Bad Carbs vs Good Carbs: Does It Matter on Keto?

May 11, 2018

Bad Carbs vs Good Carbs: Does It Matter on Keto?

Reducing your carb intake is the most important part of going keto. Nonetheless, it's still important that you eat a tiny amount of carbs to stay healthy. But does it really matter if you're eating good carbs or bad carbs on a keto diet? The short answer is: it depends.

Some keto dieters can actually benefit from so-called "bad carbs" while others will want to avoid bad carbs at all costs. To understand why this is so, you'll need to know more about carbs, including what they are, what makes good carbs vs bad carbs, and how they affect your body.

What Are Carbs?

What carbs are depends on who's defining them.

Chemists define carbs (short for carbohydrates) as biological molecules consisting of carbon, hydrogen, and oxygen atoms. The four major types of biological molecules are proteins, carbohydrates, lipids, and nucleic acids. Carbohydrates are the most abundant of the four.

Biochemists call carbs saccharides (Greek for "sugar") and they divide them into sugars, starches, and cellulose. They further divide saccharides or carbs into four chemical groups:

  • Monosaccharides- Monosaccharides are the simplest of the carbs. They are the basic unit of carbohydrates and include glucose, fructose, and galactose.

  • Disaccharides- Disaccharides are carbs formed from two monosaccharide molecules bound together. Examples of disaccharides include lactose, maltose, and sucrose.

  • Oligosaccharide- Oligosaccharides contain a small number of monosaccharides. Many oligosaccharides are known as prebiotics because they feed friendly bacteria in the gut. Examples include raffinose and stachyose.

  • Polysaccharides- Polysaccharides are carbohydrate molecules composed of long chains of monosaccharide molecules. Examples include glycogen, starch, pectin, and cellulose.

Lastly, nutritionists define carbs as one of the three macronutrients. The other two macronutrients are protein and fat. Nutritionist further classify carbohydrates as simple and complex depending on their molecular structure and how fast your body digests and absorbs them [1]. Some would say that the simple carbs are the bad carbs (more on that later).

The Standard Definition of Good vs Bad Carbs Β 

A common way of classifying carbs is into either good or bad carbs. This largely depends on whether carbs have been refined or if they're in their natural state. Refining makes carbs bad because it reduces the nutritional value of food, leaving you only with a source of empty calories. In the case of bad carbs, the biggest problem with refining, however, is the loss of fiber.

Fiber is an indigestible type of carbohydrate that is, nonetheless, essential for your health. It slows down the digestion of carbs and balance out blood glucose levels. When you eat bad carbs with little to no fiber, your blood sugar levels rise and drop dramatically. These fluctuations in blood glucose can put you run the risk of type II diabetes if left unchecked [2].

Some nutritionists also classify carbs as either good or bad based on their complexity. So-called complex carbs are considered good and the simple ones are called bad carbs. However, this is where a lot of people disagree. Simple carbs like fructose exist in fruit but most wouldn't really call fruit a bad carb.

What About Bad Carbs on Keto?

The ketogenic diet is radically different from your standard diet. This means that the keto diet has completely different criteria for determining what's good and what's bad. For example, on a keto diet, fat is good, protein is also good, but most carbs are bad – especially in excess.

So, to make things a bit easier for you, here is a list of bad carbs on keto:

  • Sugars– Table sugar, brown sugar, honey, agave nectar, and any other sweetener with calories in it is bad. The reason? It raises blood glucose levels almost instantly and to a great extent.

  • Grains– All grains, including whole grains, are not allowed on keto. Despite their high-fiber content, grains are mostly made out carbs which is why they can quickly add up on your daily carb limit.

  • Starchy food– Bananas, potatoes, beans, corn, and chestnuts are all high in starches. Starches are carbohydrates that plants use as energy storage and this makes them carb bombs.

Highly-processed food like cookies, pasta, chips, and candies are also bad carbs due to their high carb content. These bad carbs also tend to be low in essential nutrients and contain preservatives, dyes, and emulsifiers, many of which are harmful to human health.

