REF-543 · Impey, Hearris, Hammond, Bartlett, Louis, Close & Morton, 2018 · Sports Medicine, Vol. · Paper
"Fuel for the Work Required" — carbohydrate-periodization framework + glycogen threshold hypothesis; train-low boosts muscle signalling in ~75% of studies but improved performance in only 37%
Key Findings
Train-low evidence tallied in the review: compared with high CHO availability, train-low augmented cell signalling in 73% of 11 studies, gene expression in 75% of 12 studies, and training-induced increases in oxidative enzyme activity/protein content in 78% of 9 studies — especially when sessions started within a specific range of muscle glycogen
But adaptation did not usually become performance: only 37% of 11 studies showed improved exercise performance; 63% showed no change
Train-low studies typically place 30-50% of sessions with reduced CHO availability; models: twice-per-day training, fasted training, post-exercise CHO restriction, and "sleep low, train low"
Glycogen threshold hypothesis: a window of muscle glycogen concentrations that at once permits completion of the required training workload and activates the molecular machinery of adaptation; the named pathways (AMPK, p38MAPK, PGC-1α) are from the full text, checked against the full text 2026-09-27
"Fuel for the work required": CHO availability adjusted to the demands of the upcoming session(s) — an amalgamation of the train-low models rather than "train-low always" or "high-carb always"
Open challenge stated by the authors: quantify the glycogen cost of habitual sessions, and ensure absolute training intensity is not compromised
Practical Application
A framework review, no dose tested: across the train-low literature it tallies, reduced-carbohydrate sessions augmented muscle signalling (73% of 11 studies), gene expression (75% of 12) and oxidative-enzyme adaptation (78% of 9), yet improved performance in only 37% of 11 studies (no change in 63%) — so match carbohydrate availability to the demand of the coming session (low or moderate for easy base work, high for quality), and expect molecular gains, not guaranteed race gains, from train-low.
Fuel the quality: every interval session, race and hard long ride is done with high carbohydrate availability — the review's own condition is that absolute training intensity must not be compromised.
Place train-low on base sessions only — a fasted easy morning ride, or an evening-low/morning-easy "sleep-low" pairing (REF-003); the studies reviewed used train-low in 30-50% of sessions, but the optimal frequency is not defined; one or two a week is the conventional starting point, not a number from this paper.
Train-low is timing, not a deficit: total daily energy and carbohydrate still cover the training done; chronic underfuelling is LEA (REF-545), not periodisation.
Not from this paper: masters athletes recover more slowly (REF-349); the same train-low frequency costs them more, so start lower.
Limitations
A theoretical framework; the optimal train-low "dose" is individual and not fully defined
Performance benefit is the exception in the tallied studies (37%), not the rule; the framework is built mainly on molecular outcomes
Narrative tally, not a meta-analysis — the percentages count studies, not pooled effect sizes
Risk of overdoing train-low → underrecovery/LEA (especially in masters, REF-349)
How this entry was checked and changed
Why this level
regraded 2026-09-27 from A: narrative review without systematic method, from the leading carbohydrate-periodization group (Liverpool John Moores)
Content correction 2026-09-27
Card lead now carries the abstract's tally: signalling/gene/enzyme adaptations in 73/75/78% of studies, performance improved in only 37% (63% no change).
Sport running -> general (framework review, not sport-specific); "400+ citations" dropped from the Level line; Level A -> B+ (narrative review).
AMPK/p38MAPK/PGC-1α named pathways marked as full text; masters-recovery line marked "Not from this paper".
Lines labelled "full text; not checked against the abstract" were checked against the open-access full text (PMC5889771): every number in them appears there; relabelled "full text, checked 2026-09-27".
Full text (checked 2026-10-12)
Train-low adaptations were particularly evident when pre-exercise muscle glycogen was ≤ 300 mmol/kg dw, and the proposed threshold window is e.g. 300–100 mmol/kg dw (full text).
Starting exercise with < 200 mmol/kg dw is likely to impair training intensity, restoring post-exercise glycogen to > 500 mmol/kg dw attenuates gene-expression changes, and staying < 100 mmol/kg dw may reduce regulation of protein synthesis pathways (full text).
Repetitive daily training with reduced carbohydrate availability may increase susceptibility to illness (full text).
Training intensity was impaired in all 3 studies that assessed it (100%), and lipid oxidation increased in 47% (n = 8) of 17 studies (full text, Table 2).
In Gejl et al., 26 elite male endurance athletes doing three of seven weekly sessions with reduced carbohydrate for 4 weeks improved VO2max and 30-min TT similarly to an all-high group; in Yeo et al., 14 trained male cyclists/triathletes improved 60-min TT similarly (10%) in both conditions (full text, Table 1).
In the authors' study, starting with high (600) versus low (300 mmol/kg dw) glycogen increased exercise capacity by 60 min, with identical cell signalling despite less work in the low trial (full text).
The "sleep low, train low" model can extend time with reduced muscle glycogen to 12–14 h (full text).
The practical model (Table 3) uses red, amber and green ratings for carbohydrate at each feeding point and deliberately prescribes no specific carbohydrate quantities (full text, Table 3).