You’ve seen it in your numbers. Maybe you didn’t recognize it at the time.

The first two hours felt great. Heart rate stable. Speed consistent. Efficiency factor right where it should be.

Then, somewhere in hour three, everything changed. Speed dropped. Heart rate dropped with it – not because you were going easy, but because your body couldn’t sustain the effort. Efficiency collapsed. The final hour became survival.

That’s not a fitness problem. That’s a fueling problem.

And it shows up clearly in your ride data – if you know what to look for.

This guide explains the science of cycling nutrition, how to recognize fueling failures in your metrics, and how to build a nutrition strategy that keeps your data consistent from start to finish.

How Fueling Affects Your Cycling Metrics

Your body runs on fuel. When fuel runs low, performance collapses – and your data tells the story.

The Energy Systems at Play

Duration	Primary Fuel	Limitation
0-10 seconds	ATP/Creatine phosphate	Very limited stores
10 sec – 2 min	Glycogen (anaerobic)	Produces lactate, unsustainable
2 min – 2 hours	Glycogen (aerobic)	~90-120 min of hard riding
2+ hours	Fat + glycogen mix	Fat burning limited by intensity

The critical insight: Your glycogen stores are finite. A trained cyclist stores approximately 400-500g of glycogen in muscles and liver – enough for 90-120 minutes of moderate-to-hard riding.

After that, you’re dependent on:

1. Fat oxidation (slower, intensity-limited)
2. Incoming carbohydrates (what you eat during the ride)

If neither is sufficient, performance craters.

What Glycogen Depletion Looks Like in Data

The bonk has a distinct data signature:

Metric	Normal Ride	Glycogen Depleted
Speed/Power	Stable throughout	Declining despite effort
Heart Rate	Stable or slight drift	Declining (can’t generate output)
Efficiency Factor	Stable	Collapsing
RPE	Appropriate to effort	Disproportionately high

The key pattern: When both heart rate AND speed/power decline together in the latter portion of a ride, fueling is often the culprit. Your body literally cannot produce the cardiac output because it lacks substrate.

The Science of Cycling Nutrition

Carbohydrates: Your Primary Fuel

For cycling performance, carbohydrates are king. Fat provides energy at lower intensities, but anything above Zone 2 requires significant carbohydrate contribution.

Carbohydrate oxidation rates:

Intensity	Carb Burning Rate	Glycogen Duration
Zone 2 (easy)	30-40g/hour	4-5+ hours
Zone 3 (tempo)	50-70g/hour	2-3 hours
Zone 4 (threshold)	70-90g/hour	1.5-2 hours
Zone 5 (VO2max)	90-120g/hour	<1 hour

The math problem: If you’re burning 60g of carbs per hour and only consuming 30g per hour, you’re in deficit. Eventually, the tank empties.

Gut Absorption Limits

Your intestines can only absorb carbohydrates so fast:

Carb Type	Max Absorption Rate
Glucose only	~60g/hour
Glucose + fructose (2:1 ratio)	~90g/hour
Trained gut (glucose + fructose)	Up to 120g/hour

Practical implication: Consuming more than your gut can absorb causes GI distress without performance benefit. Match intake to absorption capacity.

The Role of Fat

Fat provides abundant energy but with limitations:

Advantages:

• Nearly unlimited stores (even lean cyclists have 30,000+ calories of fat)
• Sustainable for very long durations
• Spares glycogen at lower intensities

Limitations:

• Oxidation rate limited to ~60g/hour maximum
• Cannot fuel high-intensity efforts
• Requires oxygen – not available at near-max efforts

The crossover point: As intensity increases, your body shifts from fat to carbohydrate burning. At threshold and above, you’re almost entirely carb-dependent.

Recognizing Fueling Failures in Your Data

Pattern 1: The Classic Bonk

What happened: Glycogen depleted, fat oxidation can’t keep up, blood sugar crashes.

Data signature:

Segment	Speed	Heart Rate	Efficiency
Hours 1-2	28 km/h	148 bpm	1.89
Hour 3	25 km/h	142 bpm	1.76
Hour 4	21 km/h	132 bpm	1.59

The telltale: Speed AND heart rate declining together. You want to go harder, but your body can’t produce the output.

Other symptoms:

• Sudden weakness, lightheadedness
• Difficulty concentrating
• Emotional instability (irritability, despair)
• Loss of coordination

Pattern 2: The Slow Fade (Partial Depletion)

What happened: Not a complete bonk, but insufficient fueling causes gradual performance decline.

Data signature:

Segment	Speed	Heart Rate	HR Drift
First half	27 km/h	145 bpm	Baseline
Third quarter	26 km/h	148 bpm	+2%
Final quarter	24 km/h	146 bpm	Stable but output down

The telltale: Progressive speed decline with relatively stable (or slightly elevated) heart rate. Body is working harder to produce less output.

Pattern 3: The Intensity Ceiling

What happened: Enough fuel for steady riding, but no capacity for hard efforts.

