Weight Loss Naturopath & Nutritionist · Australia WideRoot-Cause Support For Women's Weight Loss.

Insulin resistance (the engine of stubborn weight)

Insulin is the hormone your pancreas releases every time you eat. Its job is to clear glucose out of your bloodstream and move it into your cells, either to be used for energy or stored for later. Healthy insulin function is fast, efficient, and quiet. You eat, insulin rises, glucose moves into the cells, insulin drops back to baseline, and your body switches to burning stored fat for energy in the gap until the next meal.

Insulin resistance is what happens when this system has been overworked for years. After repeated exposure to high circulating insulin (from frequent carbohydrate-heavy meals, snacking through the day, chronic stress raising blood sugar through cortisol, poor sleep, or all of these stacked together), your cells become less responsive to insulin's signal. They stop opening the door as easily. Your pancreas compensates by producing more insulin to push the same amount of glucose into the cells, and the higher your circulating insulin sits, the harder it is for your body to access stored fat.

Insulin is a fat-storing hormone. While it is elevated, your fat cells are locked, and no amount of dietary restriction or exercise will override that hormonal signal.

The most useful marker for this is fasting insulin, which most GPs do not run. A fasting glucose can sit within range for years while fasting insulin climbs, because the pancreas is working harder and harder to keep glucose normal. By the time fasting glucose looks abnormal on a standard panel, insulin resistance has often been present for 5 to 10 years.

From a functional perspective, fasting insulin around 4 indicates good cell sensitivity and minimal pancreatic strain. Fasting insulin around 7 or 8 is the beginning of insulin resistance, the metabolic strain is already underway (fat storage increasing, inflammation building, hormones being affected), and most GPs would call this number normal. Fasting insulin above 10 is insulin resistance, and the body is in active fat storage with impaired fat burning.

Insulin resistance gets worse with age in women because oestrogen plays a protective role in cell sensitivity. As oestrogen fluctuates and declines through perimenopause, your cells become less responsive to insulin, your pancreas produces more of it, and the fat storage pattern accelerates around the belly, the upper body, and the upper arms. This is one of the reasons women in their late 30s, 40s, and 50s start gaining weight in those specific areas for the first time despite no change in their diet or activity level. The hormonal environment has shifted, and the food and exercise that used to work no longer match the body running underneath them.

Insulin sensitivity is one of the fastest systems to respond to clinical change. Stable blood sugar, longer gaps between meals, lower carbohydrate intake, adequate protein, and resistance training all directly improve insulin signalling, often within weeks.

Cortisol and belly fat

Cortisol is the hormone your adrenal glands release in response to stress, and stress in this context is anything your body reads as a threat. Work pressure, financial worry, caregiving load, a disagreement with your partner, too much caffeine on an empty stomach, a blood sugar drop at 3pm, a poor night of sleep, an inflammatory meal, an intense workout without adequate recovery. Your body does not separate emotional stress from physiological stress. They both produce the same cortisol response.

When cortisol is short-lived (acute stress, the threat resolves, cortisol drops back down), it is a useful, even essential hormone. The problem for most women in their 30s, 40s, and 50s is that the threat never fully resolves. The job pressure does not lift. The caregiving load is constant. The sleep is interrupted. The blood sugar is unstable. Cortisol stays elevated for weeks, months, and years, and the body adapts to that elevation as the new baseline.

Chronically elevated cortisol does several things to body composition at once.

The first is direct fat storage in the abdominal area. Visceral fat cells (the fat stored deep around your organs) have approximately four times more cortisol receptors than the subcutaneous fat cells stored under your skin on your arms, legs, and hips. This means cortisol preferentially deposits fat around the midsection. The thicker waist, the belly that pinches over the waistband of your jeans, the weight that sits in front and around the sides while the rest of your body looks much like it always has, is a cortisol-driven fat distribution pattern. Calorie restriction and abdominal exercise will not override this hormonal signal while cortisol stays elevated.

