Carbs and insulin have become the biggest villains of our time. And not just in the context of weight loss, but health overall.
“But there are studies proving it…” Yes. And also no.
The problem isn’t that research doesn’t exist. The problem is how it’s read and how it’s interpreted.
When evaluating studies, context matters:
- Many were conducted on people with obesity who already had multiple metabolic issues. In these cases, insulin resistance is often a consequence of obesity – not its cause. Those results cannot automatically be applied to healthy individuals.
- Many studies focus on people with diabetes, where insulin sensitivity is already impaired. Again, that is not a model of a healthy metabolism.
- Some research is outdated or methodologically weak.
- In some cases, participants’ overall health status wasn’t clearly assessed.
- Many studies examined the relationship between carbohydrates, insulin, and obesity in the context of ultra-processed foods high in salt, refined sugar, saturated fats, and additives… without clearly separating these factors.
- Some studies were too short-term or ignored the role of protein.
- Others were conducted in vitro or on animals.
- And yes, some research has been industry-funded with specific agendas.
Then all it takes is for someone to pull facts slightly out of context, highlight certain parts, “forget” others and suddenly we have a villain. Carbs. Insulin. “Metabolism.”
Instead of looking at our own lifestyle.
So, is there solid evidence that low carb or keto is the best diet for everyone?
No.
There is no high-quality evidence proving that a low-carb or ketogenic diet is universally superior for everyone.
What truly matters are newer randomized controlled trials and meta-analyses conducted directly in humans. The larger the sample size, the longer the duration, and the more rigorous the methodology the more reliable the conclusion.
And most importantly: evaluate the full body of evidence, not just the pieces that support your existing belief.
In this article, we rely specifically on high-quality data. And that data shows that low carb and keto are not necessary for healthy individuals. They may have a place in specific medical diagnoses or confirmed insulin resistance, where evidence supports their use. But they are not a universal solution.
Theory vs. Reality
We’re not going to dive into the biochemical details of every enzyme. Most readers wouldn’t make it through and would end up pulling one sentence out of context anyway.
Instead, we’ll cover the basics:
- what insulin is
- what carbohydrates are
- what functions they serve in the body
- and what real long-term human data actually shows
When you take your car to the mechanic, you don’t need to understand every detail of the engine. You need to know what went wrong, why it happened, and how to fix it.
That’s exactly how we’ll approach this topic.
One Important Clarification:
Keto and low carb are not “bad.”
They can be appropriate for specific groups of people especially in the context of certain medical diagnoses.
The issue is that the number of people who truly fall into that category is much smaller than many believe. An estimated 70 – 80% of people who consider themselves candidates for keto don’t actually belong to that group.
Low carb can be a useful tool. But only when it’s used where it actually makes sense.
What’s harmful isn’t carbohydrates.
What’s harmful is aggressively pushing extremes, pulling facts out of context, and building marketing strategies on fear.
This article is longer. But the topic deserves it. So grab a coffee or tea — and let’s look at the facts.
Insulin
Insulin is a hormone produced by the pancreas. Its role isn’t to “make you fat,” but to regulate the flow of energy in the body.
Its main functions include:
- transporting nutrients into cells (liver, muscles, and other tissues)
- enabling the use of glucose as an energy source
- storing excess consumed energy (calories)
- regulating carbohydrate, fat, and protein metabolism
Glucose is a simple monosaccharide (blood sugar). It’s rapidly absorbed and can be used immediately as fuel. If the body doesn’t need it right away, it’s stored as glycogen in the liver and muscles.
During movement, muscle glycogen is converted back into glucose and used by muscle fibers for energy. Liver glycogen is converted into glucose and released into the bloodstream to maintain stable blood sugar levels. That circulating glucose is also a key energy source for the brain both at rest and during activity.
Without insulin, the body wouldn’t be able to efficiently store glucose in the liver and muscles. Insulin also allows carbohydrates to be used as the primary fuel source, which spares fats and proteins that the body needs for tissue repair, enzyme production, and hormone synthesis.
Important: insulin doesn’t rise only after carbohydrate consumption. Protein also stimulates insulin, and to some extent, so do fats. Demonizing carbohydrates alone simply doesn’t make biological sense.
The amount of insulin produced by itself does not determine how much fat you store. Fat storage is primarily driven by excess energy intake.
Insulin responds to diet and lifestyle. It is essential for life – people who don’t produce it (for example, in type 1 diabetes) must administer it externally. From a health and body composition standpoint, insulin is neither “good” nor “bad.” It is a regulatory hormone with a critical role in survival.
