Death of Low Carbs? Dr Alan McCubbin and Scott dive into a new research paper

ON THE SHOW TODAY

1. Low carb, high fat diets for endurance athletes back in the spotlight with a new publication. Is this really “Game over for carbs” as some have claimed?
2. Does delaying carb intake after training impairs performance the next day?
3. Should we be recommending carbs during exercise based on body weight rather than just grams per hour?

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STUDY 1: Carbohydrate  Ingestion  Eliminates  Hypoglycemia  &  Improves  Endurance Exercise  Performance in Triathletes Adapted to Very Low & High Carbohydrate Isocaloric Diets

prins-et-al-2025-carbohydrate-ingestion-eliminates-hypoglycemia-and-improves-endurance-exercise-performance-in.pdf

What is this study attempting to answer?

• Test whether 6 weeks LCHF/keto diet impairs performance compared to a traditional higher carb approach.
• Assess the impact of taking in 10g/hr carbs to prevent a fall in blood glucose during exercise in both dietary patterns.
• To evaluate the adaptations that occur to an LCHF diet over 6 weeks

Who and What did it involve?

Participants:

• 10 competitive male triathletes (18-49yo)
• 70.3 < 6h or Ironman < 13h
• Riding >160km/wk
• VO2max above 50ml/kg/min
• Training >8hr/wk and at least 5 days a week
• Eating a traditional CHO diet (>50% of calories)
• Measured blood metabolite data (ketones, BG), TTE, body comp, Carb and fat oxidation rates

Intervention:

• 6 weeks LCHF diet or traditional HCLF diet for 6 weeks.
• 2 TTE tests at 70% VO2 max at the end of the 6 weeks.
• TTE tests 7 days apart, one with 10g/h carbs during TTE, one with 0g/hr TTE.
• Washout period for 2 weeks
• Repeated the protocol with the opposite diet (LCHF or HCLF)

Did it achieve what it set out to?

• Yes, from a carb perspective: LCHF ate 40g/day (0.5g/kg/day) carbs vs 380g (4.5g/kg/day) in the higher-carb group.NOTE: Lower end of what Fuelin considers higher carb fueling.
• Unintended side effect: HCLF trial only ate 1.2g/kg/day protein (vs 1.9g/kg/day in the LCHF trial)NOTE: LOWER THAN FUELIN RECOMMENDATION FOR ATHLETES
• Maintained the same training volume throughout both trials
• No effect of trial order

Main findings?

• Table below, right-hand column
• NB. 6 did better on LCHF + CHO, 4 did better on HCLF + CHO and 1 was the same

Issues encountered with the methodology?

• HCLF - fasted, no pre-exercise CHO, 10g/hr max carbs during (not real world). Shows in the TTE results compared to past study

Take-aways?

• Performance in a 70% VO2 max TTE (~high Z2/low Z3 ride to exhaustion) is pretty poor when you fast, and carb intake is restricted to ~10g/hr during the ride itself.
• LCHF for 4 weeks doesn’t help recover this performance much from sub-optimal carb intake before and during.
• The levels of fat oxidation (0.8-0.95g/min) seen in this study with LCHF were not that spectacular compared to previously reported values, likely because the absolute exercise intensity didn’t require higher values.
• This is not so much ‘game over for carbs’ as ‘game over for keto ’ 😂
• This is playing out in the real world with a push towards higher, not lower, carb intakes in endurance and ultra-endurance sports, especially at the elite level.

Other Notes:

Comparison study: SCHABORT ELSKE J.; BOSCH, ANDREW N.; WELTAN, SANDY M.; NOAKES, TIMOTHY D. The effect of a pre-exercise meal on time to fatigue during prolonged cycling exercise. Med Sci Sports Exer. 1999. 31:464-471.

StudySchabort et al. 1999 (Noakes an author on this one too)Prins et al. 2025Height (cm)158180Body Weight (kg)7684% Body Fat1415Training Volume (km/wk)Not reported194VO2max (mL/kg/min)6156VO2max (L/min)4.64.7Wmax (W)363379 (so FTP likely low-mid 300’s)Power at 70% VO2max (W)228249 (we don’t have FTP but from estimates likely high Z2 or more likely low end Z3)Diet day/s prior to TTEHabitual diet6wks: 2500kCal/dayLCHF (0.5g/kg/d) vs HCLF (4.5g/kg/day)Breakfast prior to TTE110g CHO vs FastedAll trials fastedCarbs during the TTE0gNC: 0g/hr   10C: 10g/hrTTE Duration (meam, min)Fasted: 109 minFed: 136 minLCHF: NC: 83.3 min  10C: 109.2 minHCLF: NC: 87.4 min  10C: 100.6 minTTE (range)Fasted: 58-159 minFed: 95-210 minLCHF: NC: 48-160 min  10C: 50-214 minHCLF: 47-134 min    10C: 50-166 minTake-homeMeal before training helps improve performance significantlyWhen eating a typical moderate carb diet, training fasted results in poor TTE performance. 10g/hr carbs helps a little but not much. Changing to LCHF doesn’t really improve things in the fat oxidation sweet spot (over very suboptimal carb preparation) and if not for one large improver would be almost identical.

