How insulin works: glucose, fat storage and resistance
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Daniel UreelNASM & EREPS N4 Certified Personal Trainer · Founder Rebirth35
Read ~6 min
Pillar Metabolism
Insulin is the key to your metabolism. It regulates blood sugar, decides whether you burn or store fat — and when it goes wrong, your entire physical transformation suffers. Here's how it actually works.
Insulin: the conductor of glucose
At every meal, your digestive system breaks down carbohydrates into glucose, which passes into the bloodstream. Blood sugar rises. In response, the pancreas secretes insulin — a hormone whose role is to open cell doors so glucose can enter and be used as energy.
Without insulin, glucose remains stuck in the blood — that's the definition of type 1 diabetes. With properly functioning insulin, glucose quickly reaches the liver, muscles, and serves as fuel.
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Useful metaphor: think of insulin as an access badge. Glucose is the person. The cell is the door. Without a badge, you can't get in. With a damaged badge (insulin resistance), the door barely opens — and glucose piles up in the corridor (the bloodstream).
The spike problem: what really happens
Not everything plays out the same way depending on what you eat. A meal rich in fast carbohydrates (white bread, juice, sugary cereals, white rice) causes a sharp rise in blood sugar — and therefore a massive insulin spike to handle it.
This spike triggers a cascade of consequences:
1
Blood sugar spike
Blood glucose rises rapidly above normal. You feel fine for 30–60 minutes.
2
Massive insulin release
The pancreas sends a large amount of insulin to "handle" the spike. This response is proportional to how fast blood sugar rises.
3
Blood sugar in free fall
Insulin drives blood sugar below the normal threshold — reactive hypoglycaemia. Around 2.5–3 hours after the meal: fatigue, irritability, cravings.
4
Craving fast carbs again
Your brain, starved of glucose, pushes you to eat something sweet. The cycle starts again — several times a day.
Blood sugar — high vs low glycaemic index meal
Typical curve simulation over 4 hours post-meal
High-GI carbs (white bread, juice…)
Low-GI carbs (complex, fibrous)
Insulin blocks fat burning
Here's what most people don't know: when insulin is elevated, you cannot burn fat.
Insulin is an anabolic hormone — it promotes storage. When it's present in significant amounts in the blood, it activates an enzyme called lipoprotein lipase (LPL) that captures circulating fatty acids and stores them in fat cells. At the same time, it inhibits lipolysis — the process by which your fat cells release their reserves to be burned.
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Active lipolysis when insulin is elevated
4–6h
Typical insulin spike duration after high-carb meal
12–16h
Minimum fasting time to maximise lipolysis
In practice: if you eat every 2–3 hours based on fast carbs, your insulin stays chronically elevated — and your body never gets the chance to tap into its fat reserves. Even if you're in a caloric deficit, fat loss is impaired.
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What this means for you: it's not just about calorie count. The timing, quality of carbs, and insulin management determine whether your body is in "storage" or "burning" mode.
The path to insulin resistance
Imagine constantly triggering the same pancreatic response — spike after spike, day after day. Eventually, cells become less sensitive to the insulin signal. They need increasing doses to achieve the same effect. That's insulin resistance.
The insulin resistance cycle
How repeated spikes lead to chronic hyperinsulinaemia
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Insulin resistance is silent. It can build gradually over years before causing visible symptoms. It's the precursor stage to type 2 diabetes, but also hypertension, cardiovascular disease and visceral fat accumulation.
Concrete consequences on body composition
Fat
Visceral fat accumulation
Excess glucose not captured is converted to triglycerides and stored around organs. The most dangerous type of fat.
Muscle
Muscle mass loss
Insulin-resistant muscles absorb less glucose and amino acids. Protein synthesis reduced, catabolism increased.
Energy
Chronic fatigue
Cells lack energy despite elevated blood sugar. Fatigue, brain fog, recurring energy crashes.
Inflammation
Systemic inflammation
Chronic hyperinsulinaemia activates inflammatory pathways that damage blood vessels and accelerate cellular aging.
The strategy: stabilise blood sugar to burn fat
The good news: insulin sensitivity can be restored. Let's look concretely at the difference between a high glycaemic index breakfast and a low glycaemic index one:
❌ High glycaemic index
Classic breakfast
Orange juice (concentrated fructose)
White bread / jam on toast
Sugary cereals or commercial muesli
Sweetened coffee
Glucose spike → storage → cravings at 10:30am
✓ Low glycaemic index
Metabolic breakfast
Whole eggs (protein + fat)
Avocado or nuts (healthy fats)
Plain Greek yoghurt or skyr
A handful of berries (fibre + low fructose)
Stable blood sugar → active lipolysis → sustained energy
Exercise increases glucose uptake by muscles independently of insulin (via GLUT4). The most powerful and fastest lever available.
Lever 03
Eating windows
Reducing the window during which you eat gives insulin time to drop and allows lipolysis to activate.
Lever 04
Sleep and stress management
Sleep deprivation degrades insulin sensitivity within 2 days. Chronic cortisol stimulates hepatic glucose production and amplifies spikes.
Lever 05
Prioritise protein and fibre
Proteins extend satiety. Fibre slows glucose absorption and flattens spikes. A structured meal (protein + vegetables + healthy fats + complex carbs) generates an insulin response 2 to 3 times lower than a carb-only meal.
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Most people think losing fat is about willpower. In reality, it's first and foremost about biochemistry. Stabilise your insulin — and your body will naturally regain its ability to burn fat.
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NASM & EREPS N4 Certified Personal Trainer · Founder Rebirth35
NASM certified personal trainer and sports coach, specialised in physical transformation after 35. Understanding metabolism is at the heart of my method: knowing how your body works is the condition for making it evolve durably.