Why Your Early Morning Swim Sessions Could Be Slowing You Down
Why Swimming Burns More Than You Think
Among the three triathlon disciplines swimming often feels disproportionately taxing - and for good reason. Scientific research shows that moving through water demands up to four times more energy than moving the body through air. This is primarily due to hydrodynamic drag: the water’s density creates a resistive force that must be overcome with every stroke. For triathletes, each movement becomes a battle against this resistance, making swimming a uniquely energy-hungry part of training and racing.
Even over short distances, the energy cost of freestyle swimming can be 0.8–1.2 kcal per kilogram per kilometre, meaning a 75 kg athlete might burn 600–900 kcal in a typical endurance session.
However, most triathletes sit on the less efficient end of this spectrum. Technical inefficiencies like poor body alignment, dropped hips, or suboptimal catch mechanics dramatically increase drag and reduce propelling efficiency. As a result, they must produce much higher cardiovascular and metabolic effort just to maintain pace with technically proficient swimmers in the next lane.
These athletes often experience what feels like a paradox: the harder they try, the slower they seem to go. That’s because once drag increases additional power output yields diminishing returns in speed but exponential increases in caloric cost. It’s why swimming ‘harder’ rarely equates to swimming ‘faster’ and why technical efficiency is one of the most powerful levers for improving swim performance and energy economy.
The Science of Drag and Efficiency
Swimming efficiency is a complex equation that combines strength, body position and propulsive technique. As Zamparo et al. describe, two key variables drive energy cost:
Hydrodynamic drag – how streamlined you are through the water.
Propelling efficiency – how effectively muscular effort is converted into forward motion.
Even small flaws like a dropped elbow, a misaligned head or a wobbly kick, increase drag exponentially. Because water is nearly 800 times denser than air, inefficiencies cost far more energy than equivalent technical errors on the bike or run.
Triathletes who come from cycling or running backgrounds often start with low aquatic efficiency. As a result they burn more calories, experience higher fatigue and require greater recovery time for relatively modest training loads. Technical development is therefore a performance multiplier - it reduces the energy cost per metre and frees up capacity for harder or longer work.
Energy Balance: The Overlooked Piece
Where swimming differs most from other triathlon training is when it’s often performed - first thing in the morning and usually in a fasted state. Many athletes assume that because swimming sessions are shorter, fuelling isn’t necessary. Yet, given swimming’s high caloric cost and reliance on both aerobic and anaerobic energy systems, this approach can create a significant energy deficit early in the day.
After an overnight fast (sleep), athletes typically wake in a 300–500 kcal deficit due to basal metabolic demands during sleep. If they then complete a moderate-to-hard swim session without fuelling, that deficit can quickly swell to 1,500–2,000 kcal before their first meal. At that point, the body is running substantially behind on available energy - a deficit that many triathletes struggle to close across the rest of the day, especially if further workouts or normal daily activity follow.
Starting your day in such a large energy hole can:
Suppress glycogen availability, compromising the quality of later bike or run sessions.
Elevate cortisol and other stress hormones, impairing recovery and muscle adaptation.
Create a persistent negative energy balance that compounds over the week.
Over time, this pattern increases the risk of low energy availability (LEA) a condition that disrupts hormonal function, impairs training adaptation and elevates injury and illness risk.
Rethinking Fasted Swimming
There can be limited metabolic benefits to fasted training, but for most triathletes these are outweighed by the cost to performance and recovery, especially when swimming precedes other sessions in a two-a-day structure.
Rather than avoiding fuel entirely, athletes can take a ‘light-fuel’ approach:
A small carbohydrate snack (e.g., banana, sports drink, or slice of toast) 15-30 minutes before swimming helps maintain blood glucose and energy output.
For sessions exceeding 60 minutes or including hard efforts, mid-session carbs (30-45 g/hr) can sustain pace and reduce post-session hunger.
A carb-protein recovery snack immediately after the swim kick-starts glycogen resynthesis and supports muscle repair.
Think of fuelling your morning swim as an investment - one that supports the quality of all subsequent training that day.
Training Implications
Swimming energy expenditure draws from both aerobic and anaerobic sources. Longer sets at steady intensity primarily tax the aerobic system, while poor technique or excessive sprinting can push athletes toward anaerobic energy use, accelerating fatigue. The key for triathletes is to train efficiently, not just harder, focusing on technique, pacing and consistent energy availability.
Takeaway Tips for Triathletes
Prioritize technique: Reducing drag and improving propelling efficiency can cut energy cost dramatically.
Fuel strategically: Avoid large energy deficits from early-morning fasted swims.
Support daily balance: Track total training calories and recovery intake to maintain consistent energy availability.
Recover intentionally: Include carbs and protein within 30 minutes post-swim.
Analyze your stroke: Video feedback or coaching analysis identifies energy-wasting flaws invisible to the swimmer.
Final Thoughts
Freestyle swimming places unique energetic demands on triathletes, yet it’s often the most neglected from a fuelling perspective. The combination of high drag, low mechanical efficiency and early-day training timing can quietly erode overall performance if not addressed.
Understanding the true caloric cost of swimming and aligning fuelling with your broader daily energy balance turns the morning pool session from a drain into a performance-enhancing foundation for the day’s training.
Bevan McKinnon / October 2025
LINKS:
Zamparo P, et al. The energy cost of swimming and its determinants. Eur J Appl Physiol. 2020. PubMed link