Youโ€™re Fat. Are Hormones to Blame?

by Christian Thibaudeau

The Truth About Hormones and Overeating

Here are the cold hard facts about insulin, leptin, cortisol, and thyroid hormones.

Hormones are often blamed for fat gain or the inability to lose fat. They do play a role, but theyโ€™re not independent of overeating. In short, a lot of the hormonal problems that make it harder to lose fat are caused by what made you fat in the first place: chronic overeating.

When we think of the hormones that impact our capacity to lose fat, we think of insulin, leptin, cortisol, and thyroid hormones. Letโ€™s look at each.

1. Insulin

Itโ€™s probably the most maligned hormone in the human body. And if you ask a keto dieter about it, theyโ€™ll tell you itโ€™s the โ€œfat gain hormone.โ€

But insulin actually doesnโ€™t make you fat. Yes, itโ€™s a โ€œstorageโ€ hormone so itโ€™s easy to assume that more insulin equals more fat gain. But in reality, insulin can only lead to the storage of the energy that you consume. You canโ€™t store more calories than youโ€™ve eaten.

When we talk about insulin and weight gain, we usually talk about insulin resistance being the problem. Insulin resistance means that your cells donโ€™t respond well to insulin and as a result, you need to produce more of it to get the job done. If youโ€™re insulin resistant and eat a meal, you release more insulin than someone whoโ€™s insulin sensitive (especially if the meal was higher in carbs). As a result, insulin stays elevated for longer.

Why is that relevant? Because when insulin is elevated above baseline, the body is less efficient at mobilizing stored energy. It can still do it but to a lesser extent. This means that itโ€™s a bit harder to lose body fat.

Understand that insulin resistance doesnโ€™t lead to more energy storage. When youโ€™re insulin resistant you need more insulin to do the same job. The main difference is in the inhibition of fat mobilization because of the lengthened amount of time during which insulin is elevated.

While there are a lot of factors that can lead to insulin resistance, the two main ones are chronic insulin elevation and having the energy stores filled up. If you always produce a ton of insulin, your cells can become less sensitive to this hormone. Youโ€™ll need more and more to get the job done.

What will lead to excessive insulin production? Eating too much, too often. And overeating foods that increase blood sugar levels.

The second factor is having the energy reserves (muscles, liver, and fat cells) topped off. Your blood glucose level and/or blood fatty acids levels increase when you eat a meal. The body releases insulin to clear the blood of these nutrients. But if the energy stores are full, you canโ€™t send the nutrients anywhere. The body reacts by releasing even more insulin to try to force the body to store those nutrients. Eventually, that can lead to the creation of new fat cells.

What can cause these energy reserves to be full? Overeating!

So, overeating can easily lead to insulin resistance. Thatโ€™s why thereโ€™s a higher occurrence of Type 2 diabetes among obese individuals (1) (2) (3).

2. Leptin

Itโ€™s a hormone released by the adipocytes (fat cells). When it reaches the brain, it connects to leptin receptors and essentially tells the brain that weโ€™re well-fed. Metabolism stays normal, your appetite is kept under control, etc.

If you go on a diet, the more you lose fat the less leptin the fat cells release. Itโ€™s a way to tell your brain that you arenโ€™t getting enough energy in and need to do something about it. If leptin is low enough for long enough, the body increases hunger to force you to consume more nutrients.

The less leptin you produce, the hungrier you get. If it stays low for long enough, it can even contribute to a slowing of your metabolism.

The fuller the fat cells are, the more leptin you produce. In theory, obese individuals should produce tons of leptin, which should also kill their appetites and lead to a lightning-fast metabolism. But thatโ€™s not what happens.

In fact, obese people produce so much leptin they desensitize their leptin receptors. Their brain stops responding to leptin. Even though they produce a lot of it, the result is the same as if they werenโ€™t producing much. They get hungry, they eat to satisfy that hunger and their metabolism doesnโ€™t become powerful enough to counteract it.

In this case, yeah, hormones make it harder to lose fat. But the leptin problem they have is in fact due to their weight gain, not the other way around.

They overproduced leptin because they ate too much for too long, causing their fat cells to be completely saturated. Then when their cells were saturated, they produced new fat cells, which meant even more leptin. Over time this led to the leptin resistance that now makes it harder to lose fat.

Itโ€™s not genetic. Itโ€™s because of years of bad eating habits.

3. Thyroid Hormones

Itโ€™s easy to make a connection between low thyroid levels and obesity. After all, thyroid hormones (mostly T3) regulate the metabolic rate โ€“ a large part of your daily energy expenditure.

Thyroid hormones regulate basal metabolism, and thermogenesis, and play an important role in lipid and glucose metabolism, food intake, and fat oxidation (9). The theory is that when you have hypothyroid your metabolism slows down, which means you burn less fat and store it more easily. This leads to weight gain.

It sounds simple and elegant, but itโ€™s not as simple as that. For one thing, hypothyroidism by itself only leads to a small amount of weight gain. It certainly wouldnโ€™t explain carrying an extra 100 pounds of fat. Hypothyroidism is certainly a risk factor that increases the chances of becoming obese, but by itself, itโ€™s not enough. More importantly, in most cases, itโ€™s what you did to gain weight in the first place that caused hypothyroidism.

Excess leptin production in obese individuals is one of the main causes of hypothyroidism in those people (10). Another cause is the fat cellsโ€™ release of inflammatory cytokines which decreases the uptake of iodine, leading to lower thyroid hormone production (thyroid hormones are made from iodine and tyrosine).

So, like leptin and insulin issues, the thyroid problems seen in overweight individuals are, more often than not, caused by what made them obese in the first place or the obesity itself.

4. Cortisol

This idea that cortisol will make you fat was popularized by Charles Poliquin. In his systems, storing too much fat on your abdomen is a sign of high cortisol.

But cortisol is actually a fat-loss hormone. One of its main functions is the mobilization of stored energy (glucose, fatty acids, and amino acids). Itโ€™s not a fat-gain hormone.

If cortisol becomes chronically elevated, it makes fat loss harder by decreasing the conversion of the T4 thyroid hormone into the T3 thyroid hormone, which decrease metabolic rate. But it wonโ€™t make you obese by reducing your metabolic rate by around 5%.

You canโ€™t discount the impact of hormones on fat gain and the capacity to lose fat. However, in most obese individuals, these hormonal problems are caused either by chronic overeating or the obesity itself.

This is good news. It means you can take responsibility and do something about it. Your โ€œhormone problems,โ€ at least the ones making fat loss difficult, are fixable.


  1. Colditz GA et al. **Weight gain as a risk factor for clinical diabetes mellitus in women.**Ann Intern Med. 1995;122(7):481โ€“486. PubMed.
  2. Wannamethee SG et al. Overweight and obesity and weight change in middle-aged men: impact on cardiovascular disease and diabetes. J Epidemiol Community Health. 2005 Feb;59(2):134โ€“139. PMC.
  3. Fujimoto WY et al. Diabetes Prevention Program Research Group. Body size and shape changes and the risk of diabetes in the diabetes prevention program. Diabetes. 2007 Jun;56(6):1680-5. PubMed.

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