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Undernourished Rat Pups Show Thymus Shrinkage And Immune Cell Death

A new rat study published October 15, 2026 links early malnutrition to thymus damage, a possible clue to why poorly fed infants get sick more often.

By Dr. Axe Editorial Team · October 16, 2026 · 1 source · Watch

A study published October 15, 2026 in the British Journal of Nutrition found that undernourished rat pups had weight deficits of 40% or more and showed significant shrinkage of the thymus, the gland that trains immune cells.

Researchers induced undernutrition in nursing rat pups through competitive suckling, a method that restricts how much milk each pup can access from the mother. This produced weight deficits of 40% or more compared with normally fed pups. The team measured thymus weight, counted total thymocytes (immune cells produced in the thymus), and used flow cytometry, a lab technique that sorts and analyzes individual cells, to assess mitochondrial function, activation of caspases (proteins that execute programmed cell death) and shifts in thymocyte subpopulations. Undernourished rats showed a marked reduction in both thymus weight and thymocyte cellularity, along with significant changes in the balance of thymocyte subpopulations and a diminished mitochondrial potential, a sign that the cells' energy-producing machinery was faltering.

The authors reported that caspase 9, a signaling protein activated when mitochondria are under stress, played a significant role in early-stage thymocyte apoptosis. They describe this as suggesting a link between mitochondrial dysfunction and the thymocyte cell death seen under undernutrition, a pattern they note is consistent with prior research on thymic vulnerability to poor nutrition.

Weight deficit in undernourished rat pups

British Journal of Nutrition, 2026, Cortés-Barberena

This is a rat study using an induced-undernutrition model during lactation; it cannot establish what happens in undernourished human infants, and the record does not report funding or conflicts of interest.

Dr. Axe’s Take

The thymus is one of the first organs to suffer when a growing body doesn't get enough fuel, because immune cell production is metabolically expensive and gets deprioritized under energy stress. For a child who is underweight or recovering from illness, adequate calories and protein during early growth build the immune capacity that lasts a lifetime. I'd focus on nutrient-dense whole foods, healthy fats and enough total calories during recovery rather than restricting intake. This is rat biology, not a human treatment study, but it lines up with what shows up clinically: malnourished kids get sick more, and rebuilding immunity starts with rebuilding the body behind it.

— Dr. Axe

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Study summary

Who: Rat pups made undernourished via competitive suckling during lactation, compared with normally fed pups
Design: Animal study measuring thymus weight, thymocyte count, mitochondrial potential and caspase activation via flow cytometry
Dose: Undernutrition induced through competitive suckling, producing weight deficits of 40% or more
Primary result: Undernourished rats showed significantly reduced thymus weight and thymocyte cellularity versus well-fed pups
Secondary: Diminished mitochondrial potential in thymocytes; significant caspase 9 activation linked to early apoptosis; altered thymocyte subpopulation balance
Funding / conflicts: Funding and conflicts not reported in the abstract.
Date: Oct 15, 2026