Insulin Sensitivity and Resistance
Insulin sensitivity describes how much insulin is required to process a given amount of glucose. The higher the sensitivity, the less insulin is needed to achieve the same effect.
With insulin resistance, the pancreas has to produce more insulin to accomplish that same task.
We most commonly see this in:
Type 1 diabetes – an autoimmune condition in which the immune system destroys the insulin-producing cells in the pancreas. It is often diagnosed in younger individuals and is not necessarily related to overweight.
Type 2 diabetes – most commonly associated with overweight, obesity, and long-term lifestyle factors. The body becomes less responsive to insulin, and the pancreas compensates by producing more of it to maintain normal blood glucose levels. After years of overproduction, pancreatic function may decline.
In many cases, the key intervention is reducing body fat and improving lifestyle habits not automatically switching to an extreme low-carb diet.
There is a strong correlation between higher body fat percentage and insulin resistance. The primary solution is fat loss, not demonizing a single macronutrient.
Carbohydrates
Carbohydrates are an essential and the most readily available source of energy for the human body.
In a balanced diet, they should make up approximately 40 – 60% of total energy intake. Of that amount, the majority (around 75%) should come from polysaccharides, complex carbohydrates. The remainder comes from mono- and oligosaccharides.
Why are carbohydrates important?
Not because “the food pyramid says so,” but because they serve specific physiological functions:
- They are the primary source of energy for the body, brain, renal cortex, skeletal muscles, and other nervous tissues.
- They help maintain stable blood glucose levels, which red blood cells absolutely depend on (without glucose, they cannot survive).
- They spare protein – preventing the body from breaking down its own tissues for energy.
- In the form of fiber, they support digestion, gut microbiota, and overall digestive health.
- They are critical for brain function, as the adult brain uses approximately 120 grams of glucose per day.
A long-term lack of carbohydrates may manifest as fatigue, reduced performance, impaired recovery, digestive issues, irritability, difficulty concentrating, and weakened immunity.
Carbohydrates are not the “enemy.” They are fundamental fuel.
Main Classification of Carbohydrates
Carbohydrates are classified based on their chemical structure:
Monosaccharides
Simple sugars that are rapidly absorbed.
Examples include:
- Glucose (blood sugar)
- Fructose (fruit sugar)
Oligosaccharides
More complex structures composed of a few sugar units.
These include:
- Disaccharides – sucrose (table sugar), lactose (milk sugar), maltose (malt sugar)
- Trisaccharides – such as raffinose
- Certain oligosaccharides – such as inulin
Polysaccharides
Complex carbohydrates that should form the primary energy source in the diet.
These include:
- Starch
- Cellulose (non-digestible fiber)
- Pectins
- Inulin
- And other complex forms
These carbohydrates are absorbed more slowly, often contain more fiber, and promote longer-lasting satiety. Complex sources (whole grains, legumes, potatoes, rice, vegetables, fruit) should form the foundation of carbohydrate intake not ultra-processed products, as is often the case in reality.
Does a Low-Carb Diet Even Have a Definition?
There is no single, official definition of a low-carb diet. Everyone interprets it a little differently. In practice, however, we most commonly encounter four approaches:
Less than 50 g of carbohydrates per day
Most research already classifies this intake as a ketogenic diet. It was originally developed for the treatment of neurological disorders, especially epilepsy.
Less than 150 g of carbohydrates per day
Some people subjectively feel good at this intake level, others do not. It may be appropriate for individuals who, based on medical recommendation, need to reduce their carbohydrate intake.
Less than 250 g of carbohydrates per day
For part of the low-carb community, this is “not low enough.” From a performance and recovery standpoint, however, this level of intake usually does not pose a problem. At the same time, it does not provide specific advantages for people diagnosed with insulin resistance.
Carb cycling
Carbohydrates are consumed only on certain days or at specific times (so-called cyclical ketogenic dieting). This approach may be useful for athletes or for individuals who want to deliberately work on body composition.
With this, we close the theoretical part of definitions. Because theory is often taken out of context and simplified into the claim that “carbs make you gain weight.”
No serious scientific literature states that carbohydrates themselves are the direct cause of weight gain.
Below, we’ll look at specific studies and their real-world results.
Weight Loss and Health – What High-Quality Studies Actually Show
If carbohydrates and insulin were the main drivers of obesity, solid research would clearly demonstrate it.
