Fasting, ketodiets

Intermittent fasting (IF), often combined with a ketogenic diet remains a very popular diet strategy. Many gurus swear by the amazing health benefits, effortless weight loss and promises to live to 100 years old. Sounds like the dream, so why aren’t we all doing it?

As with all things nutrition and health, the truth is probably a bit more nuanced. Benefits of any diet are usually blown up by the media or gurus who are looking to sell you something. IF is not new, in fact, depending on how you define IF we all do it on a daily basis when we sleep and some people engage in this practice for religious reasons such as Ramadan or lent. So let’s have a look at what the differences are between fasting, the ketogenic diet and calorie restriction and how useful these methods are to lose weight.

Fasting defined

Fasting in the narrowest sense of the word simply means you are abstaining from all or some food and/or drink, for a predetermined amount of time. Timeframes typically last between 12 hours and 3 weeks and can be unbroken or intermittent (10). 

There are many fasting strategies and no clear cut rules. The 3 most popular ones in research are the 5:2 fast, so you fast 2 days per week and eat normally 5 days, the alternate day fast which means you fast and eat normally the day after and finally the daily fast. The daily fast restricts your eating window to 6-12 hours per day. For many that means skipping breakfast and limiting the eating window to 12 and 8 PM for example.

Calorie restriction

Calorie restriction (CR) refers to an imbalance between calories eaten and calories burned. If the intake is less than the calories burned that is referred to as a kcal deficit. The person will lose weight. The reverse is also true, if intake exceeds the burn the person will gain weight. 

All diets work through this principle. Starving and dieting are essentially all measures of the same coin. The difference between them is simply the intensity of it. Our bodies respond the same way.

Fueling the human body and what happens when we stop doing that?

The two main fuels our body can use are fat and carbohydrate. Both nutrients have different properties which means they are handled differently. Most tissues can use carbohydrate and fats equally, but our brain and nervous tissue, but also our red blood cells can only use glucose. Our body’s capacity to store either nutrient is also very different. 

We have an almost endless capacity to store body fat (unfortunately) but we can only store between 400-600 grams of carbohydrate in the form of glycogen. Fat is a much more efficient way of storing excess energy than glucose. Therefore, if we eat an excess of anything, protein, carbohydrate or fat, this is stored as fat mostly. This is because glucose takes about 6 times more space to store the same amount of energy as fat because to store glucose, we also hold water. For this reason, when people go on a low carb diet and these stores are depleted, many mistake the water loss for fat loss. As soon as you start eating “normally” again you regain the water weight. 

Most glycogen is stored in our muscles and about 100 grams in our liver. Because stored muscle glycogen can only be used by that muscle, our brain and red blood cells can only rely on the stored liver glycogen as a reserve. This reserve is depleted after about 24 hr without food (3). This is mainly due to the brain as it is an incredibly hungry organ. The brain takes up about 20% of the oxygen and 25% of the glucose intake of the average human (or 120 g), yet the brain represents only 2% of the total body mass (4). 

So what happens when we stop eating carbohydrates or stop eating all together, in the case of starvation? Our bodies need to figure out other ways to fuel the neverending demand for glucose by our brains and RBC’s.

There are 2 main ways of doing this: 

  1. Converting muscle protein and some parts of fat into glucose. The main store of protein in our bodies is muscle tissue, therefore, starvation leads to muscle wasting. Obviously this is not desirable so in order to slow this down our liver has another trick up its sleeve. 
  2. Increasing ketone body production. The problem with using fat for fuel is that fatty acids cannot cross the blood brain barrier, which is the reason our brains rely on carbohydrates normally. If glucose supply becomes scarce, the liver can produce ketone bodies from fatty acids which can cross this barrier. Important to understand is that the liver always produces some ketone bodies, we say ketosis is reached when ketone production increases above 0.5 mmol/L a threshold of 4 mmol/L, it’s normally less than one thirteenth of that (2). Ketones can cover up to ⅔ of the energy requirements of the brain, ⅓ still comes from glucose. Furthermore, red blood cells also cannot use ketones so there is always a minimum amount of glucose required.

