Role of Nonexercise Activity Thermogenesis in Resistance to Fat Gain in Humans
Levine et al. overfed 16 nonobese volunteers by 1000 kcal a day above their weight-maintenance needs for 8 weeks and measured where the extra energy went. Two-thirds of the increase in total daily energy expenditure came from non-exercise activity thermogenesis (NEAT): fidgeting, maintaining posture and the other physical activities of daily life.
Fat storage differed 10-fold between people, and the change in NEAT accounted for those differences: it directly predicted resistance to fat gain (r = 0.77, P < 0.001). The authors' reading is that when humans overeat, activating NEAT dissipates excess energy, and that failing to activate it may lead to ready fat gain.
Protocol
16 nonobese volunteers (n = 16), from the abstract.
Fed 1000 kcal/day in excess of weight-maintenance requirements for 8 weeks.
Measured: changes in energy storage (fat gain) and in energy expenditure, with NEAT as the component of expenditure not due to volitional exercise.
Not in the abstract: sex split, measurement methods, how exercise was controlled, the individual ranges.
Key Results
| Outcome | Result (abstract) |
|---|---|
| Surplus | 1000 kcal/day above weight maintenance, 8 weeks |
| Share of the rise in total daily energy expenditure due to NEAT | two-thirds |
| Fat storage between individuals | 10-fold differences |
| Change in NEAT vs resistance to fat gain | r = 0.77, P < 0.001; NEAT changes accounted for the differences in fat storage |
Mechanisms
Measured: the rise in daily expenditure during overfeeding came mostly from non-exercise activity, and the people whose NEAT rose most stored the least fat.
Authors' interpretation: NEAT is a route by which humans dissipate excess energy to preserve leanness; failure to activate it may allow ready fat gain.
Inference, not tested here: the part of a surplus that is not absorbed is small by comparison (REF-1126, stool loss a few per cent of intake), so the 10-fold differences in fat storage arise after absorption.
Inference, not tested: in an athlete, planned training fixes a large part of expenditure, and fatigue from training may limit how much NEAT can rise; nothing here measured that.
Practical Application
In this overfeeding study, the difference between people came from how much they moved without meaning to: in 16 nonobese adults overfed 1000 kcal/day for 8 weeks, two-thirds of the rise in expenditure was NEAT, fat storage differed 10-fold, and the NEAT change predicted resistance to fat gain (r = 0.77; B, from the abstract).Two people eating the same extra food stored very different amounts. Absorption was not measured; by inference from REF-1126 it differs little on an ordinary diet.
For a weight-conscious athlete this works in both directions: a hard block or a cut that makes the athlete sit more lowers daily expenditure outside training, which a calorie app does not see. Judge the intake by the body-mass trend, not by the app's expenditure estimate (SYNTH-87); the specific behaviour is inference, not tested.
Do not quote ranges of fat gain or NEAT in kilocalories from this paper: the abstract gives the two-thirds share, the 10-fold difference and r = 0.77 only.
Limitations
n = 16 nonobese volunteers, 8 weeks, no control group; untrained general population.
From the abstract: no measurement methods, sex split or individual values; the individual ranges often quoted from this paper are not in the abstract and are not used here.
A forced 1000 kcal/day surplus is far larger than ordinary logging error; NEAT behaviour during an energy DEFICIT or heavy training was not studied.
Correlation between NEAT change and fat gain within one group; it predicts, it does not prove mechanism.
How this entry was checked and changed
Why this level
uncontrolled pre-post, n = 16; from the abstract (closed access)