Six months of intermittent fasting: 8% weight loss, better lipids, and not much else

Intermittent fasting is usually defended on two levels at once. The first is that it produces weight loss, which is straightforward and testable. The second is that the fasting itself switches on something — repair pathways, reduced inflammation, slowed biological ageing — that ordinary calorie restriction does not.

A trial that ran for six months has now reported on both, and the two levels have come apart.

Nature Communications published the secondary outcomes on 13 December [s1]. The trial randomised middle-aged adults (30–65 years) with overweight — body mass index 24.8 to 35 kg/m² — to an intermittent fasting intervention or to continue their habitual diet, for six months [s1]. It is registered as NCT01964118 [s1].

What the fasting did

Weight came down by 8%, and body fat by 16% [s1]. Lipids improved: significant reductions in plasma LDL cholesterol, non-HDL cholesterol and triglycerides (p = 0.001) [s1].

No significant changes were observed in the other cardiometabolic risk factors measured [s1].

That is the whole clinical result. Six months, 8% of body weight, a better lipid panel, and nothing else that reached significance.

The primary outcome, which was reported earlier and was negative

The Nature Communications paper is explicit that it is presenting exploratory secondary analyses, and that the primary outcome was reported previously [s1]. That earlier report, published in Aging Biology in 2023, is the part of this trial that gets cited least and matters most.

The primary outcome was the change in serum C-reactive protein from baseline to six months. There was no difference — in CRP or in multiple cytokines and chemokines — despite the same 8% weight loss [s2]. Insulin sensitivity indexes derived from an oral glucose tolerance test improved by an amount the authors themselves described as statistically significant but clinically irrelevant [s2].

So the trial's designated test of whether fasting reduces inflammation returned nothing, and the secondary paper is an attempt to work out what was happening underneath.

What the molecular analysis found

The investigators performed untargeted plasma metabolomics and transcriptomic analysis of colon mucosa biopsies [s1]. Significant multi-omic changes were identified, concentrated in lipid metabolism, bile acid signalling and enteroendocrine regulation [s1].

The specific finding they flag is a downregulation of transcripts related to glucagon-like peptide 1 and related enteroendocrine hormones [s1]. Correlation analysis identified PPAR-α and B-cell-mediated immune processes as significantly associated with changes in non-HDL cholesterol [s1].

GLP-1 is the hormone axis that the current generation of obesity drugs targets in the opposite direction. A fasting protocol that reduces GLP-1-related transcription in the gut lining while producing weight loss is at minimum an interesting result, and it is the kind of thing that is hard to see without biopsies.

How much weight this can carry

Very little, and the paper says so in its framing: these are exploratory analyses of secondary outcomes [s1].

The sample is small. The Nature Communications paper states that 41 participants were randomised [s1]. The earlier report of the same trial states that 50 participants were randomised, 28 to intermittent fasting and 22 to usual diet [s2]. The two figures are not reconciled in the abstracts, and the difference is large relative to the trial. Readers should treat the analysed sample as smaller than the randomised one and the multi-omic subgroup as smaller still.

Whatever the exact denominator, a few dozen people is not a base from which to generalise transcriptomic findings from colon mucosa. Metabolomic and transcriptomic analyses generate large numbers of comparisons; correlations between molecular pathways and lipid changes in this setting are hypothesis-generating, not confirmatory.

There is also no calorie-matched control arm. The comparator maintained habitual diet [s1]. So the trial cannot separate effects of fasting from effects of the 8% weight loss it produced — which is precisely the separation the fasting literature keeps promising to make.

Why it matters

The practical reading is unglamorous. Six months of intermittent fasting in adults with overweight produced meaningful weight and fat loss and a better lipid profile, and did not move the inflammatory marker the trial was built around [s1][s2].

If fasting's advantage over other routes to the same calorie deficit is supposed to be metabolic rather than arithmetic, this trial did not find it. It did find molecular signatures worth chasing — in bile acid signalling and gut hormone regulation — which is a different and more honest claim than the one usually made for the practice.

What to watch

Whether the GLP-1-related transcriptional finding replicates in a larger cohort, and whether any trial runs intermittent fasting against calorie restriction matched for total energy intake. Until one does, the question of whether the timing adds anything to the deficit stays open.

This article is informational and is not medical advice.

Sources

• [s1] Barve RA, Veronese N, Bertozzi B, et al. "Cardiometabolic and molecular adaptations to 6-month intermittent fasting in middle-aged men and women with overweight: secondary outcomes of a randomized controlled trial." Nature Communications, 16(1), published online 13 December 2025. https://doi.org/10.1038/s41467-025-66366-8
• [s2] Tosti V, Barve RA, Bertozzi B, et al. "When a Calorie Is Not a Calorie: Metabolic and Molecular Effects of Intermittent Fasting in Humans; Exploratory Outcomes of a Randomized Clinical Trial." Aging Biology, published online 4 August 2023. https://doi.org/10.59368/agingbio.20230013