But it doesn’t.
Meta-analysis 2020
BMJ 2020, Johnston et al.
A massive analysis of 121 randomized controlled trials including more than 21,000 participants with overweight and obesity compared 14 different diets.
The result?
- After 6 months, both low-fat and low-carb diets had similar effects on weight loss and blood pressure reduction.
- After 12 months, the differences almost completely disappeared, in both body weight and cardiovascular risk factors.
DIETFITS (2018) – One of the Highest-Quality Studies
Gardner et al., JAMA 2018
Randomized clinical trial DIETFITS (Diet Intervention Examining The Factors Interacting with Treatment Success).
609 adults.
12 months of follow-up.
Strict dietary control.
Genetic testing.
Compared:
- a healthy low-fat diet
- a healthy low-carb diet
The result?
- No significant difference in weight loss.
- No significant difference in metabolic markers.
- No difference based on genetics or insulin secretion.
Neither approach proved to be “the better option for a specific type of person.”
Carbohydrate Intake and Mortality (2018)
Seidelmann et al., The Lancet Public Health 2018
Prospective cohort analysis + meta-analysis. This analysis followed 15,428 individuals over 25 years.
Findings:
- Both very high and very low carbohydrate intake were associated with increased mortality.
- The lowest risk was observed at approximately 50% of total energy intake from carbohydrates.
The European Society of Cardiology responded to these findings by recommending that healthy individuals avoid extreme long-term carbohydrate restriction.
Caloric Deficit vs. Macronutrient Ratio
Meta-analysis of 32 studies (2017), Hall & Guo, American Journal of Clinical Nutrition 2017
Controlled metabolic ward studies comparing low-fat vs. low-carb diets under equal calorie and protein intake.
Repeatedly (since the 1990s), controlled conditions show:
- Weight loss is driven by a caloric deficit.
- Not by the carbohydrate-to-fat ratio.
As early as 1992, isocaloric diets with extreme differences in macronutrient distribution were compared. No significant differences in energy expenditure were confirmed.
This meta-analysis, conducted under precisely controlled calorie and protein intake, showed minimal differences, with slightly greater fat loss in the low-fat group.
The “Carbohydrate–Insulin Model” of Obesity
Hall et al., American Journal of Clinical Nutrition, 2017
This model claims that high carbohydrate intake increases insulin, which then “locks” fat inside fat cells, lowers energy expenditure, and directly causes obesity.
Controlled metabolic ward studies in 2017 tested this assumption directly in hospitalized patients with obesity. Participants had tightly controlled calorie and macronutrient intake.
The result?
The data did not support the claim that carbohydrates and insulin alone are the primary cause of obesity. Some key mechanisms underlying the model were not confirmed under controlled conditions and were described as experimentally unsupported and untenable.
Keto vs. High-Carb Diet (2016)
Hall et al., American Journal of Clinical Nutrition 2016
A controlled metabolic ward study involving 17 men with overweight and obesity was conducted under strictly controlled conditions, with precisely defined calorie intake and diet composition.
Over 4 weeks, researchers compared a high-carbohydrate diet with a ketogenic diet.
Results:
- On the high-carb diet, participants lost approximately 0.8 kg, of which about 0.5 kg was body fat (the rest was water).
- On the ketogenic diet, they lost approximately 1.6 kg, but only 0.2 kg was body fat, the rest was water loss.
Despite lower insulin levels and higher ketone levels during the ketogenic diet, fat loss was not greater than with the high-carbohydrate diet.
Type 2 Diabetes (2015)
Tay et al., Diabetes Care 2015
Randomized study comparing a low-carb and a high-carb diet in patients with type 2 diabetes.
Both led to weight loss and improved fasting glucose.
The low-carb group showed advantages in lipid profile and glucose stability, but this was a specific clinical population.
Summary Across Dozens of Studies
When a caloric deficit is maintained:
- low-fat,
- low-carb,
- and other balanced diets
are equally effective for weight loss.
Low-carb and keto may appear “faster” in the first few months on the scale, but this is largely water loss, not fat loss.
After 12 months, differences largely even out.
Some studies (e.g., 2006, ➡️ Johnston et al., American Journal of Clinical Nutrition) even point to potential adverse metabolic and emotional effects of ketogenic diets.
If you look at the research without a marketing filter, the picture is fairly clear:
- 👉 Carbohydrates themselves do not cause obesity.
