Why Our Brain Is Always Hungry

New Insights Into Energy Constraints, ERM, and the “Selfish Brain"

In everyday life, we think of the brain as a powerful, almost limitless organ—capable of learning, solving problems, and creating our sense of self. But deep inside, the brain is quietly living on the edge of an energy crisis.

A new chapter published in Metabolic Neuropsychiatry (Rae et al., 2025), titled “Brain Energy Constraints and Vulnerability,” helps explain why the brain is so sensitive to stress, why it often demands energy at the expense of the rest of the body, and why this matters for our physical and mental health.

The insights from this chapter perfectly complement the ERM (Exposure-Related Malnutrition) framework and help clarify long-standing ideas like the “selfish brain,” “hungry brain,” and brain–body energy conservation.

Let’s break it down.

The Brain Lives Close to the Edge of an Energy Budget

The first key discovery from the chapter is surprisingly simple:

The awake brain operates at nearly 90% of its maximum energy capacity—even when you feel like you’re “doing nothing.”

That means:

• Only ~10% of its energy is left for repair, recovery, and resilience.

• Even strong stimulation (thinking harder, sensory input, stress) only increases brain energy use by 5–20%.

• The brain cannot significantly increase oxygen extraction or glucose uptake beyond a narrow limit.

This isn’t a design flaw—it’s a survival strategy. The brain uses strict homeostatic controls to prevent oxidative stress, glycation, and acid buildup that could damage neurons.

But this also creates a trade-off:

When stress rises or energy availability drops, something else in the body must give way so the brain can keep running.

This is where the ERM framework and classic neuro-energy theories align beautifully.

1. The “Selfish Brain” Explained by Energy Constraints

The “selfish brain” hypothesis suggests the brain will always prioritize itself—even if the rest of the body suffers.

Rae et al.’s chapter gives us the mechanistic reason why:

The brain cannot increase its fuel supply internally, so under stress it must pull energy from the body.

When the brain senses energy mismatch:

• Stress hormones rise

• The liver releases more glucose

• Muscles become insulin-resistant

• Appetite increases

• Immune and repair systems are temporarily paused

In ERM terms: The body reallocates energy toward the brain, reducing resources for growth, maintenance, and repair—especially during chronic stress.

This becomes the first step toward functional undernourishment and reduced resilience.

2. The “Hungry Brain” and Why Stress Feels Like Fatigue

The chapter highlights how extremely sensitive the brain is to even tiny drops in glucose, oxygen, or pH. A 10% drop in oxygen or glucose is enough to impair cognition.

This sensitivity underlies the “hungry brain” phenomenon:

• Chronic stress signals “not enough energy”

• Motivation drops

• Effort feels harder

• Appetite increases

• Fatigue appears without obvious physical cause

The brain isn’t being dramatic—it’s defending itself from a real biochemical threat.

From the ERM perspective, this is early-stage energy misallocation, where the brain drives behaviors that conserve or acquire energy because internal resources are stretched too thin.

3. Brain–Body Energy Conservation: Why the Body Slows Down

The chapter’s energy-budget model echoes a core ERM concept:

When total energy demand exceeds what the system can supply, the body shifts into conservation mode.

Since brain signaling consumes most of the available energy and cannot be reduced:

• metabolic rate slows

• muscle strength drops

• hormonal function declines

• digestion becomes sluggish

• immunity weakens

• recovery becomes slower

The brain stays online→ the rest of the body pays the price.

This is a clean explanation for symptoms commonly seen in chronic stress, long COVID, burnout, overtraining, and aging.

How This Chapter Strengthens the ERM Framework

The new findings integrate seamlessly with core ERM ideas:

✓ The brain has almost no metabolic reserve.

IRM Level 1–2: Small stressors disrupt repair and recovery.

✓ Chronic stress forces energy away from GMR (Growth–Maintenance–Repair).

This matches ERM’s concept of “functional undernourishment.”

✓ The body downregulates itself to protect the brain.

This explains “brain-first” energy allocation and compensatory metabolic suppression.

✓ Mental fatigue, brain fog, and low motivation are energy-defense signals.

Early warning signs in ERM.

✓ Vulnerability peaks during childhood and aging.

Exactly aligned with ERM staging: both periods have minimal energy reserve.

✓ Insulin resistance worsens energy mismatch.

Reduced glucose transport → brain energy gap → ERM progression.

Rae et al.’s chapter provides the neurochemical and metabolic physiology behind these dynamics, elevating ERM from a conceptual model to a mechanistically supported framework of brain-body energy resilience.

The Takeaway

Your brain is not lazy. It is not weak. It is not malfunctioning.

It is protecting itself because:

• its energy supply is limited

• its energy demands are enormous

• the cost of failure is fatal

• chronic stress and modern exposures push it past its adaptive range

The brain will always defend its energy—even if the body must sacrifice in response.

Understanding these constraints helps us reframe many symptoms not as “failures,” but as signals of bioenergetic strain—and as early markers of Exposure-Related Malnutrition.

This is why the ERM framework matters: it helps us detect the energy-allocation crisis early, restore resilience, and prevent long-term decline.

Rae, C. D., Barros, L. F., Behnke, A., Goyal, M. S., Herculano-Houzel, S., Peleg-Raibstein, D., Picard, M., Rothman, D., Vernon, A. C., & Sarnyai, Z. (2025). Brain energy constraints and vulnerability. In D. Öngür & J. M. Ford (Eds.), Metabolic neuropsychiatry (Strüngmann Forum Reports, pp. 29–55). Springer. https://doi.org/10.1007/978-3-032-05630-6_3