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STUDY 2: Delaying post-exercise carbohydrate intake impairs next-day exercise capacity but not muscle glycogen or molecular responses

https://onlinelibrary.wiley.com/doi/epdf/10.1111/apha.14215

What is this study attempting to answer?

• To look at whether delaying carb intake by 3hr post-exercise impacts recovery and adaptations to exercise when performing high-intensity intervals
• HIIE: 10 × 2min intervals with 1-min rest, intervals at 94% W’peak (so above LT) (mean 225W) and a cadence of 90–100 rpm.

Who and What did it involve?(SCOTT)

Participants

• 9 recreational exercisers (exercising >2.5hrs a week but not following a training program)
• Age: mean 24
• Height: 180cm   Weight: 75kg
• VO2max: 46.0 mL/kg/min or 3.5 L/min
• Wmax: 240W
• W LT: 164W
• Measured
  • blood metabolite data (plasma glucose, lactate),
  • GTX & HIIE reps,
  • RPE & HR
  • Muscle glycogen muscle biopsy
  • mRNA content of mitochondrial-related genes

Study Design (SCOTT)

• Standardised dinner the night before and breakfast the morning of HIIE
• 10 × 2min intervals (225W or 94% Wmax) with 1-min rest
• Previously shown to reduce muscle glycogen by 25-50%
• First 2hr post-exercise: 2.4g/kg CHO (Lucozade) (60kg/132lbs athlete consuming 144g or 70kg/154lbs consuming 168g) or 0 carbs (placebo)
• 8hrs post-exercise: Swapped CHO or placebo so overall CHO intake was the same (immediate vs delayed)
• Total intake in 24hr post-exercise: 7.4g/kg/day CHO, 2g/kg/day protein (more prior to exercise), 1g/kg/day fat
• 24hrs later: HIIE - as many as possible til exhaustion

Did it achieve what it set out to? (SCOTT)

• Yes, a well-designed and executed study
• Muscle glycogen was reduced ~36% after HIIE compared to before, but was completely restored by 24 hours (that was the aim)

Main findings?

• No difference in the recovery of muscle glycogen 24hr post-training whether carb intake was delayed or not (Throughout recovery, muscle glycogen was maintained between ~200 and 350 mmol/kg dm in both conditions)
• No change in markers of training adaptations
• A reduced number of intervals (by 30%, which was 5 intervals or 10 minutes of work) could be completed in 24 post-original training sessions.
• This was reduced in every single participant (rare) and was not related to the amount of glycogen in the muscle immediately before that session.
• RPE (perception of effort) was higher after the 5th and 10th intervals in the delayed feeding trial
• Authors don’t know why this occurred - possibly related to glycogen stores in different parts of the muscle (not measured as not the main focus)
  • “Indeed, intra-myofibrillar glycogen depletion is correlated with impaired sarcoplasmic Ca 2+ release rate and endurance capacity and glycogen utilization rates among different subcellular locations (i.e., intra-myofibrillar, intermyofibrillar, and subsarcolemmal) may differ during exercise”

Issues encountered with methodology?(SCOTT)

• Only that muscle biopsoes for glycogen weren’t designed to look at glycogen in different parts of the muscle.

Take-aways?

• Hard to say because these were not serious athletes
• But suggests that if you’re doing intervals in training 2 days in a row, then getting in post-session carbs early may improve performance and how you feel in the second session (but doesn’t change the adapations you get to the session).
• But probably too preliminary to make big conclusions from this.

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STUDY 3: Should we recommend in-session carbs in g/hr or g/kg/hr?

Exogenous Glucose Oxidation During Exercise Is Positively Related to Body Size

Ijaz et al. - 2024 - Exogenous Glucose Oxidation During Exercise Is Positively Related to Body Size.pdf

What is this study attempting to answer?