- 👉 Insulin is not the main villain.
- 👉 Energy balance is what determines the outcome.
What About Weight Loss in Insulin Resistance, Overweight, and Diabetes?
This is one of the most common questions.
Do people with insulin resistance play by different rules? Do they need to go low carb at all costs?
This is exactly the point where the carbohydrate discussion often gets reduced to a single sentence.
“If you have insulin resistance, you must go low carb.”
But the reality is, once again, more nuanced and much less dramatic.
Let’s look at the data.
Studies from 2006
McAuley et al., 2006, PMID: 16158081
Rima E. Kleiner, 2006, PMID: 17415320
In 2006, two studies were published examining:
- the long-term effects of popular dietary approaches on weight loss and markers of insulin resistance,
- and a study in individuals with overweight and elevated fasting serum insulin (insulin resistance), where participants were divided into two groups:
- high-protein, low-fat diet
- high-carbohydrate, low-fat diet
➡️ Both studies found no significant differences between the diets neither in terms of weight loss nor insulin levels.
With high-protein diets, proper fat composition remains important.
With high-carbohydrate diets, emphasis is placed on fiber, low glycemic index foods, and reducing ultra-processed products.
Study from 2005
McAuley et al., 2005 – American Journal of Clinical Nutrition
As early as 2005, a clinical trial was conducted in women with obesity and insulin resistance comparing:
- a diet high in fat and protein
- a diet high in carbohydrates
The result?
Body weight, triglycerides, and insulin levels decreased similarly in both groups.
To achieve comparable benefits, the authors recommended:
- increasing whole grains, fiber, legumes, vegetables, and fruit in higher-carbohydrate diets
- and more substantially limiting saturated fatty acids
What About the Rest of the Evidence?
There are many additional studies conducted in individuals with hyperinsulinemia or type 2 diabetes.
The conclusion repeats itself:
➡️ Caloric restriction is more decisive for weight loss than macronutrient distribution alone.
In other words – the deficit is the key mechanism.
This means that in type 2 diabetes or insulin resistance (which are very often consequences of obesity), it is not always necessary to automatically switch to low carb or keto.
Often, the primary solution is reducing body fat and improving lifestyle habits. After a reduction in body fat percentage, insulin sensitivity very often improves as well.
And yes – these conditions should always be managed in consultation with a physician.
Additional Studies Referenced
- 2007, PMID: 18025815
- 2007, PMID: 17298712
- 2006, PMID: 17023708
- 2005, PMID: 15800559
- 2005, PMID: 15767618
- 2002, PMID: 11874925
- 1999, PMID: 10372237
- 1996, PMID: 8561057
And one more important piece of research:
2009 Study – Causes of Obesity in the United States
Swinburn et al., American Journal of Clinical Nutrition, 2009, PMID: 19828708
This research analyzed obesity trends in both children and adults in the U.S.
The conclusion?
Excess energy intake alone was more than sufficient to explain the rise in obesity.
Not a single macronutrient.
Not insulin.
Energy surplus.
Diets Focused on the Glycemic Index
In recent years, the glycemic index (GI) has become another “boogeyman” supposedly dividing foods into good and bad. The reality, however, is far less dramatic.
Controlled research shows that, in healthy individuals, there is either no difference or only a minimal difference between low-GI and high-GI diets, especially when it comes to weight loss and overall metabolic health.
The glycemic index does not tell you whether a food is “healthy” or “unhealthy” as a whole. It only describes how quickly a given food raises blood glucose levels. It’s a tool. Not a moral compass for nutrition.
High-quality studies examining GI and glycemic load (GL) show mixed results. This means GI is not a universal parameter that everyone – especially healthy individuals without metabolic disorders must strictly follow.
Some research shows improvements in certain markers among patients with diabetes or insulin resistance. In these cases, a lower GI or GL may have practical value.
On the other hand, other studies demonstrate that in people with overweight or obesity, a low-GI diet does not lead to superior weight loss results compared to other dietary approaches – when calorie intake is the same.
Reviews published in 2017 further state that there is insufficient evidence to conclude that low-GI diets significantly reduce cardiovascular disease risk or consistently improve blood lipids or blood pressure.
However, in patients with type 2 diabetes, better fasting glucose values have been observed when following a low-GI or low-GL diet.
“But Carbs Turn Into Fat” – Meaning Lipogenesis
This statement is one of the most distorted claims in nutrition.
