Calorie Deficit vs. Fat Deficit: What People Actually Want (Pt. 3)

Being in a fat-burning state isn’t about skipping carbs or doing fasted cardio. It’s about the internal conditions of your body — the hormonal, metabolic, and physiological environment – that determines whether you burn fat or store it.

There are four key, though not exclusive, aspects of metabolic health that determine how efficiently your body uses fat for energy and how consistently you can stay in a fat deficit without relying on stress or restriction:

• Insulin sensitivity

• Metabolic flexibility

• Cortisol regulation

• Fat oxidation capacity

Together, these systems shape how your body responds to food, movement, stress, and recovery. Improving them shifts your physiology toward using fat as the preferred fuel source — making fat loss feel less forced and more natural.

In other words, exactly how it’s supposed to work in a healthy, metabolically efficient body.

Insulin is a storage hormone and it directly inhibits fat burning. When insulin is elevated (such as after a carb-heavy meal or frequent snacking), the body is signaled to store energy rather than use it. This doesn’t mean carbs are “bad” — it means that constantly spiking insulin makes it harder for your body to access stored fat as a fuel source.

Over time, this pattern — especially when combined with calorically dense, ultra-processed foods — can lead to insulin resistance and, eventually, type 2 diabetes.

To enter a fat-burning state, insulin needs to be low and stable for portions of the day — which happens naturally when your meals are balanced, your blood sugar is steady, and you’re not constantly grazing or relying on ultra-processed foods. In addition to these factors, both aerobic exercise and strength training improve insulin sensitivity — but aerobic exercise does a better job of regulating it and keeping its baseline levels low throughout the day.

This is also why people with insulin resistance often struggle to lose fat: their body’s signaling is dysregulated. Fat-burning is impaired, cravings are heightened, and energy is unstable. Until insulin sensitivity is improved, the very mechanisms required for fat loss remain suppressed.

Fixing this isn’t about restriction — it’s about restoring function. If a trainer or nutritionist thinks that a calorie deficit, 10,000 steps, and high protein will solve fat loss for someone with insulin resistance or diabetes, best of luck to them. It might “work” in the short term — mostly by shedding water and lean mass — but it’s very likely to worsen insulin resistance in the long run.

A metabolically flexible body can efficiently switch between burning glucose and fat depending on energy demands. This is how the body is designed to function, seamlessly shifting between fuel sources based on activity level, food intake, and internal conditions.

But most people today are metabolically inflexible — overly reliant on glucose, quick to fatigue when meals are delayed, and unable to efficiently oxidize fat for energy. This shows up in two key ways:

During low-intensity movement, the body taps into carbs too early, instead of primarily using fat as it should.

At rest, the ratio of fat to carbohydrate being burned is skewed — favoring carbs, even when no movement is happening.

This second point reflects poor fat oxidation efficiency — the body’s ability to use fat even at rest — which we’ll explore in more depth later.

Fortunately, metabolic flexibility can be restored. Daily aerobic training, steady blood sugar, and lower baseline stress levels — all of which improve when you consistently practice intuitive eating — support this process. When you’re metabolically flexible, you can go hours without eating (not that this is the goal, but rather an option when necessary), move through your day with stable energy, and your body burns more fat even while at rest.

Cortisol is often misunderstood. It’s not bad, we need it. In fact, cortisol plays a key role in waking us up, mobilizing fuel, and helping us respond to stress. And to the cardio critics' credit, aerobic exercise — especially at higher intensities like Zone 3 or 4 — does raise cortisol acutely.

The key distinction is acute versus chronic elevation.

When cortisol stays high for too long — due to chronic under-eating, poor sleep, emotional stress, blood sugar swings, or too much caffeine — it shifts the body into a catabolic state. This makes it harder to build or preserve muscle, while encouraging fat storage, especially around the abdomen. But the damage doesn’t stop there.

Chronic cortisol elevation can lead to:

Reduced thyroid hormone output, which slows metabolism

Impaired insulin sensitivity, making fat oxidation more difficult

Suppressed reproductive hormones, causing poor recovery, low libido, and mood swings

Increased inflammation, which further blunts metabolic efficiency

Women are more sensitive to elevated baseline cortisol and its negative effects compared to men, making it especially important for women to maintain a low, healthy cortisol baseline.

Ironically, most calorie-deficit-based diets increase cortisol — which is why they often feel harder than they should, accompanied by cravings, poor sleep, and emotional volatility.

On the flip side, low- to moderate-intensity aerobic training (like Zone 2 cardio) helps regulate cortisol, improving mood, sleep, and overall hormonal balance. Over time, this lowers baseline cortisol, helping the body feel safe — and when the body feels safe, it lets go of stored fat more easily.

Fat oxidation is the literal process of burning fat — converting stored triglycerides into usable fuel. It’s what people think they’re doing when they diet or cut calories, but unless the body is primed for it, it doesn’t happen efficiently.

Aerobic exercise is the #1 way to increase fat oxidation capacity — especially when done consistently, daily, and within the right intensity zone (like Zone 2). Over time, this improves mitochondrial function, oxygen delivery, and the body’s preference for fat as a primary fuel source.