REF-1127 · Bouchard et al., 1990 · New England Journal of Medicine, · Paper · PubMed

The Response to Long-Term Overfeeding in Identical Twins

Blimited evidenceMeasured on non-athletes and recreational people

Bouchard et al. overfed 12 pairs of young adult male identical twins by 1000 kcal (4.2 MJ) a day, 6 days a week, for 84 days within a 100-day period, after a two-week baseline: 84,000 kcal (353 MJ) of surplus per man, the same for everyone. Weight gain averaged 8.1 kg but ranged from 4.3 to 13.3 kg.

Brothers responded alike: for body weight, percentage fat, fat mass and estimated subcutaneous fat there was about three times more variance among pairs than within pairs (r ≈ 0.5, P < 0.05), and for abdominal visceral fat, after adjustment for fat gain, about six times more (r ≈ 0.7, P < 0.01).

The authors conclude that genetic factors are most likely involved, possibly governing whether surplus energy is stored as fat or lean tissue and how resting energy expenditure responds.

Protocol

12 pairs of young adult male monozygotic twins (n = 24), from the abstract.
After a two-week baseline, the men ate an extra 4.2 MJ (1000 kcal) a day, 6 days a week, totalling 84 overfeeding days within 100 days. Total excess per man: 353 MJ (84,000 kcal).
Outcomes: body weight, percentage fat, fat mass, fat-free mass, subcutaneous fat (estimated) and regional fat distribution including abdominal visceral fat; within-pair similarity tested by intraclass analysis.
Not in the abstract: physical activity during the study, how intake was supervised, the measurement methods.

Key Results

OutcomeResult (abstract)
Surplus1000 kcal/day × 84 days = 84,000 kcal (353 MJ) per man, identical for all 24 men
Weight gainmean 8.1 kg; range 4.3 to 13.3 kg
Within-pair similarity, weight / % fat / fat mass / subcutaneous fatsignificant (P < 0.05); about three times more variance among pairs than within pairs (r ≈ 0.5)
Within-pair similarity, regional fat distribution and visceral fat (adjusted for fat-mass gain)P < 0.01; variance between pairs roughly six times the variance within pairs (r ≈ 0.7)

Mechanisms

Measured: the same energy surplus produced a three-fold range of weight gain, and the response clustered within twin pairs.
Authors' interpretation: genetic factors are the most likely explanation; they "may govern" the tendency to store energy as fat or lean tissue and determinants of resting energy expenditure. Neither partitioning nor expenditure is quantified in the abstract.
Inference, not tested here: the spread is in what the body does with the surplus (partitioning, expenditure), not in how much of the food is absorbed; absorption was not measured, but stool energy loss on an ordinary diet is a few per cent of intake (REF-1126).

Practical Application

The same surplus produced very different gains: 24 young men given the identical 84,000-kcal surplus over 84 days gained anywhere from 4.3 to 13.3 kg (mean 8.1 kg), and identical twins resembled each other (B, young men, training status not reported, from the abstract). The base's inference, comparing with REF-1126: on an ordinary diet, people differ more after the gut than in it.
An athlete's weight response to eating more is personal (for eating less this is inference; a deficit was not studied): read it from the athlete's own body-mass trend over weeks, not from a calorie formula or a friend's result (SYNTH-87, SYNTH-54).
Not a licence for fatalism: the range is wide but every man gained weight on the surplus; the response clustered within twin pairs (genes are the authors' most likely explanation), affecting how much and where, not whether.
Not from this paper: how trained athletes partition a surplus; a deliberate surplus in elite athletes added mostly fat in REF-1089.

Limitations

Men only, young, n = 24; no women, no athletes.
No control group; the design shows variation and within-pair similarity, not mechanism.
From the abstract: methods, physical activity, measurement of body composition and energy expenditure are not available here.
Twins share upbringing as well as genes; the abstract attributes similarity to genetics as "the most likely explanation".
1000 kcal/day for 84 days is a large, forced surplus, far beyond normal day-to-day logging error.
How this entry was checked and changed
Why this level

uncontrolled pre-post, n = 24 (12 pairs of monozygotic twins); from the abstract (full text not retrieved)