Meet Your Metabolism

Metabolism is often described as speed: fast or slow, efficient or sluggish. That framing has some use, but it conceals more than it reveals. What metabolism actually does is coordinate. It determines how the body partitions fuel across tissues, shifts between energy substrates, and adjusts those decisions in response to hormonal signals, time of day, and the state of the tissues doing the work. The question worth asking is not how fast the system runs. It is how well the parts stay in step.

Beyond the calorie model

The calorie model of metabolism has shaped public thinking for decades. Eat less than you burn and you lose weight. Eat more and you gain it. There is truth in that framing, but it has narrowed public understanding of what metabolism does and what it means when things start to go wrong.

The body is not a passive recipient of calories. It senses fuel availability, adjusts hormone output, shifts between substrates, and coordinates those decisions across organs. When Gerald Reaven delivered his Banting Lecture in 1988, the condition he described was not a calorie problem. It was a coordination failure. Insulin resistance, elevated triglycerides, hypertension, and central adiposity were appearing together in the same patients, and Reaven argued they shared a common upstream mechanism. 1

The conditions were not separate diseases arriving by coincidence. They were arriving together because the regulatory system managing them was losing coherence.

That distinction changes what is worth paying attention to. If metabolism is about energy balance alone, the levers are intake and expenditure. If metabolism is coordination, the more informative questions are about insulin sensitivity, tissue responsiveness, body composition, and timing. Calories still matter. They are just not sufficient to explain most of what goes wrong.

Insulin as a coordination hormone

Insulin is usually introduced as a blood sugar hormone. It does regulate blood sugar, but that is one function among many. Insulin directs how the body partitions energy: whether nutrients are stored, mobilized, or directed toward growth and repair. It coordinates glucose uptake in muscle, suppresses glucose release from the liver, promotes energy storage in adipose tissue, and influences protein synthesis, lipid metabolism, and inflammatory signaling. 2

The basic rhythm is straightforward. After a meal, insulin rises. That signal redirects the body toward processing and storing incoming nutrients. Between meals, insulin falls, and the body shifts toward releasing and oxidizing stored fat. When the system is sensitive, a modest rise in insulin produces a clear response across tissues.

The trouble begins when that sensitivity erodes. Muscle becomes less responsive to insulin's uptake signal. The liver continues releasing glucose even when circulating levels are already adequate. Fat tissue, which should be suppressing lipolysis in the presence of insulin, begins to release free fatty acids at inappropriate times, further interfering with glucose metabolism in muscle and liver. 3

The pancreas compensates for all of this by producing more insulin to achieve the same downstream effect. Blood glucose may remain in the normal range for years, but the insulin output required to maintain that range keeps climbing. That rising cost is not visible on a standard fasting glucose test. 45

This is worth understanding because it reframes what early metabolic strain looks like. It does not always look like high blood sugar. It often looks like a system that is still producing acceptable numbers but requiring disproportionate effort to do so.

What happens when insulin stays elevated

Chronically elevated insulin, called hyperinsulinemia, is one of the earlier measurable signs that coordination is slipping. In many individuals it precedes any detectable rise in fasting glucose by years.6

Whether hyperinsulinemia is purely a consequence of insulin resistance or can also act as an independent contributor to metabolic deterioration remains an open question in the literature. Barbara Corkey's 2011 Banting Lecture explored this directly.

The practical significance does not depend on which direction the causal arrow ultimately points: either way, the pancreas is working harder than it should to maintain a glucose level that looks normal on paper.

The downstream consequences of sustained high insulin are broad. Fat storage increases, particularly in the visceral compartment. Fat oxidation is suppressed, because insulin is a strong anti-lipolytic signal. Inflammatory signaling tends to increase. And insulin resistance itself tends to deepen, creating a cycle where the condition promotes more of the same condition. 78

A note on clinical utility. Fasting insulin is not a standard screening metric in most settings. Its interpretation depends on assay method, population norms, and the patient's clinical picture. It is not a standalone diagnostic. But when interpreted alongside fasting glucose and HOMA-IR, it can provide directional information about pancreatic workload that glucose alone does not. 910

Why muscle matters more than most people realize

Skeletal muscle is the body's largest insulin-responsive tissue and the primary destination for blood sugar after a meal in a metabolically healthy person.11 That makes the quantity and quality of muscle one of the most consequential variables in how well the body handles carbohydrate.

When muscle is well-trained and insulin-sensitive, glucose is absorbed efficiently with modest insulin output. When muscle mass is low or the tissue has become resistant, the same meal demands more insulin to clear the same glucose load. The workload on the pancreas increases. The system has less margin.

This is one reason resistance training improves insulin sensitivity in intervention studies even when body weight does not change. 12

The tissue itself becomes a more effective glucose sink, absorbing more glucose per unit of insulin. It is also why muscle loss, whether from aging, prolonged inactivity, aggressive calorie restriction, or illness, has metabolic consequences that extend well beyond strength or appearance. Less muscle means less glucose disposal capacity. The system compensates by requiring more insulin to clear the same load, and over time that compensation erodes other parts of the metabolic chain.