The second is appetite hormone disruption. Cortisol increases ghrelin (the hormone that signals hunger to your brain) and reduces leptin (the hormone that signals fullness). The result is that you are hungrier more often, harder to satisfy, and reaching specifically for sugar, refined carbohydrates, and fatty foods because your body is asking for quick fuel for a perceived emergency. The 3pm pantry visit, the evening cravings, the feeling of being hungry an hour after a meal that should have been filling, are hormonal responses to cortisol-disrupted appetite signalling.

The third is the cortisol-insulin loop. Cortisol triggers your liver to release stored glucose into the bloodstream (a process called gluconeogenesis), which raises blood sugar even when you have not eaten. That blood sugar rise triggers insulin to clear it, and now you have both cortisol and insulin elevated at the same time. Both are fat-storing hormones. Both promote inflammation. Both block fat burning. Your body is locked into active storage mode while simultaneously losing the ability to access what is already stored.

I see this loop in clinic constantly in women who have done everything right. The food is good. The training is happening. The weight will not move because the hormonal environment created by chronic stress is promoting fat storage from every direction at once.

Thyroid function and metabolism

Your thyroid gland sits at the front of your neck, and it controls the speed at which your body burns energy. It produces primarily T4, which is the inactive storage form of thyroid hormone. For your cells to use it, T4 has to be converted into T3, the active form. Approximately 60% of that conversion happens in your liver, another 20% happens in your gut (influenced by your microbiome), and the rest happens in muscle and other peripheral tissues. T3 is what determines your resting metabolic rate, your body temperature, your energy production, your hair and skin quality, your bowel motility, and how efficiently every cell in your body operates.

The thyroid problem I see most in women presenting with stubborn weight gain is not low thyroid hormone production. The thyroid gland is often working fine. The problem is that T4 is not being converted into T3 efficiently, which means even though the bloods look adequate, the active hormone your cells actually need is sitting low.

The standard thyroid panel from a GP usually only checks TSH and T4. If both are within range, you are told your thyroid is fine, even when every symptom you have (the fatigue, the cold hands, the weight gain, the brain fog, the dry skin, the thinning hair, the constipation, the slow recovery from exercise) is pointing toward functional hypothyroidism.

The conversion of T4 into T3 is impaired by chronic cortisol, elevated insulin, liver congestion, gut dysbiosis, and nutrient deficiencies (selenium, zinc, iron, iodine, and tyrosine). Almost every woman presenting with stubborn weight gain has at least three of those running at the same time. Cortisol also increases the conversion of T4 into reverse T3, which is an inactive form of thyroid hormone that competes with active T3 at the receptor sites on your cells. So even the small amount of T3 that is being produced has fewer receptor sites available to bind to, and the metabolic signal is weakened from both directions.

The functional thyroid panel I run in clinic includes TSH, free T4, free T3, reverse T3, and TPO and thyroglobulin antibodies. The pattern I find most often in women with stubborn weight gain is TSH normal, T4 normal, T3 low, and reverse T3 elevated. The thyroid gland is producing the hormone. The conversion is where the breakdown is occurring, and the cause is almost always sitting in the liver, the gut, the cortisol load, or the nutrient status, which is why the work is rarely just thyroid.

Resting metabolic rate is the number of calories your body burns while doing nothing, and active T3 is one of the primary regulators of that number. When T3 sits low, resting metabolic rate drops, and your body becomes more efficient at storing what comes in and slower at burning what is already there. This is one of the reasons women with functional hypothyroidism describe feeling like their metabolism has slowed even when their food and exercise have not changed. The number on the bloods is not telling the full story.

Hormonal shifts and body composition (oestrogen dominance, low progesterone, perimenopausal weight)

The hormonal layer of stubborn weight gain is often the layer that surprises women the most, because the weight changes happen at the same time as cycle changes, sleep changes, mood changes, and skin changes, but they are rarely connected by the practitioner the woman is seeing. Each symptom gets treated as a separate issue. What's underneath all of them is hormonal.