What Are Good Carbs on Keto?

Proponents of the standard diet recommend that 45-65% of your calories comes from carbs [3]. On a keto diet, however, only about 5% of your calories should come from carbs and these calories need to be the good kind.
So, what is considered a good carb on keto? Well, here are some examples with explanations:
  • Leafy vegetables– Leafy greens are mostly water and fiber with only a little bit of carbs. They're also rich in vitamins, minerals, and antioxidants [4] to help support your overall health.
  • Nuts and seeds– Both are low in carbohydrates and high in fiber and fat. Examples include walnuts, almonds, pecans, coconuts, sesame seeds, and flaxseed.
  • Berries – You are allowed to eat a small number of berries on keto. These fruits are lower in carbohydrates than other fruits and can add plenty of antioxidants to your keto diet.
  • Fiber supplements– Fiber is a carb that you are not only allowed but advised to eat on keto. Psyllium husk and acacia fiber are two supplements you may want to consider.
  • Avocados – Avocado is a unique keto food item. It's high in both fiber and fat but low in carbs. It's a type of berry that most people eat in savory dishes.
On a keto diet, it's best to eat unrefined, complex carbs to keep your blood glucose levels low. These carbs will give your body enough energy to keep you healthy and your functioning high. They also support gut health and are rich in important micronutrients.

What About Glycemic Index and Glycemic Load?

The glycemic index (GI) is a measure of how carbs raise blood glucose. The GI ranks food in numbers from 0 to 100, with 100 being the glycemic index of pure glucose. The GI represents how much your blood sugar levels will rise two hours after eating a carb food.
Foods low on the GI scale (below 55) are nuts, vegetables, and berries while those high on the GI scale (above 70) are high-fructose corn syrup, white bread, and white potato. On a keto diet, you should stick to foods lower on the GI scale.
As for glycemic load (GL), it is an extension of the GI. It is different from the GI in that it takes the quantity of the carbs eaten into account. The purpose of the GL is to give you a more accurate picture of the effect a food item has on your blood sugar. To calculate the GL of a food, you simply multiply the GI value of the food by its amount of carbohydrate in grams the serving of food contains and divide that by 100.
One frozen bagel has a glycemic index of 72 and 29 grams of carbohydrates (72 x 29 / 100 = 20.8). So, the GL of one bagel is 21 making it a high GL food.
Here is how foods are ranked depending on their GL:
  • Low GL – 10 or less
  • Medium GL – 11 to 19
  • High GL – 20 and above

The Only Exception

Foods made with bad carbs and that are high on the GI scale are obviously those you need to avoid on a ketogenic diet. The only exception to this rule is if you're following the targeted ketogenic diet (TKD).
The TKD is your standard keto diet but with timing of carb intake around workouts. For example, on a TKD you would eat your usual standard ketogenic diet every day, but on the days when you work out, you would eat up to 50 grams of carbohydrate before your workout.
The goal of the TKD is to get enough glucose for stronger workouts but without being kicked out of ketosis. The types of carbs you eat doesn't really matter. In fact, most keto experts suggest you go for the simpler, high GI foods like Twinkies, fruits, donuts, and other things some might consider bad carbs. This is because they'll give your body easy access to glucose and burn it out just as fast.


Bad carbs are those carbs that shouldn't really exist in our diet. They include the heavily-processed foods that, while tasting good, wreak havoc on your health. And when it comes to the keto diet, avoiding bad carbs matters even more.
However, followers of the targeted ketogenic diet sometimes eat "bad carbs" before a workout to get a quick glucose shot. This is more for practical than health reasons. In most other cases, we suggest sticking to foods that are lower on the GI and GL scale, high in fiber, and dense in nutrients.
You can find a list of low-carb food here and here if you want to know which carb-containing foods are keto approved. You can also take our free Ketocademy course to learn which foods are best to eat to reach and stay in ketosis. Β