Muscle matters for a second reason that receives less attention. It is one of the body's most mitochondria-dense tissues and the primary site where the body oxidizes both glucose and fat for energy. When mitochondrial density or function in muscle declines, the body's capacity to shift between those two fuel sources becomes impaired. That impairment has been measured directly in people with insulin resistance and type 2 diabetes, using respiratory exchange ratio data that shows a blunted ability to increase fat oxidation during fasting conditions. 13

The connection between tissue quality and fuel switching is one of the clearest in metabolic physiology, and it leads to a broader concept called metabolic flexibility.

How fat tissue changes the signaling environment

Fat tissue produces signaling molecules that influence insulin sensitivity, inflammation, and energy regulation throughout the body. The metabolic consequences depend heavily on where fat is stored.

Visceral fat, which accumulates around the abdominal organs, releases inflammatory cytokines including TNF-alpha and IL-6 that interfere with insulin signaling in muscle and liver. 14

As visceral fat increases, the coordinated response to insulin across tissues becomes less precise. Subcutaneous fat is generally less disruptive, which is part of why two people at the same body weight can have markedly different metabolic profiles depending on fat distribution.15

Under conditions of lipid oversupply, intermediates called ceramides accumulate in tissues and can impair insulin signaling directly.16

The broader point: body weight as a single number conceals most of the metabolic information that matters. Two people at the same weight can have very different metabolic profiles depending on how much of that weight is muscle versus fat, and where the fat sits. Composition and distribution are more informative, though harder to assess in routine clinical encounters. Waist circumference, tracked consistently, captures more of this picture than scale weight does.

Metabolic flexibility and why it degrades

A well-coordinated metabolism shifts between fuel sources depending on availability. After a carbohydrate-rich meal, the body increases glucose oxidation. Between meals, it shifts toward fat. This capacity to transition smoothly between substrates is called metabolic flexibility. 1718

As insulin resistance progresses, that flexibility tends to degrade. The body becomes less efficient at accessing stored fat between meals, partly because elevated insulin suppresses lipolysis, and partly because the enzymatic and mitochondrial machinery for fat oxidation has been underused and has declined in capacity.1920

The practical experience is familiar to many people even if they do not have a name for it: difficulty going more than a few hours without eating, sharp energy drops in the afternoon, a persistent sense that the body resists using its fat stores despite carrying more of them than it needs.

Metabolic flexibility is not restored by any single intervention. It tends to improve when insulin sensitivity improves, which usually involves some combination of regular resistance and aerobic exercise, reduced chronic overfeeding, meal timing that allows insulin to fall meaningfully between eating windows, and adequate sleep. 21

The value of those inputs is cumulative. None is dramatic on its own. The body gradually becomes better at switching between substrates because the conditions that prevented it from doing so, principally sustained hyperinsulinemia and impaired mitochondrial function in muscle, begin to resolve.

A limitation: metabolic flexibility is a research construct that describes a real physiological process, but it is not measured in clinical practice. There is no routine test. Its improvement is usually inferred from downstream markers rather than observed directly.

Practical payoff

None of the above replaces medical evaluation. But a few informal observations, tracked over time, can provide directional information about whether coordination is intact or beginning to slip.

Energy stability through the day. Tolerance for going several hours between meals without sharp drops. Waist circumference trending over months. Fasting insulin, when available and interpreted in clinical context, as a complement to fasting glucose. These are heuristics, not diagnoses. They are worth tracking because they ask a question that a fasting glucose alone does not.

Reframing

Metabolism is the ongoing coordination between insulin, muscle, fat tissue, liver, and circadian timing. When that coordination holds, the body handles what it receives. When it begins to slip, the problems accumulate quietly, sometimes for years before a lab result crosses a clinical threshold. The shift back toward coordination tends to be gradual and undramatic, visible first in how the day feels and only later in what the labs confirm.

FAQs

Is metabolism really about calories?
Calories contribute to energy balance. How the body partitions, stores, and mobilizes those calories depends on insulin sensitivity, muscle mass, hormonal timing, and substrate switching capacity. The calorie framework is part of the picture, not the whole of it. 2223

Is fasting insulin a useful early marker?
It can add information when interpreted alongside fasting glucose, HOMA-IR, and clinical context. It is not a standard screening metric in most settings, and assay variability complicates interpretation. But it provides a signal about pancreatic workload that glucose alone does not. 2425

Does exercise improve metabolic health even without weight loss?
Resistance training improves insulin sensitivity and glucose disposal capacity independently of changes in body weight.26

Why has insulin become so central to the metabolic conversation?
Because insulin resistance ties together conditions that were previously treated as separate problems. Reaven's original insight was that the clustering of those conditions pointed to a shared upstream mechanism. 2728