Data signature:

• Steady-state metrics look normal
• Unable to respond to climbs, surges, or attacks
• HR doesn’t reach typical high values during hard efforts
• “Legs just weren’t there” feeling

The telltale: Your max HR during hard efforts is lower than usual. The body can’t generate peak output without glycogen.

Pattern 4: Dehydration-Induced Drift

What happened: Insufficient fluid intake causes blood volume to decrease, HR to rise.

Data signature:

Segment	Speed	Heart Rate	HR Drift
Hour 1	28 km/h	145 bpm	Baseline
Hour 2	28 km/h	152 bpm	+5%
Hour 3	27 km/h	158 bpm	+9%

The telltale: Speed remains relatively stable, but heart rate progressively climbs. Same output, increasing cardiac cost.

The physiology: Dehydration reduces blood plasma volume. Heart must beat faster to deliver the same oxygen. Eventually, performance falls as the system strains.

Building Your Fueling Strategy

Before the Ride: Topping Off the Tank

Timing matters:

When	What	Why
3-4 hours before	Large meal (100-150g carbs)	Full digestion, liver glycogen topped
1-2 hours before	Smaller snack (30-50g carbs)	Top-off without GI discomfort
15-30 min before	Optional small amount (15-25g)	Immediate availability

Pre-ride meal examples:

Meal	Carbs	Notes
Oatmeal + banana + honey	~80g	Easy to digest, sustained release
Toast + peanut butter + jam	~60g	Quick to prepare
Rice + eggs	~50g	Lower fiber, gentle on stomach
Pasta (moderate portion)	~70g	Classic choice, avoid heavy sauces

What to avoid:

• High fat meals (slow digestion)
• High fiber (GI distress risk)
• Unfamiliar foods (not race/hard ride day)
• Large meals within 2 hours of start

During the Ride: Maintaining the Supply

Fueling guidelines by duration:

Ride Duration	Fueling Needed	Carbs/Hour
<60 minutes	Usually none	0-30g optional
60-90 minutes	Optional/moderate	30-40g
90-150 minutes	Recommended	40-60g
150+ minutes	Essential	60-90g
3+ hours (hard)	Maximum	80-120g (trained gut)

When to start: Begin fueling at 30-45 minutes, not when you feel hungry. By the time you feel depleted, it’s too late – absorption takes 15-30 minutes.

Fueling sources comparison:

Source	Carbs	Pros	Cons
Energy gels	20-30g	Concentrated, fast	Can cause GI issues, need water
Energy bars	30-45g	Sustained release, satisfying	Harder to eat at intensity
Sports drink	30-60g/bottle	Hydration + fuel combined	May not match needs separately
Bananas	25-30g	Natural, easy to digest	Bulky, less precise
Rice cakes	20-30g	Savory option, real food	Requires preparation
Dates	15-20g each	Natural sugars, compact	Sticky, can be too sweet

Practical approach:

• Set timer for every 20-30 minutes
• Consume 20-30g carbs each interval
• Alternate between sweet and savory to prevent flavor fatigue
• Practice your fueling strategy on training rides, not events

The Glucose-Fructose Advantage

For rides demanding >60g/hour intake, use products with multiple carbohydrate sources.

Why it works: Glucose and fructose use different intestinal transporters. Combining them allows higher total absorption.

Target ratio: 2:1 glucose to fructose (some products use 1:0.8)

Products typically using multiple transporters:

• Most commercial energy gels and drinks
• Check labels for maltodextrin (glucose source) + fructose

Hydration: The Other Half of the Equation

Dehydration degrades performance even when carbohydrate intake is adequate.

Sweat Rate Reality

Conditions	Typical Sweat Rate	Fluid Need/Hour
Cool (15°C), moderate effort	500-700 ml/hour	400-600 ml
Moderate (20-25°C), hard effort	800-1200 ml/hour	600-900 ml
Hot (30°C+), any effort	1000-1500+ ml/hour	750-1200 ml

You cannot fully replace sweat losses during exercise. The gut can only absorb 600-800 ml/hour maximum. In hot conditions, you’re fighting a losing battle – minimize the deficit rather than eliminating it.

Dehydration Performance Impact

Dehydration Level	Performance Impact
1% body weight	Minimal – often unnoticed
2% body weight	~10% performance decline
3% body weight	~20% performance decline, impaired thermoregulation
4%+ body weight	Significant health risk, severe impairment

For a 70kg cyclist: 2% dehydration = 1.4kg fluid loss = ~1.5 liters

Electrolytes: When They Matter

Plain water is sufficient for rides under 90 minutes in moderate conditions.

When to add electrolytes:

Factor	Electrolyte Need
Duration >90 minutes	Recommended
Hot conditions	Recommended
Heavy sweater	Essential
Salty sweat (white residue)	Essential
History of cramping	Recommended

Key electrolytes:

Electrolyte	Role	Typical Need/Hour
Sodium	Fluid balance, nerve function	300-600mg
Potassium	Muscle function	50-100mg
Magnesium	Muscle contraction	10-30mg

Practical approach: Use electrolyte drink mix or tablets in one bottle, plain water or carb drink in another.

