Macros Part 1: Carbohydrates - Your Body's Essential Fuel
In the world of nutrition, carbohydrates have long been a controversial topic. Demonised by some, misunderstood by many, they are often the first thing people cut when trying to lose weight or "eat clean." So let’s break down what carbohydrates are, how your body uses them and why they’re so important in your diet.
What Are Carbohydrates? The Building Blocks of Energy 🧱
Carbohydrates are one of the three macronutrients (alongside protein and fats) that your body needs in large amounts. Chemically, they are made up of carbon, hydrogen, and oxygen atoms. They come in various forms:
Sugars (Simple Carbs): These are the simplest forms, like glucose, fructose (fruit sugar), and lactose (milk sugar). They provide quick energy.
Starches (Complex Carbs): These are long chains of sugar molecules found in foods like grains, potatoes, and legumes. They take longer to break down, providing sustained energy.
Fibre: A type of carbohydrate that your body can't digest. It's crucial for digestive health, blood sugar control, and satiety. While it doesn't provide you with direct energy, it is the primary fuel source for your beneficial gut bacteria [1], a fascinating process I’ll discuss in a future blog post!
How Carbs Work: The Journey from Plate to Power 🔋
When you eat carbohydrates, your digestive system gets to work, breaking them down into their simplest form: glucose. Glucose is absorbed into your bloodstream, causing your blood sugar levels to rise, which signals your pancreas to release insulin [2].
Immediate Energy: This rise is not inherently bad; in fact, it is the desired and perfectly normal outcome of digestion, signalling fuel is available [2].
When the rise becomes a negative: The blood sugar spike becomes a concern only when it's too rapid or too high, often due to highly-refined, low-fibre carbs consumed without accompanying protein or fat.
Glucose Utilisation: A Prioritised Cascade
Your body manages glucose based on a continuous hierarchy of necessity, with processes happening simultaneously. The glucose is then distributed based on this priority:
Survival Priority (Immediate & Non-Negotiable): The body first ensures that the organs that must have glucose receive it.
Brain and CNS: Glucose is immediately taken up by the brain to maintain essential cognitive function. While the brain can adapt to use ketone bodies for up to 60% of its energy, it still requires the remaining 40% to come from glucose [5].
Red Blood Cells (RBCs): Since RBCs cannot use fat for energy, they always get their 100% energy requirement met first.
Activity Priority (Muscles): Glucose is directed to tissues currently demanding energy.
Active Muscles: If you are currently moving, your muscles absorb glucose to fuel that activity. This process requires insulin, which acts as a key to allow glucose entry into the muscle cells.
Storage and Stability (The Overflow): Once immediate energy demands are met, the remaining glucose is directed to storage:
Liver Glycogen (Stability): The liver takes up glucose to refill its glycogen stores. This is crucial for maintaining stable blood sugar between meals.
Muscle Glycogen (Reserve): Muscle cells take up glucose to refill their glycogen stores, providing the reserve fuel for your next workout or sprint [3].
Conversion to Fat (The Last Resort): Only when glycogen stores are completely full will the significant excess glucose be converted to fat [4].
Why You Can't Live Without Them: The Essential Roles of Carbs
Carbohydrates aren't just "good for energy"; they perform several non-negotiable roles that keep your body and brain functioning optimally.
1. The Brain's Essential Fuel Source and the RBCs' Only Fuel 🧠: Your brain is a glucose guzzler! It requires a constant, steady supply of glucose to function properly. While the brain can adapt to use ketone bodies (derived from fat) for up to 60% of its energy during severe restriction, it still requires the remaining 40% to come from glucose [5].
Crucially, your red blood cells (RBCs) have no mitochondria and rely on glucose for 100% of their energy. Therefore, even if you are "fat-adapted," your body must maintain a baseline supply of glucose to survive.
Without enough readily available glucose, you might experience:
Brain fog and difficulty concentrating
Irritability and mood swings
Fatigue
2. Muscle Power and Performance Fuel 💪: Whether you're an athlete hitting the gym or just navigating your daily routine, carbs are crucial for muscle function.
High-Intensity Exercise: Glycogen is the primary fuel for high-intensity, short-duration activities (like sprinting or weightlifting) and sustained endurance exercise. Depleted glycogen stores lead to early fatigue and reduced performance [3].
3. The Body's "Backup Plan" 📉: This is where the 40% glucose minimum becomes critical. If you drastically cut carbohydrates, your body must still make that essential glucose for your brain (and 100% for your red blood cells).
Gluconeogenesis: The body resorts to a process called gluconeogenesis (literally "making new glucose"). Your body primarily makes this new glucose by breaking down protein [4].
The Consequence: If that protein isn't coming from your diet, your body will start breaking down its own muscle tissue to get the necessary amino acids. Severe carb restriction can lead to muscle loss — reducing your metabolism and overall strength, which is detrimental for long-term health and weight management.
The Health Cost of Carb Restriction: Losing Fibre and Nutrients 💊
Beyond losing muscle and energy, severely restricting quality carbohydrates comes with a significant nutritional cost:
Micronutrient Deficiencies (The Vitamins and Minerals): Foods that contain carbohydrates are also the primary foods that contain the micronutrients and fibre we require. When you eliminate food groups like whole grains, beans, and many fruits, you risk deficiency in critical micronutrients [6].
