Dr. Axe Wire · Study decoded · nutrition

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.

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

Take the Cellular Health Quiz →

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 stated in the paper: Universidad Autónoma Metropolitana Unidad Iztapalapa supported the study, and the National Council of Science and Technology (CONAHCYT) gave one author a postgraduate scholarship. Conflicts stated in the paper: The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this paper.
Date: Oct 15, 2026