Insulin and Leptin – very different effects in lean versus overweight people

The hormone insulin (involved in storing fat) and leptin (involved in burning fat) work very differently in lean people than in overweight people. This is why excess fat such as is found in “bullet proof coffee” or “fat bombs” results in overweight (or obese) people that follow a Low Carb High Fat diet gaining weight—whereas lean people will simply burn it off. This article explains the role of these hormones and how they impact lean people and overweight people very differently.

When we eat, the hormone insulin is released which signals our body to do two things; (1) it tells our cells to uptake energy (in the form of glucose) and (2) to store excess energy as fat. Insulin is the major driver of weight gain. If we are lean, when we eat more than usual and increase our body fat stores, the body responds by increasing secretion of a hormone called leptin.  Leptin acts as a negative feedback loop on the hypothalamus area of our brain, reducing our hunger, causing us to eat less and preventing us from gaining too much fat.

The problem occurs when we become insulin resistant.

Insulin Resistance

When we eat a diet that is high in carbs and we eat every few hours (3 meals plus snacks), insulin is released each time we eat (in order to cause our cells to take in energy and store the excess as fat). If we continue to eat this way, over time our body is inundated with insulin, so it sends signals to down-regulate the insulin receptors, making our cells less sensitive to insulin signals. This is called insulin resistance. When we are insulin resistant, our body releases more and more insulin to deal with the same amount of glucose in the blood.

Leptin Resistance

Consistently having high levels of insulin, will also keep stimulating the release of leptin, which normally results in us becoming less hungry and eating less. However, when we are insulin resistant, we keep producing more and more insulin, which results in us producing more and more leptin. Over time, this consistently high leptin level will result in the same type of down-regulation of hormonal receptors that occurred with insulin, resulting in leptin resistance.

Leptin resistance interferes with the negative feed back loop on our hypothalamus which normally reduces our hunger, causing us to eat less. When we are leptin resistance, even when we’ve eaten a great deal of food, we don’t feel satiated — even when our abdomens are straining from feeling full. As a result, we just keep eating, as if there is no “off” switch.

It is this leptin resistance that results in obesity.

Obese people aren’t obese because they lack will-power, but because their body is responding to signals from very powerful hormones produced in response to the types of foods they eat.

Difference between a High Carb Diet and a High Fat Diet

When people consume diets high in carbs it stimulates insulin to be released. In response to all the insulin, energy that is not immediately needed for activity is stored as glycogen in the liver and muscle cells, and the remainder is shipped off to our adipose cells (fat cells), to be stored as fat. When eating a high carb diet, getting excess calories into fat cells is easy, getting the fat out of fat cells, not so much.

When people eat a diet high in fat and low in carbs, the fat is absorbed in the intestines as chylomicrons and is shuttled through the lymphatic system to the thoracic duct, going directly into the blood circulation. From there, the fat is either burned for energy or goes into our fat cells, to be stored. It is important to note that the fat does NOT go to the portal circulation of the liver and as a result, fat needs no help from insulin to be absorbed.

That’s good, but if excess fat gets stored in fat cells, doesn’t eating fat make one fat?

Not for lean people, because lean people are leptin sensitive and obese people are leptin resistant. When overweight or obese people eat excess fat, it is a different matter.

Lean People versus Obese People

If a lean person eats a diet high in fat and low in carbs, the excess fat will be stored in fat cells, but insulin does not go up. So a lean person does not become insulin resistant, as described above.  As their fat mass goes up, leptin also goes up. Since the lean person is sensitive to leptin, the negative feedback loop acts on the brain causing them to stop eating, allowing their body weight to go back down. Even if a lean person deliberately eats more and more fat when they aren’t hungry, what happens is their body’s metabolism goes up, and they burn off the extra calories.

If an overweight or obese person eats a diet high in fat and low in carbs with moderate amounts of protein, insulin levels don’t go up — which is good of course, however from years of eating high carb low fat diets and from eating a carb rich foods every few hours, overweight and obese people are insulin resistant. This means that their blood glucose levels remain high for long periods after they’ve eaten and as importantly, it also means that they are also leptin resistant. In this case, if they eat too much fat – such as drinking “bullet-proof coffee” or having “fat bombs”, they will respond (as the lean person does) by making more leptin, but the problem is, they are not sensitive to leptin! Their brain doesn’t respond to the signals from leptin, so when an obese or overweight person eats excess fat, beyond that which is naturally found in a low carb high fat foods, their appetite doesn’t drop – nor does their metabolism go up to burn off the excess fat being stored in fat cells. They simply get fatter.

Weight Loss

For those that are overweight or obese and insulin resistant, it is important to keep in mind that with insulin resistance comes leptin resistance. Leptin resistance by definition means that the signals to stop eating don’t work.  The “off switch” is defective.  As well, the body doesn’t respond to signals from leptin to up-regulate metabolism, so when an overweight or obese person on a low carb diet eats too much fat, they gain weight.

Since increasing carbs is not an option and increasing protein results in glucose being synthesized from the excess (gluconeogenesis), the way to lower insulin resistance (and thus leptin resistance) is by extending the amount of time between meals.  This is known as intermittent fasting – a topic that will be covered in a future article.

Have questions?

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To our good health!


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Ebbeling CB, Swain JF, Feldman HA, et al. Effects of Dietary Composition During Weight Loss Maintenance: A Controlled Feeding Study. JAMA”¯: The journal of the American Medical Association. 2012;307(24):2627-2634. doi:10.1001/jama.2012.6607.

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