B Vitamins & Iron: Whole grains and legumes are powerhouse sources of B vitamins (essential for energy production) and Iron (crucial for oxygen transport). Eliminating them impairs your entire energy metabolism.
Magnesium & Antioxidants: Fruits and vegetables provide essential Magnesium (needed for muscle function and recovery) and a wide range of inflammation-fighting Antioxidants and Vitamin C.
Digestive Dysfunction (Lack of Fibre): Fibre, which comes mainly from whole grains, fruits, and vegetables, is essential for a healthy digestive tract.
Gut Health: Fibre acts as a prebiotic, feeding the beneficial bacteria in your gut. A lack of this nutrient can quickly lead to a loss of microbial diversity and negatively impact your overall gut health.
Bowel Regularity: Without adequate bulk and roughage from fibre, a common side effect of low-carb diets is chronic constipation [1] and other digestive discomforts.
The take-away is this: Low-carb diets often encourage replacing nutrient-dense whole foods with higher amounts of fat and protein sources, which may lack the diversity of vitamins, minerals, and, most notably, fibre needed for long-term health.
The Implications for Athletes and Active People:
For those who rely on high-intensity or prolonged activity, cutting carbs is especially problematic:
Impaired High-Intensity Performance: Muscle glycogen is the turbo-charger for sprinting, lifting, and interval training. While a low-carb, high-fat (LCHF) diet will make you highly "fat-adapted" (meaning your body burns fat much more efficiently, even at moderate intensities), studies show this adaptation often sacrifices your ability to burn carbohydrates at high intensities [7]. Your ability to go hard—which is critical for race performance or intense training—is blunted.
No Clear Performance Advantage: For endurance events, studies generally show that LCHF adaptation does not lead to a clear enhancement of performance compared to a high-carbohydrate approach. In fact, it often impairs exercise economy, meaning you use more oxygen to achieve the same speed or power [7].
Muscle Loss: If you drastically cut carbohydrates, your body must still get glucose for your brain. It resorts to a process called gluconeogenesis (literally "making new glucose"). Your body primarily makes this new glucose by breaking down protein [4]. If that protein isn't coming from your diet, your body will start breaking down its own muscle tissue.
Why People Start by Cutting Carbs (and What Actually Happens) 🧐
Many people choose low-carbohydrate diets because they see rapid initial results. However, this is largely due to misleading metabolic changes:
The Myth: People believe the quick weight drop is fat loss.
The Reality: The rapid loss is mostly water weight. When you cut carbs, your body quickly depletes its glycogen stores. Glycogen is hydrophilic and therefore binds to large amounts of water (about 3 grams of water for every 1 gram of glycogen). When glycogen goes, the water goes with it. This quick "win" is often followed by a stall or rebound.
It is important to remember this fundamental principle of energy balance: An excessive intake of any or all calorie-providing macronutrient(s)—be it fat, carbohydrates, or protein—will inevitably lead to a calorie surplus, resulting in weight gain [2]. This is precisely why, if you are looking to lose weight, the focus should be on lowering total calories, not on cutting out entire food groups!
The Bottom Line: Choose Wisely, Don't Eliminate ⚖️
Carbohydrates are not the enemy. They are fundamental to your body's energy production, cognitive function, and athletic performance. The key isn't to eliminate them, but to understand and choose them wisely.
Focus on quality carbohydrates that are minimally processed and rich in fibre:
Whole grains (oats, brown rice, quinoa, whole-wheat bread)
Fruits
Vegetables
Legumes (beans, lentils)
Limit highly refined sugars and processed carbs that offer little nutritional value beyond quick energy, unless you are specifically using them to rapidly fuel or recover from intense exercise. For quick energy needs before, during or immediately after a hard session, easily digestible sugars are a really important tool in an athlete's toolbox.
By embracing carbohydrates as an essential part of a balanced diet, you'll be fuelling your body for optimum performance, both mentally and physically.
NEXT UP: Macros Part 2! We dive into the importance of protein—your muscle-building powerhouse!
References
[1] Fibre as prebiotic; gut health/constipation. Monash University. (n.d.). Prebiotic diet - FAQs. Department of Gastroenterology. https://www.monash.edu/medicine/translational/gastroenterology/prebiotic/
[2] Calorie surplus causes weight gain; normal glucose signal. Institute of Medicine (US). (1995). Weight-Loss and Maintenance Strategies. NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK221839/
[3] Glycogen storage location and role as fuel for exercise. Achten, J., et al. (2018). Fundamentals of glycogen metabolism for coaches and athletes. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC6019055/
[4] Fat storage regulation; Gluconeogenesis/protein breakdown. Achten, J., et al. (2018). Fat oxidation, lipolysis, and muscle anabolism... Clinical Nutrition ESPEN. https://pmc.ncbi.nlm.nih.gov/articles/PMC6019055/#sec2
[5] Brain's preference for glucose; 40% minimum. Mergenthaler, P., et al. (2013). Sugar for the brain: the role of glucose... PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC3900881/
[6] Micronutrient density in whole foods. Better Health Channel (Victorian Gov). (n.d.). Cereals and wholegrain foods. https://www.betterhealth.vic.gov.au/health/healthyliving/cereals-and-wholegrain-foods
[7] LCHF impaired high-intensity performance/economy. Burke, L. M. (2015). Re-Examining High-Fat Diets for Sports Performance... Sports Medicine. https://pmc.ncbi.nlm.nih.gov/articles/PMC4672014/
