Today's Peptide News — September 27, 2026

Researchers at the University of Barcelona, led by Marion Peyrou, report in Biomedicine & Pharmacotherapy that tirzepatide, the dual GIP/GLP-1 receptor agonist sold as Mounjaro and Zepbound, directly activates brown adipose tissue in obese mice. The key design choice was a pair-fed control group: untreated mice were given exactly the same amount of food as tirzepatide-treated mice, which let the team separate the drug's own effects from those caused simply by eating less.

Tirzepatide-treated animals showed brown fat activation beyond what reduced intake alone produced, along with greater energy-burning capacity and increased release of batokines, signaling molecules secreted by brown fat that benefit whole-body metabolism. The authors note that earlier attempts to switch on brown fat pharmacologically often failed because of cardiovascular side effects, whereas tirzepatide has shown cardiovascular benefits in humans.

Why it matters: the finding suggests tirzepatide's metabolic effects may extend beyond appetite suppression into energy expenditure, which could help explain its strong effects on glucose and lipids. The authors caution that mouse and human fat biology differ substantially and that human data are needed. A good PeptideWiki angle: "Not just appetite: does tirzepatide also turn up the metabolic thermostat?", explaining pair-feeding as a study design and what brown fat is.

A study in Childhood Obesity from Ann & Robert H. Lurie Children's Hospital of Chicago analyzed national claims data from 2017 to 2022 and identified 2,031 GLP-1 receptor agonist users aged 10 to 17 with no prior deficiency diagnosis. Within one year of starting treatment, 16.8% were diagnosed with a nutritional deficiency, most commonly vitamin D deficiency at 12.4%.

Liraglutide (Victoza, Saxenda) accounted for 78.6% of prescriptions, dulaglutide 10.4% and semaglutide 9.1%. Only 5% of patients received nutritional counseling within 30 days of starting therapy, and fewer than 25% within six months. Senior author Justin Ryder highlighted vitamin D, iron and calcium as particular concerns during adolescent growth.

Why it matters: pediatric GLP-1 use is growing fast, and appetite suppression during puberty carries different stakes than in adults. The data are observational and predate widespread semaglutide and tirzepatide use in teens. A PeptideWiki angle: a short explainer on why GLP-1 therapy should come paired with proactive nutrition monitoring, especially in young people.

The FDA has approved Eli Lilly's insulin efsitora alfa, now branded Onswik, a once-weekly basal insulin for adults with type 2 diabetes. It could spare patients more than 300 injections a year compared with daily basal insulin. The US decision follows approvals in Europe, Mexico and Japan.

Approval rests on the Phase 3 QWINT program in roughly 3,400 adults, which showed non-inferiority to once-daily insulin glargine and insulin degludec. In QWINT-2, Onswik lowered A1C by 1.34% versus 1.26% for degludec. The label advises against use in type 1 diabetes because of increased hypoglycemia risk.

Onswik goes up against Novo Nordisk's weekly insulin icodec (Awiqli), which was approved in the US in March after earlier rejections. Lilly says Onswik will launch in the US in the coming months.

Eli Lilly's legal fight with the FDA over whether its triple-agonist obesity peptide retatrutide counts as a biologic was scheduled for oral argument before the Seventh Circuit Court of Appeals on September 24. The FDA says retatrutide does not meet its definition of a protein, which requires more than 40 alpha amino acids. Lilly argues that its 41-residue molecule qualifies, or at least is "analogous to" a protein.

The stakes are commercial. A biologics license application brings about 12 years of exclusivity against biosimilars, compared with roughly five years of new-chemical-entity exclusivity under a standard new drug application. A district court upheld the FDA's protein finding in 2025 but sent the "analogous to a protein" question back to the agency, and Lilly appealed.

Lilly says it plans to submit retatrutide in the first quarter of 2027 through the biologics pathway. The outcome could also set a precedent for how future incretin peptides near the 40-amino-acid line are regulated, and for whether compounders can make copies.

The FDA has sent a warning letter, dated September 18, to Empower Clinic Services, which does business as Empower Pharmacy. Inspectors at its Houston site found sterile products "prepared, packed or held under insanitary conditions" that could be contaminated or made harmful to health.

The agency also said Empower compounded tirzepatide and semaglutide products that appear to be essentially copies of FDA-approved drugs, made "regularly or in inordinate amounts." That would violate Section 503A. The FDA flagged "significant difference" prescriber statements that were repeated word for word across many orders and suggested that pre-selected menu options on telehealth platforms undercut the individualized judgment the law requires.

Empower has 15 working days to respond. The letter signals continued FDA enforcement against mass-market GLP-1 compounding, even as debate grows over which other peptides pharmacies should be allowed to compound.

A major analysis suggests that Phelan-McDermid syndrome, a genetic disorder caused by loss or disruption of the SHANK3 region on chromosome 22 and closely tied to autism, may affect about 1 in 7,300 people. That would mean more than 45,000 Americans, far more than the number currently diagnosed.

The researchers warn that thousands of people are likely living without a diagnosis. A diagnosis matters for medical monitoring and for eligibility for emerging targeted therapies and clinical trials.

Scientists found that feeding C. elegans worms a magnetotactic bacterium, a microbe that produces tiny internal magnets, extended their average lifespan by more than 43%. It also helped protect their neurological and intestinal health.

The team traced much of the effect to suppression of ferroptosis, an iron-dependent form of cell death driven by lipid peroxidation that is increasingly linked to aging and neurodegeneration. The work is early and limited to worms, but it adds to interest in ferroptosis as a target for longevity research.

Researchers used CRISPR to remove the surface protein CD33 from donor blood-forming stem cells. CD33 is a common target on acute myeloid leukemia cells, but therapies that attack it also destroy healthy blood cells that carry it.

Transplanting CD33-free donor cells could let doctors use CD33-directed drugs or cell therapies aggressively after a transplant, clearing residual cancer while sparing the new, edited blood system. The approach could widen the therapeutic window for hard-to-treat blood cancers.

A large study that tracked more than 95,000 people found that those who spent more time in REM sleep had lower risks of 83 conditions, including dementia and heart failure.

The findings are observational and cannot show that REM sleep itself is protective. Still, they add to growing evidence that sleep architecture, not just total sleep time, may be a meaningful marker of long-term health.