate timing is one of the most practically relevant aspects of sports nutrition.
Pre-training (2–4 hours before): A moderate-to-large meal containing complex carbohydrates (1–4g/kg body weight) provides the fuel reserves needed for moderate-to-high intensity training. This is the window for foods like oats, rice, potatoes, and whole grain bread.
Within 60 minutes before training: A smaller, easily digestible carbohydrate source (banana, white rice, sports gel) tops up glycogen without digestive discomfort.
During training (over 60 minutes): Carbohydrate intake during prolonged exercise (30–60g per hour for sessions over 60–90 minutes) significantly extends endurance performance and delays fatigue. Sports drinks, gels, or easily digestible real foods serve this purpose.
Post-training: Consuming carbohydrates (0.8–1.2g/kg body weight) alongside protein in the 30–60 minutes after training accelerates glycogen resynthesis, supports insulin-mediated amino acid uptake, and reduces muscle protein breakdown.
Why Low-Carb Diets May Hinder Performance for Active Individuals
Ketogenic and very low-carb diets have demonstrated metabolic benefits for sedentary individuals and may be appropriate in certain clinical contexts. For active individuals engaged in moderate-to-high intensity training, the evidence for performance is less favourable.
The fundamental issue is fuel availability. At exercise intensities above 65–70% of VO2 max — which includes most recreational sport, resistance training, and interval work — the body's capacity to oxidise fat at a sufficient rate lags behind the energy demands of performance. Glycogen is required.
Studies on trained athletes transitioning to ketogenic diets consistently show:
- Reductions in high-intensity power output
- Elevated RPE (perceived exertion) at equivalent workloads
- Impaired sprint performance
- Reduced ability to complete high-volume training blocks
Some adaptations to fat oxidation do occur after 3–4 weeks of low-carb adaptation, and endurance performance at lower intensities may be maintained. But for anyone whose training involves significant periods above Zone 2 intensity, carbohydrate restriction is likely to impair training quality and adaptation.
The conclusion is not that everyone needs to eat large quantities of carbohydrates. It is that carbohydrate intake should be matched to training demands. A sedentary individual has modest carbohydrate needs. An athlete training 8–12 hours per week has very high ones. Demonising an entire macronutrient ignores this reality.
Carbohydrates are not your enemy. They are the fuel your body evolved to run on at the intensities that make fitness actually improve.