Yes, there is a process called de novo lipogenesis = the creation of fat from non-fat sources. In theory, the body can convert glucose into fatty acids, which are then stored as body fat.
But the reality is far less sensational and far more logical.
Fat storage happens most efficiently from dietary fat itself. Converting carbohydrates into fat is metabolically demanding and inefficient for the body. The body will preferentially use carbohydrates as an immediate energy source or store them as glycogen in the liver and muscles.
Carbohydrates increase insulin, just like protein does, and to some extent fat as well. But insulin does not “create fat out of thin air.” Fat storage is a consequence of excess energy intake, not simply the presence of carbohydrates.
Two logical conclusions follow:
- Carbohydrates are not converted into fat in large amounts “just because.” It’s inefficient for the body.
- Without a long-term caloric surplus — from any source — fat stores would not increase.
This is not about one macronutrient. It’s about energy balance.
Who Is Low Carb or Keto Actually Suitable For?
Let’s be clear:
For healthy individuals, low carb and keto are not necessary. In most cases, they are unnecessary and long-term, they can even be counterproductive.
Carbohydrates have important, irreplaceable functions in the body. And when we say “carbohydrates,” we mean high-quality, complex sources: rice, potatoes, fruit, vegetables, whole grains, legumes.
We do not mean ultra-processed “junk” like cookies, overbaked granola bars, or chips.
There are, however, specific groups of people for whom carbohydrate restriction may have medical relevance:
1️⃣ People with Type 1 Diabetes
This is an autoimmune condition where managing carbohydrate intake is part of treatment.
2️⃣ People with Diagnosed Insulin Resistance
The key word here is diagnosed, not self-diagnosed.
Some studies suggest that individuals with insulin resistance may lose weight more easily on a low-carb diet.
At the same time, research shows this is not a universal rule.
In many cases, insulin resistance is a consequence of higher body fat percentage. And the solution?
Fat loss.
After weight reduction, insulin sensitivity often improves — even without extreme carbohydrate restriction.
3️⃣ Some Individuals with Overweight and a Metabolic Disorder
But again, not everyone.
Most people with overweight do not need keto or low carb.
They need a caloric deficit and a long-term sustainable approach. Macronutrient distribution often plays a secondary role.
Multiple studies confirm that the difference between a balanced diet and a low-carb diet for weight loss is minimal when energy intake is controlled.
After body fat is reduced, insulin sensitivity often improves as well.
Why Was This Article Written?
The goal was simple: to summarize the facts.
We are not taking away anyone’s right to eat low carb or keto if it works for them. Everyone can choose their own way of eating.
But one important point:
Don’t push your approach onto others just because it worked for you.
For someone else, it may be unnecessarily restrictive, psychologically stressful, or metabolically inappropriate.
And most importantly:
Neither keto nor low carb will save you if you’re not in a caloric deficit.
The Biggest Problem? Demonizing Carbohydrates.
What’s most concerning is the oversimplification:
“Carbs are the culprit.”
“Insulin is the enemy.”
That’s not how nutrition works. There is a fundamental difference between:
✔ complex carbohydrates (fruit, vegetables, rice, potatoes, legumes, whole grains)
and
❌ ultra-processed products loaded with salt, sugar, saturated fats, and additives.
No high-quality study shows that carbohydrates and insulin directly cause overweight and obesity.
Likewise, no study proves that keto or low carb are ideal for everyone.
If anything, the opposite is true: for healthy individuals, long-term extreme carbohydrate restriction may pose unnecessary risk.
What About Weight Loss?
Yes, with low carb you may lose slightly more weight in the first few weeks. But most of that is water, not fat.
Long-term, the results between diets are very similar.
Even in obese individuals with insulin resistance, research repeatedly shows that reducing body fat is what matters most, not extreme macronutrient ratios.
It’s always wiser to consult a physician about your health status than to follow an internet trend.
So What Actually Makes Sense to Limit?
If you’re going to reduce something, start with:
- refined sugar
- ultra-processed products
Sugar in reasonable amounts doesn’t have to be a problem. But beyond quick energy, it provides little nutritional value.
All available evidence points in the same direction:
Choose your diet based on:
- personal preferences
- long-term sustainability
- your health status
- food quality
Not fear.
Carbohydrates are not the enemy.
The enemy is long-term energy surplus, poor food quality, and extreme oversimplification of complex topics.
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Conquering Fat Logic: how to overcome what we tell ourselves about diets, weight, and metabolism, Nadja Hermann
