When Carbon Cannot Simply “Burn”: An Old Mitochondrial Idea That Looks Different Through Bioenergetic Impedance

More than 20 years ago, Michael MacDonald and colleagues published a perspective that challenged a simple view of mitochondrial metabolism in pancreatic β-cells.

The conventional story was straightforward: glucose enters the cell, mitochondria oxidize it, ATP rises, ATP-sensitive potassium channels close, calcium enters, and insulin is released.

MacDonald and colleagues argued that this was only part of the story.

Their central observation was striking.

In glucose-stimulated β-cells, a large fraction of glucose-derived pyruvate does not simply proceed through pyruvate dehydrogenase toward acetyl-CoA and complete oxidation. Roughly half is carboxylated to oxaloacetate through pyruvate carboxylase, expanding tricarboxylic acid-cycle intermediates through anaplerosis.

Those intermediates are then exported from mitochondria through cataplerosis, where they contribute to NADPH production, lipid synthesis, redox regulation, protein modification, membrane dynamics and other processes involved in insulin secretion.

At the time, the central question was largely:

What are these mitochondrial metabolites doing as signals?

Today, we can ask another question:

Why does the cell need to reroute so much carbon in the first place?

That question leads toward a broader view of metabolism—and potentially toward a different way of thinking about aging.

The mitochondrion is not simply a furnace

A common mental model of metabolism treats mitochondria almost like furnaces: carbon comes in, gets completely oxidized, and ATP comes out.

Real cells are much more complicated.

Carbon enters through glucose, fatty acids, amino acids and other substrates. But oxidative metabolism has finite capacity. It depends on oxygen delivery, mitochondrial respiratory machinery, redox balance, substrate transport, enzyme kinetics, protein import and assembly, and the cell's current functional demands.

The amount of substrate arriving therefore does not necessarily equal the amount that can be completely oxidized at that moment.

MacDonald and colleagues showed an elegant example of what cells can do instead.

Rather than directing all pyruvate through one oxidative route, β-cells redistribute carbon through:

pyruvate → oxaloacetate → malate / citrate / isocitrate / α-ketoglutarate → cytosolic export

The exported products then support other functions.

Malate and isocitrate can transfer reducing equivalents and contribute to NADPH generation. Citrate can export acetyl-CoA equivalents into the cytosol, supporting malonyl-CoA, lipid metabolism and membrane-related processes. α-Ketoglutarate can participate in signaling and hydroxylation reactions. Citrate itself may participate in metabolic oscillations that coordinate glycolysis with mitochondrial metabolism.

Seen this way, incomplete carbon oxidation is not necessarily metabolic failure.

It can be adaptive carbon redistribution.

Anaplerosis and cataplerosis as pressure management

One particularly revealing part of the 2005 perspective is the authors' discussion of why accumulated TCA-cycle intermediates must be exported.

They point out that excessive accumulation would alter TCA-cycle enzyme activity and mitochondrial NAD/NADH balance. In other words, carbon cannot simply continue entering a metabolic network without consequences. The expanding pool has to be processed, redistributed or exported.

That gives anaplerosis and cataplerosis an interesting systems-level interpretation:

increasing substrate input → internal metabolic pressure → redistribution and export → preservation of useful cellular function

The exported carbon is not simply waste. The cell converts it into useful products—redox capacity, membrane components, signaling molecules and other resources that help maintain insulin secretion.

This is remarkably close to the broader logic we describe in our forthcoming perspective on bioenergetic impedance.

From metabolic rerouting to bioenergetic impedance

Our perspective proposes bioenergetic impedance as a systems-level construct describing a time-dependent constraint on energetic throughput and recovery when energetic pressure exceeds available biological capacity.

The important point is that impedance is not synonymous with mitochondrial dysfunction.

Nor is lactate production, metabolic rerouting, inflammation, oxidative stress or any particular metabolite itself “impedance.”

Instead, these processes can occupy different positions in the sequence.

A limitation in substrate handling, oxygen delivery, oxidative capacity or redox regulation may constrain throughput. Redox backpressure or metabolite accumulation may reflect that constraint. Carbon rerouting may then become a compensatory response that helps the cell continue functioning despite the mismatch.

In simplified form:

energetic demand → throughput constraint → pressure / congestion → compensatory rerouting → preserved function

That makes the β-cell described by MacDonald and colleagues a useful historical example.

The cell is not simply failing to oxidize glucose.

It is reorganizing carbon flow.

More metabolism can sometimes mean more compensation

This distinction matters because we often interpret metabolic abnormalities too quickly.

If a cell increases glycolysis, lactate production, anaplerosis, lipid synthesis or mitochondrial biogenesis, the instinct may be to label the process dysfunctional.

But a compensating system can actually look metabolically more active.

The central question is not simply whether a pathway has increased or decreased.

The deeper question is:

What problem is that pathway solving?

If oxidative throughput cannot fully accommodate incoming carbon and reducing equivalents, alternative routes may preserve ATP production, redox balance, biosynthesis and cellular output.

Our forthcoming framework explicitly treats metabolic rerouting in this way: as a possible adaptive response to constrained throughput, rather than as evidence of impedance by itself.

This also helps explain why apparently opposite metabolic states can arise from similar underlying pressures.

One tissue may increase glycolysis.

Another may increase fatty-acid oxidation.

Another may restrict substrate entry through insulin resistance.

Another may divert carbon toward lactate, lipid storage or biosynthesis.

The pathways differ, but the systems problem may be similar: incoming energetic demand has to be reconciled with finite throughput and recovery capacity.

The missing dimension is time

The major extension of the bioenergetic-impedance framework is not simply carbon rerouting.

It is recovery.

