If you are just starting the ketogenic diet or are thinking of starting it, you may want to know whether you can eat fruits and berries. I will try to explain some of the reasons why you may want to reduce or even stop eating certain fruits and berries. I don’t have a background in the medical field, and this can be a complex subject, but I will do my best to relay as accurate information as possible. Many of the details of fructose metabolism are well above my pay grade, but hopefully I am able to explain the information I learned in a way that anyone can understand.
When I first started the ketogenic diet, it was really hard for me to understand why fruits and berries could be bad for a ketogenic diet. I knew they had sugars in them, but I also know that they have a lot of vitamins and minerals in them. Please keep reading to understand why many keto experts recommend eating fruits very sparingly.
Fruits and berries are worse than glucose carbs?
Well, they can be. Let me try to explain why fruits and berries can be worse than some carbohydrates. Keep in mind that fructose is a form of carbohydrate. When I refer to carbohydrates, I am referring to foods that are known as traditional (glucose) carbohydrates like pasta. To begin with, you need to know that all fruits, berries, some vegetables and honey have fructose (fruit sugar) in them.
The fructose found in vegetables is relatively nominal, so we will just focus on the fruits and berries. Fructose is metabolized differently in our bodies than glucose. Remember that carbohydrates are broken down into glucose molecules in our bodies, so from here on out I will just reference glucose because that is what carbohydrates will be broken down into. So when you see me comparing glucose to fructose you will know that I am referring to glucose as a broken down form of carbohydrates. Hopefully that wont be too confusing. Below I will try to explain the differences in how the two sugars are metabolized and why it makes a difference.
Fructose is metabolized differently
Anytime you eat something that has fructose, the fructose is sent to the liver from the small intestine. The liver is the only place where fructose can be metabolized. When the liver has metabolized the fructose, it will be converted to glycogen (a form of sugar that can be utilized by the body) and it will stay in the liver as liver glycogen, along with lactate, uric acid, free radicals and triglycerides ( a form of fat) that are created as a byproduct of fructose metabolism. The process of metabolizing fructose is called fructolysis.
Triglycerides can build up in liver cells and damage the liver. Any of the triglycerides that are released into the blood stream and contribute to plaque in your artery walls. If you are in ketosis, the triglycerides can be broken down into ketones for fuel.
Free radicals are very damaging to cell structures, enzymes and even your genes. There are many different things that can cause free radicals in your body. Ideally you want to learn what those are, and try to eliminate all that you can. They just cause chaos in your body.
Uric acid is an acid that can turn off the production of nitric oxide. Nitric oxide is a substance that helps protect artery walls from damage.
Once the fructose is metabolized, it can only be used by very few cells in your body. This is because fructose needs glut-5 transporter to allow it to enter a cell. Very few cells have the glut-5 transporter as compared to glut-4 transporter that is used for glucose.
How glucose is metabolized
When you eat a glucose carbohydrate, it will go in your stomach an go down to your small intestine. There the carbohydrates are broken down into monosaccharides (mainly glucose). The glucose then enters the blood stream. Some is sent to your liver and the rest is sent to the rest of your body. If there is too much sugar in your blood stream, your pancreas will release insulin to help maintain a balance. The insulin will act as a transportation vessel for glucose and will transport the glucose to your cells utilizing glut-4 transporters. When glucose is stored, it will be converted to glycogen. The liver, muscles and kidneys are able to break down glycogen into glucose that can be used when they need it. The glycogen stored in muscles will be used by the muscles. The glycogen stored in the liver will be used to maintain blood sugar levels between meals. So when ever your body needs energy, the stored glycogen is converted back to glucose and used.
While there are quite a few steps that have to take place in order for this whole process to take place, keep in mind this is just a brief overview.
Overview of the differences in fructose and glucose metabolism.
- Fructose can only be metabolized in the liver. Glucose is broken down in the small intestine and only about 20% is metabolized in the liver. Our muscles, skeleton and other systems metabolize the other 80 %.
- Fructose once converted to glycogen can only be used by the liver. Glucose can be converted to glycogen and used by nearly every cell in the body.
- Easier for the liver to store glycogen from fructose than from glucose.
- Since nearly every cell in the body can utilize glucose, it is easy to quickly use it up. Fructose can only be used by the liver, and will take longer to use it up.
