Introduction
You have probably heard this before. Avoid carbs. They spike your insulin. Insulin makes you fat. It sounds logical on the surface, gets repeated everywhere, in gyms, on podcasts, in diet books, and it has led millions of people to fear carbohydrates, obsess over meal timing, and spend money on expensive low insulin diet programs.
But is any of it actually true? A growing body of research, including a major new four year study, has now put these ideas to a proper test. And the results might completely change the way you think about insulin.
What Even Is Insulin
Insulin is a hormone made by your pancreas. Think of it like a key that unlocks your body’s cells, letting glucose, which is the sugar from food, enter them to be used as energy. Every time you eat, your blood sugar rises and insulin rises with it to move that sugar into your cells. This is completely normal. This is how your body is supposed to work. Insulin rising after a meal is not a bug. It is a feature. It is your body doing exactly what it should.
The Distinction That Changes Everything
Before going further, there is one critical thing you need to understand. Scientists always separate two very different situations. The first is fasting levels, meaning your glucose or insulin measured after you have not eaten for eight to ten hours. The second is postprandial levels, which simply means your levels after eating a meal.
High fasting glucose or high fasting insulin is a genuine red flag. It suggests your body is struggling to manage blood sugar even at rest, which can point to insulin resistance or pre-diabetes. But glucose and insulin rising after a meal is expected. That is normal physiology. Even during exercise and certain phases of sleep, glucose levels naturally go up and then come back down. The problem is not glucose going up after eating. The problem is glucose staying elevated for too long, too often, over months and years. That chronic pattern is what becomes a real risk factor.
Why Are People Afraid of Insulin Spikes
Two big ideas have dominated the fitness world for years. The first is that insulin spikes after eating signal your body to store fat, which leads to weight gain over time. The second is that repeated insulin spikes cause your tissues to stop responding to insulin properly, eventually leading to insulin resistance and Type 2 Diabetes. There is also a broader theory built around these ideas called the Carbohydrate Insulin Model of obesity. It argues that high carbohydrate intake drives insulin up, insulin drives fat into storage, and the result is hunger, slowed metabolism, and eventual weight gain. It was taken seriously for a while because parts of it seemed to stack up on the surface. But when researchers actually tested these predictions in controlled studies, the story fell apart.
What Insulin Actually Does In Your Body
Here is where the science gets interesting and where a lot of popular nutrition content gets it wrong. Yes, insulin does increase the uptake of fat into fat cells. That part is true. But what often gets left out is everything else insulin is doing at the same time. Insulin also prioritises glucose entry into your muscles and other metabolically active tissues. It promotes glucose burning in those tissues. It is not simply a fat storing hormone. A far more accurate description is that insulin is a nutrient partitioning hormone. It directs fuel to where it is most needed, prioritising glucose for your muscles while managing fat in storage tissue.
If you reduce carbohydrates to lower insulin, you do weaken the signal to store fat. But in an energy matched situation, there is now simply more dietary fat available to be stored instead. The net effect on fat deposition does not change the way the theory predicts it should. This alone is a significant problem for the idea that insulin is the primary driver of fat gain.
The Study That Changes the Conversation
Scientists recruited around 300 women and gave them a defined dose of glucose to drink. They measured how much insulin each woman’s body released in response and then tracked multiple health markers across four full years, adjusting results for other known diabetes risk factors to isolate the specific effect of insulin spike size as clearly as possible.
At baseline, women with larger insulin spikes had higher waist circumference, more inflammation, and more insulin resistance. That sounds concerning on its own. But they also had better beta cell function, which refers to the cells in your pancreas that produce insulin, along with lower fasting glucose and lower glucose levels after eating. A single snapshot in time cannot tell you what caused what. For that, you need to follow people forward.
After four years, the women with the largest insulin spikes went on to have better beta cell function, lower fasting glucose, lower glucose after meals, and significantly lower risk of developing pre-diabetes or Type 2 Diabetes. How much lower? Five times lower. People with the biggest insulin spikes had a fivefold reduction in diabetes risk compared to those with the smallest spikes. BMI, waist circumference, inflammation, insulin resistance, and lipid levels showed no significant differences between the groups over time. In the researchers’ own words, a robust insulin response after a glucose challenge does not indicate poor cardiometabolic health. It actually predicts favourable metabolic function in the years ahead.
Does High Insulin Actually Predict Weight Gain
This is where the evidence becomes even more decisive. If the insulin theory were correct, people with higher baseline insulin responses to meals should struggle more with weight loss than people with lower responses. That is a clear, testable prediction and researchers tested exactly this. They placed obese women into a calorie deficit and measured their fasting insulin, insulin resistance, and insulin response to meals before the intervention began. After 30 days, the people who successfully lost weight and those who did not showed no significant difference in any baseline insulin measure. In fact, those who failed to lose weight had slightly lower fasting insulin and lower insulin responses to meals than those who succeeded. That is the opposite of what the insulin theory predicts. A separate study with 76 obese adults over three months found the same thing, with the ability to lose weight showing no variation based on baseline insulin levels or insulin resistance. The factors determining whether someone succeeds at weight loss are far more important than their insulin levels.
Why Bigger Insulin Spikes Might Actually Be Better
One compelling explanation is that the size of an insulin spike reflects the health of your pancreas. A healthier pancreas can produce more insulin quickly when the body needs it. That is a sign of capacity and strength, not damage. It means your body is responding efficiently to food. A smaller insulin spike, on the other hand, might reflect a pancreas with limited reserve, and that is the scenario more associated with diabetes risk over time. Think of it this way. A bigger insulin spike may simply be a sign that your pancreas is working well, not a warning that something is going wrong.
What Foods Actually Cause Insulin Spikes
Most people assume the answer is carbohydrates, full stop. And yes, carbs do raise insulin. But the reality is far more nuanced than that. Scientists have tested this directly by giving people different foods and measuring the insulin response to each one, and the results are genuinely surprising. Oatmeal did not raise insulin significantly more than fish or beef. Apple triggered a response similar to fish. Pasta came in lower than most people would expect. Doughnuts triggered about half the response of jelly beans, sitting much closer to fish than to candy. Lentils stayed low. Potatoes and jelly beans triggered the largest responses.
One major reason for these unexpected patterns is that protein also triggers insulin secretion. So the carbohydrate content of a food is only one piece of a much larger picture. Judging whether a food is healthy based purely on how much it raises insulin acutely is a deeply flawed approach. The distinction that matters is not whether a food raises insulin temporarily. It is whether your insulin and glucose remain chronically elevated over time. One is normal physiology. The other is a genuine health concern.
The Bottom Line
Chronically high fasting glucose and chronically high fasting insulin are real warning signs for metabolic disease, and both are best addressed through maintaining a healthy body weight and exercising regularly. These are the signals worth taking seriously. But a normal insulin rise after eating a balanced meal is not something to fear, and avoiding a food simply because it raises insulin acutely is not supported by the evidence.
The idea that insulin spikes cause weight gain, trigger insulin resistance, or lead to diabetes is not supported by current research. The four year study discussed here suggests that larger insulin spikes may actually predict better metabolic health over time, not worse. Insulin is not a villain. It is a nutrient partitioning hormone doing an incredibly complex and important job in your body every single day. Does diet quality matter? Absolutely. But treating a completely normal physiological response to a meal as something dangerous is not science. It is fear dressed up as logic. Stay curious, stay evidence based, and do not let fear of a biomarker replace a grounded and balanced approach to how you eat.
https://www.thelancet.com/journals/eclinm/article/PIIS2589-5370%2823%2900540-0/fulltext
