What Are Carb Blockers and Do They Work?

Carb blockers are a category of dietary supplements.

But they operate differently from most other weight-loss pills available.

They aim to prevent certain carbohydrates from being digested, theoretically letting you consume carbs without absorbing (some of) the associated calories.

But do they really deliver on that promise? This is an in-depth look at carb blockers and how they influence weight and overall health.

What Are Carb Blockers?

Carb blockers, sometimes called starch inhibitors, can impede the enzymes required to break down certain carbohydrates.

Some forms are marketed as slimming supplements. They’re typically derived from a class of substances known as alpha-amylase inhibitors, which naturally occur in some foods.

These inhibitors are often extracted from beans and labeled as Phaseolus vulgaris extract or white kidney bean extract (1, 2, 3).

Other types exist as prescription drugs called alpha-glucosidase inhibitors (AGIs), which are prescribed to manage high blood glucose in type 2 diabetes (4).

For the purposes of this piece, “carb blocker” refers to the dietary supplement made from bean extracts, not the prescription medications.

Bottom Line: The carb blocker discussed here is a dietary supplement derived from beans, marketed for weight control.

How Do Carb Blockers Work?

Digestible carbohydrates can be classified into two primary groups: simple and complex carbs.

Simple carbs occur naturally in foods like fruit and dairy products.

They also appear in processed items such as soft drinks, sweets and flavored yogurts.

Complex carbs, conversely, are found in pasta, bread, rice and starchy vegetables like potatoes.

Complex carbohydrates consist of chains of simple sugars that must be cleaved by enzymes before being absorbed.

Carb blockers contain agents that inhibit some of the enzymes responsible for breaking down these complex carbs (3).

Consequently, those carbs can pass into the large intestine without being degraded or absorbed. They don’t contribute calories or elevate blood glucose.

Bottom Line: Carb blockers hinder enzymes that digest complex carbs, stopping those carbs from supplying calories or raising blood sugar.

Carb Blockers May Support Weight Loss

Carb blockers are often promoted as weight-loss aids, claiming you can eat large quantities of carbs without taking in their calories.

Yet their actual effectiveness is limited and research findings are mixed.

How Effective Are Carb Blockers?

Carb blockers only stop a fraction of the carbs you consume from being digested. At best, they appear to inhibit 50–65% of the carb-digesting enzymes (5).

It’s key to understand that blocking enzymes doesn’t necessarily equate to blocking the same share of carbs.

One trial using a potent carb blocker discovered that although it could suppress 97% of the enzymes, it only prevented 7% of dietary carbs from being absorbed (6).

This could occur because carb blockers don’t directly block absorption; they may simply delay enzymatic digestion.

Moreover, the complex carbs targeted by these supplements represent only a portion of most people’s carbohydrate intake.

For many trying to slim down, added sugars in processed foods are a larger concern. These added sugars are generally simple carbs like sucrose, glucose or fructose, which are not affected by carb blockers. You might consider alternatives such as keto sweeteners when reducing sugar intake.

Bottom Line: Carb blockers only block a small portion of carbs from being absorbed, and their impact depends on the types of carbs you consume.

What Does the Evidence Say?

Some trials indicate carb blockers can produce modest weight loss.

Studies lasting 4–12 weeks reported that participants taking carb blockers lost about 2–5.5 lbs (0.95–2.5 kg) more than controls. One study observed as much as 8.8 lbs (4 kg) greater weight loss versus placebo (7, 8, 9, 10).

Interestingly, those who consumed the most carbs tended to be the ones who experienced weight loss while using these supplements (11).

That’s logical, since a higher share of complex carbs in the diet increases the potential effect of carb blockers.

Still, average weight loss for those on carb-heavy diets was modest — about 4.4–6.6 lbs (2–3 kg) on average (7, 8, 9, 10, 11).

Conversely, other trials found no meaningful difference in weight loss between those taking supplements and control groups, making it difficult to draw firm conclusions (11, 12).

Bottom Line: Some research shows carb blockers can lead to 2–9 lbs (0.95–4 kg) of weight loss, while other studies find no benefit.

Carb Blockers Might Reduce Appetite

Beyond reducing carb digestion, carb blockers may influence hormones that regulate hunger and satiety (2, 6).

They may also slow gastric emptying after eating (2, 6).

This may be partly because bean extracts can contain phytohaemagglutinin, a compound that can raise levels of certain fullness-related hormones (2).