Oestrogen and progesterone work in a paired rhythm across your cycle, and that rhythm changes through your 30s, 40s, and into menopause. Progesterone is the calming hormone. It supports deep sleep, stabilises mood, regulates the second half of your cycle, and balances the effects of oestrogen. Progesterone starts to decline earlier than oestrogen, often in the late 30s, which is why many women notice their first signs of hormonal change as worsening sleep, increased anxiety, shorter luteal phases, and more pronounced PMS years before they would consider themselves perimenopausal. Lower progesterone also means oestrogen sits relatively higher even when absolute levels have not increased, which is the pattern called oestrogen dominance.

Oestrogen dominance contributes to weight gain in several specific ways.

The first is fluid retention, particularly through the cycle and especially in the lead-up to a period. The bloating, the puffiness in the fingers and the face, the 1 to 2kg the scale picks up that disappears a few days into the next cycle, are oestrogen-driven fluid shifts rather than fat gain.

The second is fat distribution. Oestrogen promotes fat storage in the hips, the thighs, the buttocks, and the breasts. Higher relative oestrogen often shows up as weight gain in those areas alongside the abdominal weight from the cortisol and insulin pattern.

The third is liver and gut load. Used oestrogen has to be cleared by the liver and excreted via the bowel, and when liver function is congested or the gut microbiome is out of balance, oestrogen recirculates instead of being eliminated. This worsens oestrogen dominance regardless of how much oestrogen the ovaries are actually producing. Insulin compounds this further, because elevated insulin promotes the conversion of testosterone and DHEA into oestrogen in fat tissue, which means the more insulin resistance is present, the more oestrogen is being produced in the body's own fat cells. The cycle reinforces itself.

Through perimenopause, oestrogen does not decline smoothly. It swings, and that erratic pattern compounds the weight picture by repeatedly disrupting insulin sensitivity. As oestrogen levels drop further into menopause, the protective effect oestrogen has on insulin sensitivity is lost, fat redistribution shifts further toward the abdomen, and the progesterone decline that started in the late 30s becomes more significant.

The clinical pattern I see most often in women in this stage is a combined picture: low progesterone, fluctuating or relatively elevated oestrogen, declining DHEA from chronic stress, impaired thyroid conversion, and insulin resistance, all running at once. The weight gain is the visible end of a hormonal sequence that has been compounding for 5 to 10 years.

Gut health and weight (microbiome, inflammation, water retention)

Your gut is doing more for your weight than you can see from the outside. The microbiome (the trillions of bacteria living in your digestive tract) influences how much glucose your body extracts from a given meal, how much insulin is released in response, how efficiently used hormones are cleared, how much inflammation is circulating, and how much water your body holds. None of this shows up on a scale as a separate number, but every part of it shapes the number you see.

The first connection is microbial composition. Some bacterial populations are more efficient at extracting energy from food than others. Two women eating the same meal can absorb different amounts of glucose depending on the makeup of their microbiome, which means the same calorie intake can produce a different metabolic outcome. Women with low microbial diversity (often from years of antibiotics, restrictive eating, low-fibre diets, or chronic stress) tend to have higher inflammatory markers, less efficient blood sugar regulation, and stronger cravings for sugar and refined carbohydrates because the bacteria themselves signal for the food that keeps them dominant.

The second connection is short-chain fatty acid production, particularly butyrate. Beneficial gut bacteria produce butyrate when they ferment fibre from vegetables, legumes, nuts, and seeds. Butyrate strengthens the gut wall (think of it like re-grouting tiles in a bathroom, sealing the gaps so nothing leaks through into the bloodstream), reduces intestinal inflammation, and improves insulin sensitivity directly at the cellular level. When the microbiome is depleted or out of balance, butyrate production drops, the gut wall becomes more permeable, inflammatory compounds cross into the bloodstream, and the liver has to manage that inflammatory load alongside everything else it is processing.

