Some explanations for stubborn fat are well supported. Some are plausible but unproven. And some are sold harder than the evidence justifies, including by people in my own field.
I've worked with people on fat loss for over three decades, and I run a practice built around functional testing. So I have a stake in the idea that hidden factors like hormones, gut and toxicity hold people back. You should know that before you read this.
It's also why I've tried to grade each explanation honestly. Some of what my field says about fat loss is well supported. Some of it isn't, and you deserve to know which is which before you spend money on tests or supplements, including mine.
Strong means consistent evidence from several independent sources. Moderate means good evidence with real gaps. Weak means limited or conflicting. Unsupported means the claim is made widely but the evidence isn't there.
The claim: The wrong macronutrients, foods or meal timing block fat loss.
What the evidence shows: In tightly controlled metabolic-ward studies, where every mouthful is measured, low-carb and low-fat diets matched for calories and protein produce very similar fat loss. The DIETFITS trial found the same over a year in 609 people living normally: 5.3 kg lost on low-fat, 6.0 kg on low-carb. Neither genes nor insulin response predicted who would do better on which. But look at the spread within each group, from about 30 kg lost to 10 kg gained. The diet mattered much less than how each person responded to it and stuck with it.
Food quality affects how much you eat. In a small but tightly controlled NIH study, 20 people spent two weeks on an ultra-processed diet and two weeks on a minimally processed one, with the same nutrients available. On the ultra-processed diet they ate about 500 calories a day more, without trying. Adequate protein also improves fullness and protects muscle while you lose fat.
The claim: The wrong kind of exercise, like too much cardio or not enough intensity, stalls fat loss.
What the evidence shows: Exercise alone produces less weight loss than people expect. The body compensates, partly through appetite and partly because total daily energy use doesn't rise as much as the exercise suggests. Where exercise matters most is resistance training, which protects muscle while you're losing fat. Physical activity is also one of the strongest predictors of keeping weight off long term. Fasted cardio has no clear advantage over fed cardio.
The claim: Mindset, all-or-nothing thinking and emotional eating sabotage progress.
What the evidence shows: Across diet trials, how well people stick to the plan predicts results better than which plan it is. Behavioural approaches like self-monitoring, planning, and strategies for emotional eating have reasonable trial support. All-or-nothing thinking, where one slip means the whole plan is off, is a well-documented pattern in people who regain. So is weight stigma, including from health professionals, which tends to make things worse.
The claim: Poor sleep makes fat loss harder.
What the evidence shows: Short sleep increases hunger and food intake, and that's been replicated across many studies. One small study adds a further twist: 10 people on the same calorie-controlled diet for two weeks lost about half as much fat, and more muscle, when given 5.5 hours in bed rather than 8.5. That finding needs repeating in larger groups, but it fits everything else we know.
The claim: Some prescribed drugs cause weight gain.
What the evidence shows: This is well established and often overlooked. Weight gain is a recognised effect of steroids such as prednisolone, insulin and some other diabetes drugs, several antipsychotics, some antidepressants, some anti-epileptics, gabapentin and pregabalin, and some older beta-blockers.
The claim: Chronic stress raises cortisol, which drives belly fat, and "adrenal fatigue" blocks fat loss.
What the evidence shows: Very high cortisol, as in Cushing's syndrome, causes central weight gain. That's real but rare. Chronic stress is linked to higher long-term cortisol exposure and to obesity, and it clearly affects eating, sleep and motivation.
"Adrenal fatigue" isn't a recognised diagnosis. A 2016 systematic review found no evidence that the adrenal glands become exhausted. I still use the term with clients, but as a way of explaining a pattern, not as a diagnosis. It describes a progression many people recognise: high cortisol output under sustained demand, then a flat, low pattern with poor recovery. What's actually happening is better described as HPA axis dysfunction: changes in how the brain and pituitary regulate cortisol and how it's cleared, not glands that have run out. That research is legitimate. What hasn't been validated is using commercial cortisol patterns to diagnose a treatable condition or predict fat loss.
The claim: An underactive thyroid blocks fat loss, even when blood tests are "normal."
What the evidence shows: Overt hypothyroidism causes modest weight gain, much of it fluid, and treating it produces modest loss. It's always worth ruling out. Subclinical hypothyroidism, where TSH is slightly raised and T4 is normal, has at most a small effect on weight, and there's no good evidence that treating it helps. Taking thyroid hormone to lose weight when your thyroid is normal is ineffective and harmful.
The claim: Hormone imbalances, like menopause, PCOS or low testosterone, make fat loss harder.
What the evidence shows: Menopause shifts fat towards the abdomen and is associated with modest weight gain. HRT doesn't appear to cause weight gain. PCOS is closely tied to insulin resistance, which can make weight management harder. Low testosterone in men is associated with more body fat, and treating genuine deficiency reduces fat mass modestly, though losing weight itself also raises testosterone.
The claim: Imbalances in dopamine, serotonin and other neurotransmitters drive cravings and block fat loss, and can be tested and corrected.
What the evidence shows: The brain's reward and appetite systems are central to eating behaviour. That's well-established science, and it's one reason GLP-1 drugs work. But measuring neurotransmitters in urine, as some functional tests do, doesn't reflect brain levels, because most of what's in urine comes from the kidneys and gut. There's no good evidence that correcting urinary levels with amino acids improves fat loss. I trained in this approach, and I think the underlying biology is real while the testing and protocols are ahead of the evidence.
The claim: Environmental chemicals are "obesogens" that make you gain fat and block its loss.
What the evidence shows: Several hormone-disrupting chemicals cause fat gain in animal studies. In people, the evidence is mostly observational. In one diet trial of 621 people, blood PFAS levels didn't affect how much weight people lost, but women with the highest levels regained around 2 kg more afterwards, and their resting metabolic rate fell further. When you lose fat, stored pollutants are released into the blood, but whether that slows fat loss in practice isn't established. There's no good evidence that detox protocols improve fat loss.
The claim: An imbalanced microbiome blocks fat loss.
What the evidence shows: In a well-known experiment, gut bacteria from human twins, one lean and one obese, were transferred into mice. The obese twin's bacteria produced more fat gain, and the effect depended on what the mice ate. That's striking. But in people, trials transferring gut bacteria from lean donors haven't produced weight loss. A 2023 meta-analysis of nine randomised trials found no difference in weight compared with placebo, although some short-term improvements in insulin sensitivity appeared. No probiotic has reliably produced meaningful fat loss either. Gut health matters for plenty of other reasons: symptoms, inflammation, absorption and quality of life.
The claim: Damaged mitochondria or a "broken metabolism" stop fat loss.
What the evidence shows: Metabolic adaptation is real. When you lose weight, your metabolic rate drops, sometimes more than your smaller body size alone would predict, and hunger hormones rise. This is the body defending its weight, and it's one reason keeping weight off is harder than losing it. But there's little evidence that mitochondrial supplements or "metabolism repair" protocols make a meaningful difference.
GLP-1 drugs like semaglutide and tirzepatide produce more weight loss than any other non-surgical treatment, typically 15 to 20% of body weight in trials. That's a fact, whatever you think of the pharmaceutical industry. They also have real downsides: side effects, some muscle lost along with the fat, and regain when people stop. In the main semaglutide trial, people regained about two-thirds of the weight they'd lost within a year of stopping, and most of the improvements in blood markers went back towards where they started. For some people they're the right tool. Whether they're right for you is personal, and a decision to make with your doctor.
Some weight changes need medical assessment before anything else:
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