In one rodent study, phytohaemagglutinin led to a marked reduction in food intake — the treated rats ate 25–90% less. However, this effect lasted only a few days (2).

By day eight the appetite-suppressing effect had faded and intake reverted to baseline. After stopping the carb blocker, rats even overate by up to 50% and regained their prior weight (2).

There may be other mechanisms by which carb blockers reduce appetite.

Similar experiments found a carb blocker supplement reduced food consumption in rats by 15–25% over a longer period and even lessened their preference for high-fat, high-sugar foods (2).

Human research is limited, but a recent trial reported that a concentrated, standardized bean extract reduced subjective hunger, likely by lowering levels of the hunger hormone ghrelin (6).

It’s unclear whether commercially available carb blocker supplements produce the same effect or whether any observed appetite suppression meaningfully contributes to weight loss in people.

Bottom Line: Animal and limited human studies suggest carb blockers can lower appetite and cravings, but more research is required.

Carb Blockers May Improve Blood Sugar Control

Although carb blockers are typically sold for weight loss, they may have a more pronounced effect on blood glucose regulation.

By preventing or slowing the digestion of complex carbs, they reduce the post-meal rise in blood sugar that follows carb absorption into the bloodstream.

However, this benefit applies only to the share of carbs actually impacted by the blocker.

Carb blockers may also influence hormones involved in glucose regulation (5).

In several trials involving healthy participants, carb blocker supplements produced smaller post-meal blood sugar spikes and faster returns to baseline (1, 5, 13).

Bottom Line: Studies indicate carb blockers can blunt the post-meal rise in blood sugar and help it return to normal more quickly.

Carb Blockers Increase Resistant Starch in the Gut

Carb blockers confer another indirect benefit — they raise the amount of resistant starch that arrives in the large intestine.

This occurs because fewer carbs are absorbed in the small intestine, leaving more starch to travel through the gut.

Resistant starches are starches that cannot be digested by small intestinal enzymes, similar to fiber.

They’re naturally present in foods like raw potatoes, unripe bananas, legumes and some whole grains (14).

When resistant starch reaches the large intestine, gut microbes ferment it and produce gases and beneficial short-chain fatty acids.

When carb blockers stop complex carbs from being broken down in the small intestine, those carbs act like resistant starch.

Multiple studies have linked resistant starch to reduced body fat, improved gut microbiota, and better blood sugar control and insulin sensitivity (7, 15, 16).

Resistant starch may also boost post-meal fat oxidation (17).

Bottom Line: When carb blockers let carbs pass into the large intestine undigested, those carbs behave like resistant starch, which has been associated with various health benefits.

Are Carb Blockers Safe?

Carb blockers are generally regarded as safe, but it’s important to purchase them from trustworthy suppliers.

Safety and Side Effects

In terms of adverse effects, carb blockers are considered fairly safe.

However, when bacteria in the large intestine ferment the undigested carbs, the resulting gases can produce uncomfortable symptoms.

These may include diarrhea, bloating, flatulence and cramping (1, 5).

These effects are typically mild and resolve over time, but they can be enough to cause some people to stop using carb blockers.

Also, individuals with diabetes who use insulin should consult a healthcare professional before taking carb blockers, since adjustments to insulin dosing may be necessary to avoid hypoglycemia.

Bottom Line: Carb blockers are usually safe, although they can lead to uncomfortable gastrointestinal side effects.

Supplement Regulation

Another concern is the regulation of supplements.

Manufacturers are responsible for the safety and contents of their products, and the supplement industry has a history of fraudulent practices.

FDA testing of various herbal supplements found that only about 17% contained the primary ingredient listed on the label (18).

In the past, the FDA has even detected dietary supplements that were tainted with prescription drugs that had been pulled from the market because of safety concerns.

These medications were sometimes added to make supplements appear more potent.

For this reason, many over-the-counter carb blockers may not actually contain what their labels claim.

When buying supplements, it’s wise to research and choose products from reputable companies.

Bottom Line: Although carb blockers are typically safe, there’s uncertainty about whether supplements truly contain the ingredients listed on their labels.

Should You Use a Carb Blocker?

A few trials suggest carb blockers can produce modest weight loss, curb appetite and lower post-meal blood sugar.

However, the evidence is not strong enough to confirm a lasting benefit, and they are likely most useful for people consuming a moderate-to-high carbohydrate diet.

Keep in mind that carb blocker supplements are just that — supplements. They cannot replace a healthy lifestyle.

Balanced nutrition and regular physical activity remain essential for achieving and maintaining long-term results.