Post-Ride: Recovery Nutrition

What you eat after riding affects your next ride’s performance.

The Recovery Window

First 30-60 minutes: Glycogen synthesis rate is elevated. Muscles are primed to absorb nutrients.

Target intake:

• Carbohydrates: 1-1.2g per kg body weight
• Protein: 0.3-0.4g per kg body weight

For a 70kg cyclist:

• 70-85g carbohydrates
• 20-30g protein

Recovery Meal Examples

Meal	Carbs	Protein	Notes
Chocolate milk (500ml)	~50g	~16g	Quick, convenient, effective
Rice + chicken breast	~60g	~35g	Complete meal
Pasta + meat sauce	~70g	~25g	Classic recovery meal
Smoothie (banana, berries, protein, oats)	~60g	~25g	Easy to consume when not hungry
Eggs + toast + orange juice	~45g	~20g	Breakfast option

When Recovery Nutrition Matters Most

High priority (don’t skip):

• After rides >2 hours
• After high-intensity sessions
• When training again within 24 hours
• During heavy training blocks

Lower priority (flexibility okay):

• Easy recovery rides <60 minutes
• Rest day before next session
• If next meal is within 2 hours anyway

Using Your Data to Refine Fueling Strategy

Pre/Post Comparison Method

Protocol:

1. Complete similar rides with different fueling approaches
2. Compare late-ride metrics between approaches
3. Identify which strategy produces better data consistency

What to compare:

Metric	Well-Fueled Target	Under-Fueled Sign
HR Drift	<5% over ride	>8% (especially if dehydrated)
Late-ride EF	Stable vs. early ride	Declining >10%
Speed consistency	<5% decline	>10% decline in final third
HR + Speed together	Stable	Both declining simultaneously

Tracking Fueling in Your Training Log

Record fueling data alongside ride data:

Field	Why It Matters
Pre-ride meal	Correlate with early-ride feel
During-ride intake (g carbs)	Compare to performance sustainability
Fluid intake	Correlate with HR drift
Conditions (temp, humidity)	Context for hydration needs
Late-ride feel	Subjective check against data

Over time, patterns emerge:

• “When I consume <40g/hour on 3+ hour rides, my efficiency drops >15% in the final hour”
• “My HR drift exceeds 8% when I drink <500ml/hour in temperatures above 25°C”

The Analyzer’s Role

Upload your rides to the Apple Health Cycling Analyzer and examine:

For each long ride:

• Is efficiency factor stable throughout, or declining late?
• Does HR drift stay under 5%, or climb higher?
• Do hard efforts late in the ride show appropriate HR response?

Across multiple rides:

• Which long rides maintained efficiency best?
• What did you do differently (fueling, hydration) on successful rides?
• Do your “fade” rides share common patterns?

Quick Reference: Fueling Cheat Sheet

By Ride Duration

Duration	Before	During	After
<60 min	Normal meal 2-3h prior	Water only	Normal eating
60-90 min	Carb-focused meal 2-3h prior	0-30g carbs, 500ml fluid	Snack + meal within 2h
90-150 min	100g+ carbs 3h prior, snack 1h prior	40-60g/hour, 500-750ml/hour	Recovery meal within 1h
150+ min	Large carb meal 3-4h prior, top-off 1h prior	60-90g/hour, 500-1000ml/hour	Recovery meal immediately

Warning Signs During Ride

Symptom	Likely Cause	Immediate Action
Sudden weakness	Glycogen depletion	Consume fast carbs (gel, drink)
Difficulty concentrating	Low blood sugar	Eat immediately, reduce intensity
Excessive HR rise	Dehydration	Drink, add electrolytes
Muscle cramping	Electrolyte imbalance, fatigue	Electrolytes, reduce intensity
Nausea	Too much intake or intensity	Reduce both, sip fluids
Legs “dead” to hard efforts	Glycogen depleted	Carbs won’t help immediately – ride easy

Data Patterns Summary

Pattern	Likely Cause	Prevention
HR + speed both declining late	Glycogen depletion	Increase carb intake during
HR rising, speed stable	Dehydration	Increase fluid intake
Can’t reach high HR on efforts	Glycogen depleted	More carbs, start earlier
Late-ride efficiency collapse	Under-fueling overall	Systematic fueling plan
GI distress	Too much intake or wrong foods	Reduce volume, train gut

Fuel the Data You Want to See

Your ride data doesn’t lie. When efficiency collapses in the final hour, when heart rate and speed decline together, when you can’t respond to that climb – the data captures it all.

The fix isn’t more fitness. It’s better fueling.

Build a systematic nutrition strategy. Track what you eat alongside how you ride. Upload your data to the Apple Health Cycling Analyzer and look for patterns. Find the fueling approach that keeps your efficiency stable from the first kilometer to the last.

Your legs can only perform as well as your nutrition allows. Feed them properly, and watch your data improve.