Temporary rerouting can be entirely adaptive.

A meal, exercise bout, infection, fasting period or other challenge can transiently push demand beyond immediate capacity. The system responds, redistributes resources, clears accumulated pressure and then returns toward—or sometimes beyond—its previous adaptive capacity.

Our perspective describes this as adaptive impedance cycling.

Challenge produces a temporary constraint. Compensation occurs. Recovery resolves the constraint. Flexible network organization is restored or recalibrated, and reserve may even expand through hormetic adaptation.

The biological meaning changes when recovery remains incomplete.

Then the sequence becomes:

challenge → constraint → compensation → incomplete recovery → residual constraint → next challenge

Repeated often enough, compensatory pathways may no longer be temporary.

The system begins the next challenge while still carrying unresolved metabolic burden from the previous one.

That is where compensation can gradually become lock-in.

Why this matters for aging

This is where an old metabolic observation becomes relevant to contemporary aging science.

Aging research has traditionally focused heavily on accumulated damage: mitochondrial dysfunction, oxidative stress, inflammation, senescence, altered nutrient sensing and other hallmarks.

These remain important.

But increasingly, another question is emerging:

Why do these processes persist rather than resolve?

A cell can tolerate substantial metabolic stress if it can recover.

It can temporarily reroute carbon.

It can increase antioxidant production.

It can mobilize lipids.

It can alter endocrine signaling.

It can activate inflammatory and mitochondrial stress pathways.

These responses may initially be protective.

The problem may arise when the system repeatedly needs them and never fully returns to a flexible baseline.

In our framework, aging can therefore be viewed partly as a progressive shift from:

transient compensation → successful recovery

toward:

persistent compensation → incomplete recovery → reduced reserve → greater constraint during subsequent challenges.

Our Figure 2 describes precisely this distinction: adaptive impedance cycling on one side and progressive impedance lock-in on the other.

Aging may therefore be less about one “bad pathway” than about failed resolution

This changes how many familiar aging phenomena can be interpreted.

Elevated lactate may not initially be a defect; it can regenerate NAD⁺ and sustain glycolytic throughput when oxidative metabolism cannot fully accommodate incoming carbon.

Insulin resistance may not begin simply as metabolic failure; under nutrient excess it can restrict further substrate entry into an already pressured system.

Inflammation can mobilize substrates and coordinate defense.

Senescence can arrest damaged cells.

Lipid storage can temporarily sequester surplus carbon.

Stress hormones can redistribute resources toward immediate survival.

Each can be adaptive within the appropriate temporal context.

The question is whether the system can switch them off again.

That is why recovery dynamics may eventually become as important to aging science as identifying the pathways themselves.

From isolated pathways to network physiology

The other major step is scale.

A cell does not operate alone.

If one tissue becomes constrained, its compensatory response changes the environment experienced by other tissues.

Metabolites, cytokines, mitochondrial stress signals, GDF15, FGF21, autonomic signaling, endocrine responses and vascular redistribution can communicate local energetic conditions throughout the organism.

The brain may alter appetite or activity.

The liver may mobilize or store substrates.

Adipose tissue may buffer circulating fuel.

Muscle may alter glucose uptake.

The immune system may redirect resources toward defense.

The vascular system may redistribute perfusion.

Thus a local energetic problem can become a network-level adaptation.

Our perspective therefore moves from local bioenergetic impedance to propagation and ultimately to reorganization of physiological networks. The decisive question again is whether this reconfiguration remains temporary and context-appropriate—or becomes persistent and inflexible.

The older literature may have been showing us pieces of the same picture

MacDonald and colleagues were not describing aging, and they were not proposing bioenergetic impedance.

Their work should not be retroactively presented as proof of the framework.

But it provides a valuable example of something metabolic physiology has been observing for decades:

cells do not simply maximize complete oxidation of available fuel.

They continually redistribute carbon according to capacity, demand, redox state and functional priorities.

Anaplerosis, cataplerosis, lactate formation, lipid synthesis, substrate cycling and other apparently inefficient processes may sometimes represent ways of maintaining useful biological work when direct oxidative throughput cannot—or should not—accept everything arriving at once.

The new question is whether these individual observations can be connected through a common systems principle.

Bioenergetic impedance proposes that they sometimes can.

From carbon routing to the mechanics of resilience

The 2005 β-cell perspective asked how mitochondrial metabolites help control insulin secretion.

Two decades later, we can extend that question:

What does metabolic rerouting tell us about the relationship between energetic demand, throughput capacity and recovery?

That shift in perspective may be useful far beyond diabetes.

It provides a way to connect mitochondrial biology with stress physiology, metabolic adaptation, network physiology and aging.

The key issue is no longer simply whether a pathway is activated or suppressed.

It is whether the organism can:

conduct the demand, buffer the mismatch, reroute when necessary, resolve the resulting pressure, and recover enough capacity for the next challenge.

Aging may increasingly be understood not simply as the accumulation of molecular damage, but as the progressive difficulty of completing that cycle.

And perhaps some of the pathways we have long called “dysfunctional” are better understood first as compensations that have been asked to remain active for too long.

References

MacDonald, M. J., Fahien, L. A., Brown, L. J., Hasan, N. M., Buss, J. D., & Kendrick, M. A. (2005). Perspective: Emerging evidence for signaling roles of mitochondrial anaplerotic products in insulin secretion. American Journal of Physiology-Endocrinology and Metabolism, 288(1), E1–E15. https://doi.org/10.1152/ajpendo.00218.2004

Tippairote, T., & Hoonkaew, P. (in press). Bioenergetic impedance: A network physiology framework for stress adaptation, maladaptation, and recovery. Frontiers in Network Physiology.