The ketogenic diet aims to reproduce this state of ketosis but without starvation. Ketone production is triggered by a lack of glucose in the blood. This is either due to starvation but can also be achieved by reducing carbohydrate and protein intake. There are different variations of the macros but typically carbohydrates make up no more than 20-50g net carbs.

Optimal ketone zone
Adapted from Volek & Phinney (2012). Ref 6.
Different versions of ketodiets
Different versions of ketodiets. Adapted from the charliefoundation.org

The classic keto diet is the most restrictive with 90% of kcal coming from fat. The diet is defined by its ratio of fat to net carbohydrates + protein (14). The classic ketogenic diet has a ratio of 4 grams fat to 1 g protein + carbohydrate combined. Because fat has more kcal than protein and carbohydrates that works out as follows:

4 x 9 = 36 kcal of fat for every 1 x 4 = 4 kcal of protein + carbohydrate.

In other words: 90% of kcal comes from fat and only 10% from protein and carbohydrate. On a 2000 kcal diet that means you can only eat 25 g carbohydrate, 200 g fat & 25 g protein. 

Over the years, less extreme versions of this diet have been developed, such as a 3:1 or 2:1 ratio. The more equal the ratio, the more carbohydrate is allowed and the less restrictive.  Which one is right for you depends on your motives. 

A practical difficulty of the ketogenic diet is that you don’t know if you are in ketosis, unless you are measuring ketones in blood, breath or urine. The goal should always be to try and eat as much carbohydrate as possible and still reach ketosis. That requires a bit of trial and error. The main advice is often to start with no more than 50 g net carbohydrates and start measuring from there. Note that depending on the part of the world you’re in, labeling is different. In the UK fiber is not included in the fiber content, but in the US and some tracking apps it is included, so you’d need to reduce fiber from the carbohydrate content in those cases.

It takes anywhere between 7 and 14 days to become “keto adapted” and the body is adapted to burning fat. Some people experience some nausea, sometimes coined the ketoflu during that time. The body goes through some metabolic changes that alter the way in which our metabolism works. Less energy is derived from protein and more from ketones and glycerol.  

So will a ketodiet turn me into a fat burning machine?

A ketodiet is a means to an end not a goal in itself. Burning fat and losing fat are two different things. What you are burning (fat versus carbs) is secondary to how much total energy you are burning. As you know the body can run on 2 main fuels: fat and carbohydrate, each have their own pros and cons. Burning fat only leads to losing body fat if you have created a negative energy balance at the end of the day. 

I always like to compare your body to your wallet. You can receive and spend money in cash or card. Cash is quick and easy, but nobody carries more than 100 euros cash. You keep the big money in your bank account which is slightly less accessible but you can keep an infinite amount there, the same way there seems to be no limit to the amount of body fat we can store. 

Cash is similar to carbs, fat is similar to card. Freeing up energy from fat is a metabolically slower process, this is why you are burning mostly fat when you’re at rest and you’re not burning through that many calories. Card payments take a bit longer, maybe you need to go find an ATM. Not all shops accept card. Everybody likes cash, all your body’s tissues love carbs. Some tissues won’t take card, like your heart and red blood cells.

You only become poorer if your spending exceeds what you earn, regardless in which way you spend: cash or card. The same thing goes for your bodyweight: whether you burn/eat your calories in fats or carbs, you will only lose weight if your intake is less than what you burn. Saying burning fat always makes you lose fat is like saying that card payments make you poorer than cash payments.

Keto and fat loss in practice

A ketogenic diet without caloric restriction does not lead to fat loss (20). Some weight loss is often reported but that makes sense if you remember that this is most likely water loss as glycogen stores deplete. Ketogenic diets in practice do often lead to fat loss for 2 reasons: depending on how strict you are, the ketogenic diet is very restrictive in food options. Only high fat foods can be eaten, some meat and fish in small amounts and even vegetables need to be limited. Anything that contains carbohydrate or sugar is off limits. Our diets tend to be rich in carbohydrates so for many switching to a keto diet will change and reduce their intake drastically. 

Furthermore, high fat diets are often found to be more satiating and therefore reduce hunger.  Although this is not a consistent finding and sometimes attributed to a high protein intake which isn’t something you can attribute to a keto diet (24). 

