Fruit is nature's candy. Naturally sweet and juicy, it's a dietary favorite and a pretty big staple in Western culture. Most fruit is also decidedly not ideal on a ketogenic protocol. But why?
We get that question over and over. After all, isn't the sugar in fruit "natural?" Doesn't it have tons of fiber? The answer to those questions are yes, and it depends on the fruit, respectively. Unfortunately, those answers do not necessarily make for a low-carb-friendly food. Remember, sugar itself is completely natural. Regular old table sugar is usually derived from the juice of the sugar cane or sugar beet. It's essentially juice that's been clarified and dried into crystals.
Perfectly natural. Still not keto.
The big disconnect seems to be in the commonly held notion that fructose- what we call "fruit sugar"- is somehow better for you than regular table sugar. But is that so? There does seem to be some scientific evidence that fructose is metabolized differently. It is not readily used by most cells in the rest of the body and so it is primarily processed by the liver instead. There also seems to be no blood glucose or insulin response from fructose. All of this would seem to add up to fruit being friendly.
Not so fast.
The problems with fruit are two-fold. First, as any diabetic who tests regularly will tell you, they do see an increase in blood glucose after eating fruit. Increases in insulin response follow increases in blood glucose, both of which we try to keep low on keto. Second, there is scientific evidence that increased fructose consumption not only leads to weight gain, occurring primarily in visceral belly fat, but has other, very serious adverse health consequences.
Let's take them one at a time.
Blood Glucose Increases After Consuming Fruit
It does seem to be true that fructose in isolation does not affect blood glucose or insulin release. However there are two extremely important words in that sentence: in isolation. Fructose by itself does not affect glucose or insulin, but we almost never ingest fructose by itself. In fruits and vegetables, fructose molecules are usually bonded to glucose molecules, creating a disaccharide called sucrose.
So when you're eating fruit, the sugar you're getting is not just fructose, it is also straight up glucose.
Let's look at the carbohydrate content in an apple. According to the USDA, a 100 gram serving of apple contains a total of 13.8 grams carbohydrate. About 3.4 of those grams are dietary fiber (so much for the tons of fiber myth!). The rest of that is sugar. 10.4 grams of it to be exact. Just over half of that (5.9 grams) is fructose, which means just under half is glucose.
So when you're eating an apple half of the sugar you get is not, in fact, the supposedly diabetic-friendly fructose. It is glucose and glucose will, indeed, raise your blood sugar and stimulate an insulin response. Over time, chronically elevated blood glucose and insulin leads to increased fat storage, obesity, insulin resistance, and diabetes with all its complications.
Fructose and Poor Health Outcomes
Scientists have been hard at work studying the effects of high fructose corn syrup (HFCS) on the human body. Since the introduction of this cheap, bulk sweetener in the 1970s obesity has exploded to epidemic levels. Logically, you would assume that the consumption of something so high in fructose content- which is not supposed to raise blood glucose or insulin- shouldn't be strongly linked to obesity, visceral body fat, diabetes, and heart disease (just to name a few). It is, however, and so we will take a look at why.
As mentioned before, fructose is almost never present in isolation. Even HFCS is not fructose alone. In HFCS the ratio of fructose to glucose is about 50/50, although that can depend a bit on brand. That ratio is actually fairly comparable to that of fructose to glucose in quite a few fruits as well (figs or grapes, for example). In many other fruits, like apricot, pineapple, or plums the amount of fructose is very minuscule compared to glucose, which means what you are consuming when you eat those fruits is mostly glucose.
As we pointed out earlier, because you're not just getting fructose by itself, but are actually consuming it in combination with glucose, the body is going to have a glucose reaction and all the stuff that comes with it (fat storage in the cells being the largest immediate concern). The other important factor when looking at something like fructose or HFCS, and the strong links to a plethora of health problems is what fructose does in the liver when it is metabolized. Fructose metabolization in the liver interacts with glycerol, which then increases the production of triglycerides. We are all aware that high triglycerides is a major risk factor in cardiovascular disease.
Studies in animals have consistently confirmed that increased consumption of dietary fructose is strongly linked to hyperlipidemia (elevated lipid levels) and this is largely confirmed in studies that have followed both women and children on diets that are fructose-heavy. Some studies have suggested that higher fructose consumption plays a role in reducing particle size of LDL cholesterol. Small LDL particle size is a primary factor in heart disease.
So What Does It All Mean?
The nutshell is this:
- Natural does not mean good or better.
- Fructose, "fruit sugar," is just as natural as table sugar.
- Fructose is almost never consumed in isolation.
- Even in nature fructose almost always appears along with blood sugar and insulin raising glucose. Most fruits have a ratio of half or less fructose to glucose. The majority of sugar in many fruits is glucose.
- High fructose consumption raises bad cholesterol and is linked to obesity, increases in visceral body fat, and increased risk for cardiovascular disease.
All of these reasons are why fruit is generally on the "Not Keto-Friendly" list. Keto does allow for exceptions where berries are concerned. They are typically some of the lowest in overall sugar as well as the most nutrient dense, and so they may be enjoyed in small quantities upon occasion.
The more you know, the better armed you will be to make great keto decisions.