Do Carbohydrates Cause Insulin Resistance?

Carbohydrates are a major part of many people’s diets, but some claim that eating too many carbs can lead to insulin resistance. This article examines the evidence on whether carbohydrates and high carb diets cause insulin resistance.

What is Insulin Resistance?

Insulin is a hormone that helps regulate blood sugar levels. When you eat a meal containing carbohydrates, the carbohydrates are broken down into glucose and enter the bloodstream, causing blood sugar levels to rise. In response, the pancreas releases insulin. Insulin enables cells throughout the body to take up glucose from the blood for energy.

Over time, some people develop insulin resistance. This means the cells become less responsive to the effects of insulin. As a result, blood sugar levels stay elevated for longer after eating. To compensate, the pancreas secretes more insulin. High blood insulin levels combined with high blood glucose is characteristic of insulin resistance.

Insulin resistance is closely linked to excess body fat, inflammation, and type 2 diabetes. Finding ways to prevent and reverse insulin resistance is therefore important for reducing risk of diabetes and related health problems.

The Carbohydrate-Insulin Hypothesis

Some people believe that eating too many carbohydrates, especially refined carbs and sugars, is the primary cause of insulin resistance. This idea is known as the carbohydrate-insulin hypothesis.

According to this hypothesis, when you frequently eat high carb meals, your blood sugar and insulin levels spike repeatedly throughout the day. Over time, your cells become resistant to the constantly elevated insulin levels. This leads to hyperinsulinemia (chronically high blood insulin) and progressively worsening insulin resistance.

Proponents of low carb diets argue that restricting carbohydrates prevents these frequent insulin spikes, thereby reversing insulin resistance.

But is there solid evidence to back up this carbohydrate-insulin hypothesis? Let’s examine what the science says.

Does Hyperinsulinemia Cause Insulin Resistance?

For the carbohydrate-insulin hypothesis to be true, chronically elevated insulin levels (hyperinsulinemia) need to cause insulin resistance. Research suggests hyperinsulinemia can indeed induce insulin resistance, but the conditions in these studies differ from normal physiological responses to eating carbs.

Evidence from Clamp Studies

Hyperinsulinemic-euglycemic clamp studies involve infusing insulin intravenously for hours to days while keeping blood sugar levels normal. This causes persistent hyperinsulinemia without hyperglycemia.

Studies using this technique find that prolonging hyperinsulinemia for days reduces insulin sensitivity by 20-40%. This suggests persistent high insulin can cause insulin resistance. However, clamps involve much more severe and sustained hyperinsulinemia compared to carb-rich meals.

Evidence from Insulinoma Patients

People with insulinomas have insulin-secreting tumors that cause hyperinsulinemia. These patients develop insulin resistance, likely due to their chronically elevated insulin levels. But again, insulinomas lead to more severe hyperinsulinemia than occurs after high carb meals.

In summary, severe hyperinsulinemia sustained for hours or days can induce insulin resistance. But it’s unclear whether the temporary insulin spikes from high carb meals have the same effect.

Do High Carb Diets Cause Insulin Resistance?

To directly test whether carbohydrate intake causes insulin resistance under normal conditions, we need to look at clinical studies that compare diets high and low in carbs.

Ideally, these studies should:

• Vary carb intake substantially between diet arms
• Measure insulin sensitivity objectively
• Control for weight loss between groups

Controlling for weight loss is important. Otherwise, improved insulin sensitivity on low carb diets may simply reflect weight loss rather than a direct effect of reduced carbs. Table 1 summarizes key intervention trials fulfilling these criteria:

Table 1. Impact of Low Carb vs High Carb Diets on Insulin Sensitivity

Low Carb %	High Carb %	Insulin Sensitivity Measure	Study Details	Protein Matched	Results
10%	75%	Hyperinsulinemic-euglycemic clamp	10 participants, 5 weeks crossover	Yes	No difference
4%	85%	Clamp	10 participants, 5 weeks crossover	Yes	Lower on low carb
20%	60%	Clamp	9 participants, 4 weeks crossover	No	Higher on low carb
10%	55%	Clamp	29 overweight adults, 10 weeks parallel arms	No	No difference
5%	55%	Clamp	18 men, 5 weeks crossover	Yes	Mixed results
15%	55%	Clamp	17 men, 12 months parallel arms	No	No difference
26%	52%	Frequently sampled IV glucose tolerance test	307 adults, 1 year parallel arms	No	No difference

If high carb intake caused insulin resistance, we would expect to see consistently higher insulin sensitivity on the low carb diets. But that was not the case. Of the 7 balanced studies compared, 3 found no difference, 2 favored the high carb diet, and only 2 favored the low carb diet.

Additional similar studies not included in the table showed similarly mixed results. Overall, these data do not indicate high carb intake independently causes insulin resistance under normal conditions for most people.

Why Don’t Carbs Appear to Cause Insulin Resistance?

There are several potential reasons why carbohydrates do not seem to cause insulin resistance, despite increasing insulin levels:

• The hyperinsulinemia from high carb meals is temporary rather than sustained.
• Other dietary factors may counteract negative impacts of insulin spikes.
• Carbs may slightly worsen, but other factors like visceral fat play a bigger role.
• Differences in study methodologies contribute to mixed results.

In summary, severe persistent hyperinsulinemia can induce insulin resistance, but temporary insulin elevations from high carb meals do not appear to independently cause insulin resistance for most people.

Do Low Carb Diets Increase Insulin Sensitivity?

Many studies show improved insulin sensitivity on low carb and ketogenic diets. However, most of these studies are confounded by concurrent weight loss.

Among weight-controlled trials, low carb diets do not consistently improve insulin sensitivity compared to high carb diets. This suggests reducing dietary carbs does not directly increase insulin sensitivity in the absence of weight loss.

Takeaways

• Prolonged and severe hyperinsulinemia can cause insulin resistance, but post-meal insulin spikes do not appear to.
• In weight stable individuals, high carb diets do not independently lead to insulin resistance for most people.
• When matched for calories and weight loss, low carb diets do not consistently improve insulin sensitivity compared to high carb diets.
• Focusing on carb quantity is likely less important than food quality and weight management for preventing insulin resistance.

Improving Diet Quality, Managing Body Weight, Reducing Metabolic Dysfunction

While excessive carbs and frequent insulin spikes are concerning for those with impaired glucose metabolism, available evidence does not support the idea that carbohydrates independently cause insulin resistance or that very low carb diets reverse it in those without diabetes.

Rather than focus on macronutrient ratios, improving diet quality, managing body weight, and reducing metabolic dysfunction through lifestyle changes appear more important for optimizing insulin sensitivity and metabolic health.