Foods you can eat on keto
Foods you can eat on keto

This is reflected in fat loss studies in free living conditions. Ketogenic diets are often found more effective in reducing body weight than other low calorie diets (22, 23). However, it must be noted that much of the research names ketogenic diets but does not measure ketones. Often the protein content of the diets is also high so it’s difficult to tell whether the effect is due to being in ketosis, a high protein content or the calorie content.

A randomised controlled trial (30) among 120 overweight subjects compared a low carbohydrate diet with exercise to a low fat diet for 24 weeks. They found that the low carb diet group lost more weight (-12.9% versus -6.7%) and fat (-9.4 kg versus -4.8 kg). However the diet wasn’t tightly controlled, relied on self reporting and the keto group also consumed more protein and less kcal even though they were not instructed to restrict kcal. Therefore there’s again an interference effect of a kcal difference.

And what about fasting?

Fasting does not necessarily lead to fat loss. It seems like a trivial distinction but it is important to understand the difference. Fat loss is determined by the energy balance. It is still possible to fast and gain weight, if you manage to eat more than you need in your eating window. Say for example you burn 2000 kcal on a day, regardless of whether you eat this across 3 meals during the day or in 2 meals one in the morning and one in the evening, your weight won’t shift. 

In practice many people lose weight when they fast, simply because it is much easier to eat 2100 kcal in three 700 kcal meals, instead of two 1050 kcal meals. Although people will have slightly larger meals, it is unlikely that the meal size increases to 1050 kcal meals, they might increase it to 900 kcal as they are more hungry, but at the end of the day that means they still have created a kcal deficit of 300 kcal, which is what enables the fat loss. If your goal is fat loss, keeping track of your intake remains essential.

Which method is right for you?

Ketosis is the state that arises in some degree after calorie restriction or carbohydrate restriction either through dieting, fasting or a ketogenic diet. Whether it leads to fat loss depends on whether this is combined with a calorie deficit. Whether either of these strategies is useful to you, depends on who you are and what your goal is. Let’s look at how to determine which strategy is right for you and which will only make your life more complicated.  

Fasting yes or no? That depends..

Fasting can be a particularly useful tool for busy people who struggle with portion control, who do not have the time or will to track quantities for every meal. As long as you can manage not to obsess over mealtimes during the time outside your eating window, it can be a viable option. This also brings me to the downside: if you are sensitive to strict rules it can cause an obsession with mealtimes and trigger binge eating. I also do not recommend this strategy if you already have a history of disordered eating, for the same reason. Fasting is probably also not the best strategy if you’re struggling with hormonal dysfunction such as RED-S or diabetes, but this widely depends on the extremity of the fasting protocol. Furthermore, if you are looking to perform your best in high intensity workouts and if optimal performance is your goal, you’re probably better off without fasting.

Keto yes or no? That depends…

There’s not one definition of keto. Medical research suggests that you need 4mmol/L ketones whereas newer sport nutrition advice suggests anywhere between 0.5 and 3 mmol/L can be optimal/sufficient (6). Where you are on this spectrum is also a very relative measure. It takes about 2 weeks of uninterrupted keto dieting to get there. Bumping up carb intake can undo a lot of the work. Obviously, the more extreme, the more difficult it becomes to sustain. How many carbs you can eat and reach ketosis depends. Some can eat up to 120  g net carbs and reach ketosis, this is a lot easier than say 30g from a practical point of view. 

It requires dedication

You only really know if you are in ketosis by measuring ketones in wee, breathe or blood. Not all methods are equally reliable. Blood testing is best but also most invasive. You need to experiment with different carb contents to find your sweet spot as to how much carbohydrate you can eat whilst maintaining elevated blood ketones. 

Either way restricting your carbohydrate intake that low and simultaneously increasing your fat intake is hard. Most foods are a combination of macros and carbohydrates are in pretty much everything. Following a ketogenic diet requires constant measuring of carbohydrate intake and careful planning of your meals. That makes it by definition difficult to sustain in the long run. Add up the restricted food choices and for may it becomes too much of a hassle to sustain. Especially if your only goal is just to shed a few pounds.

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