- Glycogen produced from glucose can be stored in our muscles and in our liver. The majority is stored in the muscles. The glycogen stored in muscles is used when your muscles need energy quickly (you exercise). Glycogen stored in the liver is used to keep your blood sugar levels steady in-between meals. The glycogen created by fructose can only be stored in the liver. It is important to understand here that our body can not produce ketones if our liver has glycogen. Only when the glycogen in the liver is depleted can the liver start producing the ketones we need for ketosis. (Note: you do not need to deplete the glycogen stores in your muscles for ketosis.) Since fructose is only stored in the liver, we will have more glycogen that needs to be depleted in order for us to get into ketosis. A much smaller amount of glycogen from glucose is stored in the liver, you can use it up much faster. So in the simplest terms, eating fructose will potentially kick you out of ketosis longer than just eating glucose carbs because it will take you longer to use up the fructose.
- Fructose needs to be transported by glut-5 transporter, which fewer cells have and in a much smaller quantity than the glut-4 transporter used by glucose. Because there are less glut-5 transporters, cells can not accept the fructose into cells nearly as quickly as glucose.
- Fructose is not insulin dependent where as glucose is. Glucose consumption will affect your blood sugar because it creates an insulin response. Fructose does not create a insulin response.
- Fructose is metabolized much faster than glucose. It is believed that this is because the liver has to prioritize its metabolism over most of its other functions. This means that your liver can not produce ketones if it is busy metabolizing fructose. Once the fructose is metabolized, then your body has to use it up before the liver will start making ketones. Fructose is mainly stored in the liver in the form of glycogen, and that glycogen is mainly used to maintain blood sugar levels between meals, hopefully you can see how this would take much longer to use up.
- When we eat glucose, our insulin increases the number of transporters for glucose. Since fructose does not affect insulin, the number of transporters does not change because our pancreas does not have fructokinase.
- It is believed that fructose is a bigger contributor to insulin resistance than glucose because of the way it is metabolized and the fact that not many cells can utilize it.
- Fructose is unique among different sugars because it causes a depletion of Adenosine Triphosphate (ATP) and increases the generation and release of uric acid. ATP molecules are the high-energy molecules that stores the energy we need for nearly every bodily function.
- In order for fructose to be utilized, it has to undergo more steps to become usable than what glucose need to.
Hopefully you aren’t completely confused yet. The important thing is to understand why fructose is worse than glucose in how our bodies processes them. I am by no means saying that you shouldn’t eat any fruits or berries. If you use restraint, some of the above can be part of your diet.
If you want to read more about the difference between carbohydrates and sugars, check out my blog here.
Which fruits and berries are ok on the keto diet?
Please keep in mind that you do not need fruits or berries to have a complete diet. If you really want to eat some fruits or berries, then you will need to use restraint. You will want to look for fruits and berries that are high in fiber, low carb and have a lower glycemic index and a glycemic load of less than 10.
Some examples of acceptable fruits are: avocados, lemons, coconut meat, cantaloupe, plums, grapefruit, honeydew, starfruit, pear, watermelon, limes and peaches.
Fun fact: cantaloupe seeds are packed with omega-3 fatty acids.
Examples of berries: black berries, boysenberry, gooseberry, strawberries, and raspberries.
The big key to eating any of these fruits or berries is to stay below your daily carbohydrates. Depending on how strict your diet is, several of the fruits and berries that I have listed above would be a no go. I can have small quantities and still stay in the carb range that I want to stay in.
If you are looking to buy frozen or canned fruits, you might want to check out my blog of different names manufacturers use for sugars here.
Berries tend to be much lower on the glycemic index and glycemic load than fruits. So they tend to be better on the ketogenic diet than most fruits. That being said, you still need to eat them in moderation. In order to control my portions, I have to portion out my serving, otherwise I would easily eat more than intended.
I think that it would go without saying, but I will mention it regardless. You should not drink any fruit juices or sodas. Both are very high in fructose and have none of the fiber that would help offset the fructose in them. They will kick you out of ketosis, and even slow down how quickly you can get back into ketosis. Many fruit juices will also have sugar added to them to make them sweeter.
If you would like to know some more about fructose and how it may affect your health, check out
these two articles. The first is an article from The American Journal of Clinical Nutrition that discusses different case studies to determine if fructose is bad for you.
The second article is from Journal of Obesity and Weight Loss Therapy, and they released this article that explains the risk of eating too much fructose (the study focuses more on High Fructose Corn Syrup that is used to sweeten soft drinks, but is a great reference as to how fructose works in our bodies).
