What does it mean to ‘be in ketosis’? And when you are not in ketosis, what does your body do with dietary fat? Can fat only be burned for energy when in keto, or just more efficiently? Doesn’t our body go in and out of keto / fat burning naturally when food intake varies? Low carb vs keto?
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Keto is when your body has next to no quick use energy coming in in the form of simple and complex sugars – ie carbs. So in order to perform its regular function, it activates fat burning processes to utilize the body’s stored energy.
Keto is “more efficient” because it burns fat while the body is sedentary, or not being active.
It also only activates after an extended period of time without carbs, so it can take a day or two to put your body into ketosis. This is because your body has become accustomed to receiving carbs and therefore waits for the next intake. After none are received, the body sounds the alarm bells, essentially, and starts burning fat so you don’t die.
Your body stores energy primarily in three forms: proteins, fats and glycogen (kind of like starch/carbs). Glycogen is preferentially utilised/used at higher rates and before fat which is preferentially utilised/used at higher rates than proteins. At any given moment, your body will break down some amount of all three and also produce some amount of all three. When we aren’t eating, this balance shifts towards breaking these down for energy than producing them.
Because we only store so much glycogen in our body, eventually that runs out. So the primary source of energy stores becomes fats. Fats can be broken down into free fatty acids or turned into ketones; when we have glycogen these ketones are low because our primary energy source is the glycogen or stuff that isn’t stored. Free fatty acids can’t enter the brain, but ketones can, and we want to keep some amount of glucose around because red blood cells can only use glucose. So to get around these limitations, our body goes into ketosis where fat is mobilised in the form of ketones to keep our brain alive. So ketosis is where ketone levels go up because we don’t have glycogen stores anymore.
When you’re not in ketosis, dietary fat is still being mobilised and used up, but mostly in the form of free fatty acids rather than ketones. Ketosis doesn’t burn fat more ‘efficiently’, rather its the primary source of energy that isn’t proteins. Our body will go in and out of ketosis but we generally have enough glycogen stores to last us longer than for ketosis to really happen, unless our carb intake becomes so low that we’re reliant on ketosis.
A keto diet is designed around achieving ketosis, whilst a low carb diet doesn’t necessarily meet ketosis. A keto diet is important in the control of some types of seizures. Low carb (and ‘keto’) diets kind of came into existence because of the supposed benefits of higher fat and lower carb diets.
To answer “can fat only be burned for energy when in keto, or just more efficiently?”:
Two high-profile keto researchers, Gary Taubes (author of “Why We Get Fat and What To Do About It” and “The Case against Sugar”) and Peter Attia (runs a blog with a ton of very science-y in-depth keto articles) were convinced that keto was more efficient at burning calories compared to other diets. Both are big proponents of the carbohydrate-insulin hypothesis, which is the idea that higher insulin levels lead to higher levels of stored fat. Carbs and sugar spike insulin, so the idea is, eat low carb –> lower insulin levels –> less fat gain for the same number of calories.
Dietary studies are hard for many reasons. One is that they often rely on self-reporting of calories, which is always a ballpark and can vary wildly, messing up the data. Taubes and Attia were convinced that previous keto studies done by others were not giving the diet a fair shake, and wanted to investigate further. In response, they founded NuSI (Nutrition Science Initiative), whose goal was to fund research into the carbohydrate-insulin hypothesis.
The first study they funded went as follows: To test whether keto was really more efficient for weight loss, they set up a metabolic ward – where calories fed to participants and calories burned by participants are tightly controlled and monitored. Participants can’t leave, so they can’t eat anything not given to them. The study was designed specifically to test the carbohydrate-insulin hypothesis.
For the first two weeks, they were given a high-carb diet. For the second two weeks, they were fed a keto-style diet with the same number of calories as the first two weeks. The results: “Body fat loss slowed during the [ketogenic diet] and coincided with increased protein utilization and loss of fat-free mass”. So, they lost less fat and more fat-free mass (i.e. muscle).
Link: https://academic.oup.com/ajcn/article/104/2/324/4564649?login=false
NuSI funded a couple more studies after this, and predictably, none of these ended up validating the carbohydrate-insulin hypothesis either. Taubes left NuSI in 2015 and the organization no longer exists.
So basically, no, keto will not help you burn fat any faster.
As a final note: one reason keto *seems* like it is effective is that your body stores one part of glycogen (carbs) with four parts water, so you can lose a tremendous amount of (water) weight by cutting carbs. People that don’t know this will see the number on the scale go down quite quickly when first starting a low carb diet and assume that this is due to fat loss.
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