The third connection is the estrobolome, a specific group of gut bacteria responsible for clearing used oestrogen from the body. When the estrobolome is dysbiotic, packaged oestrogen is reactivated in the gut and recirculated instead of being eliminated, which contributes to oestrogen dominance, fluid retention, and the hormonal weight pattern that often shows up alongside heavier or more painful periods.

The fourth connection is inflammation and water retention. A compromised gut wall allows bacterial fragments and partially digested food particles into the bloodstream, which the immune system responds to with low-grade systemic inflammation. Inflammation increases insulin resistance (worsening the storage signal), promotes water retention (showing up as puffiness in the face, fingers, and ankles), and slows recovery from exercise (making it harder to build the muscle that supports metabolic rate). The bloating that worsens through the day, the puffiness in the morning, the sense that your body is holding more fluid than it should, are often gut-driven rather than dietary in origin.

I see this in clinic regularly, and the gut work is rarely optional in stubborn weight cases. Until the microbiome is supported, butyrate production is restored, the gut wall is repaired, and the estrobolome is rebalanced, the metabolic and hormonal work has a ceiling.

Muscle mass and metabolic rate (and why under-eating slows everything down)

Muscle is the engine of your metabolism. Every kilogram of muscle on your body burns calories at rest, supports insulin sensitivity, regulates blood sugar, produces compounds that reduce inflammation, and makes your body a more efficient fat-burner around the clock. The more muscle you carry, the higher your resting metabolic rate, the better your body handles carbohydrates, and the easier it is to maintain a healthy body composition through hormonal change. Muscle is the single most underrated factor in long-term weight management for women, and it is the one that most diet advice ignores.

The problem is that the way most women have been told to lose weight (eat less, do more cardio, repeat for years) actively destroys muscle. Sustained calorie restriction signals your body to break down muscle tissue for energy, particularly when protein intake is too low to preserve it. Cardio without resistance training does very little to build or protect muscle. And every cycle of restrictive dieting followed by regain replaces some of the lost weight as fat instead of muscle, which is why women who have spent 10 or 15 years yo-yoing often find themselves with a smaller frame, higher body fat percentage, and slower metabolism than when they started.

Cortisol compounds this directly. Chronic cortisol breaks down muscle tissue (a process called muscle catabolism) to release amino acids the body uses for glucose production through the liver. So women who are under chronic stress, eating low-protein meals, restricting calories, and doing high-intensity cardio without recovery are losing muscle from multiple directions at once. The metabolic rate drops year on year, and the same eating habits that maintained weight 5 years ago now produce slow weight gain, because the body running underneath is no longer the same body.

GLP-1 medications add a specific layer to this picture. Studies on rapid weight loss through GLP-1 use have shown significant muscle mass loss alongside the fat loss, often 25 to 40% of the total weight lost being lean tissue. For women already low in muscle from years of dieting, this can mean coming off the medication with a much lower metabolic rate than they started with, which is one of the reasons regain after stopping a GLP-1 is so common and so fast. The work of supporting muscle mass before, during, and after GLP-1 use is one of the most important pieces of long-term outcomes, and it is the part most prescribing pathways do not address.

Building and maintaining muscle requires adequate protein, resistance training, and enough food to support the work. The instinct most women have when the scale will not move is to eat less. The clinical reality for women with stubborn weight gain is that under-eating is often part of why the weight will not move, and restoring metabolic rate requires feeding the body enough to rebuild the tissue that does the burning.

I see this in clinic constantly. Women who are eating 1200 calories, doing daily cardio, and gaining weight, because the muscle has gone, the cortisol is high, the thyroid is suppressed, and the body has adapted to survive on less. The intervention is rarely more restriction. The intervention is rebuilding the metabolic foundation.