• Previous research suggested that the amount of carbs from the drinks, gels, bars, etc. you consume during exercise that you can digest, absorb, send to the working muscles and oxidise (i.e. turn into energy) was unrelated to body weight.
• This is possibly counter-intuitive as bigger bodies would be expected to have bigger gastrointestinal tracts, be used to eating more food (inc. carbs) and have to digest and absorb that from meals and snacks, and have larger livers and muscles for taking up and using those carbs.
• The data from the past is not from deliberate studies of this; however, it’s just taken from other studies that were designed for different purposes, merged, and analyzed for trends.
• Potential confounders - energy demand
  • Running: bigger bodies → more calories required to maintain the same pace
  • Cycling: More muscle → more power (relative e.g. FTP and absolute (W) - not always true

Aims of this study:

• Establish whether larger/bigger athletes display higher rates of exogenous glucose oxidation than smaller athletes.
• Establish whether the higher absolute exercise intensity contributes to any potential increases in exogenous glucose oxidation with larger/bigger athletes.

Who and What did it involve? (SCOTT)

Participants:

• 15 cyclists in England, aged 18-60
• VO2peak between 40-75ml/kg/min
• Comparison of smaller (<70kg) and larger (>70kg) athletes (same relative VO2max (69ml/kg/min) and LT1 (2.3 W/kg)
  • Smaller athletes (7 males, 2 females) - Ht 178cm, Wt 67kg, VO2max 3.9L/min, LT1 156W
  • Larger athletes (6 males) - Ht 190cm, Wt 87kg, VO2max 5.1L/min, LT1 198W

Study Design

• Arrived at the lab fasted
• Rode for 2h at 95% of LT1 (top end of Z2)
  • Small 149W | 2.2W/kg | 57% VO2max
  • Large 180W | 2.1 W/kg | 59% VO2max
• Consumed pure glucose at 90g/hr for the 2 hours (more than you can theoretically oxidise to ensure the amount given wasn’t limiting potential in the larger participants)
• The glucose drinks were enriched with C13 (an isotope) to track it through the body
  • This allows you to distinguish between carbs burned from glycogen stores and carbs burned from the drink.
• Small participants did only one trial, but larger participants did a second trial at the same absolute power output as the smaller participants:
  • Small 149W | 2.2 W/kg | 57% VO2max
  • Large 143W | 1.7 W/kg | 46% VO2max

Did it achieve what it set out to? (SCOTT)

• Yes and No:
  • 1 trial comparing same relative but different absolute power
  • 1 trial comparing same absolute but different relative power
  • But both absolute AND relative power can influence carb use during exercise

Main findings? (SCOTT)

TrialSmallLarge (relative)Large (absolute)Power (W)149180143Power (W/kg)2.22.11.7Calorie Expenditure (kCal/hr)644855 or 819 (diff. in diff. figures!)677% VO2max57%59%46%Carbs Consumed (g/hr)909090Fat oxidation (g/min)0.320.450.33Fat as % of total calories292927Endogenous Carb Ox (g/min)1.41.81.4Endogenous as % of calories515049Exogenous Carb Ox (g/min)0.530.770.65Exogenous Carb Ox (g/hr)324639Exogenous as % of calories202223% oxidised (total)355143Peak Ex. Carb Ox. (g/hr)496054% Oxidised (peak)54%68%60%

• Of the 90g/hr consumed, peak glucose oxidation was Small 49g/hr (54%) | Large 60g/hr (68%)

When absolute intensity was the same (i.e. 149W small vs 143W large)

• Of the 90g/hr consumed, peak glucose oxidation was Small 49g/hr (54%) | Large 54g/hr (60%) - very small difference, not enough to change recommendations
• Total calorie expenditure: Small 644 kCal/hr | Large 677 kCal/hr (difference in GME?)

Issues encountered with the methodology?

• Ideally to isolate the effect of body weight from absolute AND relative exercise intensity, you would have to find bigger and smaller athletes with the SAME absolute power at different zones (e.g. same FTP, same Z2, etc.). This would mean relative power is the same absolute power for both groups.
• Could have had even larger weight differences if more female athletes recruited (small was average 67kg)

Take-aways?

• I don’t think it tells us much about scaling carb intakes for body weight from a practical recommendation point of view to be honest
• It suggests that absolute intensity will certainly drive the need for carbs during exercise (a feature of the new algo for Fuelin), which is sometimes but not always related to body size (i.e. need for carbs scales to calorie expenditure, not body weight)
• It's still unclear if two athletes with the same FTP but vastly different body weights (e.g., a 60kg and an 80kg athlete both with an FTP of 250W) need different amounts of carbs during exercise. I suspect no, but the research doesn’t give us a clear answer.
• Speculation (per Aitor Viribay): Gut training and the ability to digest and absorb carbs during exercise might be partly driven by the need for the carbs in the first place (i.e. absolute intensity - kCal/hr)

Follow up Tweet from one of the authors:

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