Complete Guide to Fat Fast | KetoDiet Blog (2024)

What Is Fat Fast?

Although most of you probably know how the ketogenic diet works, I'll give you a quick overview before I get to the fat fast itself. Ketogenic and other low-carb diets have been around for a long time. Most low-carbs diets, including the ketogenic diet, can are used for weight loss and offer numerous health benefits. In fact, very low-carb ketogenic diets have been used to treat neurodegenerative diseases such as epilepsy or Alzheimer's for decades. Currently, they are also shown to have potential use in cancer treatment.

We have all been told that all calories are the same no matter where they come from. This advice has been proven to be false and yet despite all the evidence, the guidelines still remain the same: "Exercise more, eat less and focus on eating low-fat foods." Sadly, I keep seeing this piece of advice on most packaged foods — just another reason to avoid them!

Here is the general rule that applies to the ketogenic diet: When you restrict carbohydrates to a certain level (20-50 grams of net carbs), your body starts producing ketones and you enter ketosis (ketosis is not the same as ketoacidosis which is dangerous).

Once you become fat-adapted, your body uses your fat stores and ketones for energy and you won't need to eat extra protein to spare muscle mass. This doesn't mean you should avoid protein — your protein intake should be adequate and based on your individual needs. Although your body still needs some glucose, it can produce it on demand via gluconeogenesis.

Fat Fast is a type of fasting often recommended for those who are already keto-adapted and reach a weight loss plateau. The idea is that you eat about 80-90% of your calories from fat while keeping your calorie intake low, up to 1000-1200 kcal a day. This fast should be followed for no more than 3-5 days. If you follow the fat fast for a longer period, you run the risk of getting your body into starvation mode, losing muscles and getting deficient in essential nutrients.

When you go on a fat fast and you are already keto-adapted, your ketone level will likely get very high (mine got as high as 3.2 mM) because most of the energy (that is limited to 1000-1200 kcal) comes from fats, and you begin burning your own body fat due to the calorie deficit and very low insulin levels.

Recent studies show that in terms of weight loss and muscle sparing, "traditional" fat fast (90% kcal from fat) doesn't seem to have more benefits than a fast with less fat and more protein. This doesn't mean that traditional fat fast doesn't work. It only means that even if you eat slightly more protein, you may achieve the same or better results. Since protein is the most sating macronutrient, it will help you keep hunger at bay during the fast.

Here's a simple calculation: the adequate protein intake from 1200 kcal for an average person is about 15-20% (or even more for physically active people and those with less body fat and more lean mass). Since you'll be eating ~ 5% or less calories from carbs on a fat fast, you will be left with 75-80% of calories from fat. Not a big difference from the "traditional" fat fast.

Studies from the 1950's and 1960's (first by Kekwick and Pawan,¹ later by Benoit²) showed that a fat fast is best for fat loss and preserving muscle mass when comparing three diets (1000 kcal diet with 90% fat vs. 1000 kcal diet with 90% protein vs. 1000 kcal diet with 90% carbohydrates). However, these studies have been shown to be largely flawed. While Kekwick and Pawan admitted that participants may have been cheating and the study was poorly controlled, Benoit's results seemed to have been overexaggerated. In his study from 1965, Dr. Benoit compared results achieved in obese participants by putting them on a fat fast and on a complete fast. Interestingly, those following a fat fast experienced twice as much fat loss compared to those following a total fast during which they lost more muscle mass and less body fat. In later reviews of this study, some experts pointed out that the participants would have to have enormous energy expenditure to reach such weight loss results.

An excerpt from a review of studies from 2006 on protein-sparing effects of very low-carb diets:

"Because of metabolic adaptations to prolonged changes in diet composition, the results of such short-term studies cannot be applied to longer-term situations... Although more long-term studies are needed before a firm conclusion can be drawn, it appears, from most literature studied, that a VLCARB is, if anything, protective against muscle protein catabolism during energy restriction, provided that it contains adequate amounts of protein."

Keep in mind that prolonged calorie restriction with inadequate amount of protein can lead to deficiencies, muscle mass loss and a decreased BMR. That's why you shouldn't do the fat fast for more than 5 days. It's also important to mention that no study seems to take keto-adaptation / fat-adaptation into consideration. The body works differently for sugar-burners and those who are fat-adapted and the logical conclusion would be that fat-adapted people would lose less muscle mass during a fat fast.

When to Consider the Fat Fast

Here are the rules for when to follow the fat fast:

If you are already keto-adapted

Otherwise, it will be very difficult for you, as your body is not used to the ketogenic diet. If you are not keto-adapted, try following the ketogenic diet for 3-4 weeks and only use the fat fast technique if you reach a weight loss plateau. You can start by following my weekly diet plan here:

Weight loss plateau

If for any reason your weight is stalling for more than 2-3 weeks and you have eliminated all the potential reasons. Make sure you check your keto ratio to eliminate the most obvious reasons for weight stalling such as too many carbs, inadequate amount of protein, fats or calories in your diet. You may want to try Intermittent Fasting (guide to IF is here).

After a "cheat day"

If you had a "cheat day" and would like to get back on track the next day. I would personally not use the fat fast for this situation but it is up to you. I would just keep eating healthy, low-carb foods from the day following a "cheat day" instead of giving my body such a calorie shock.

How Do I follow a Fat Fast? What Should I Eat?

Because most of your calories will come from fat, you have to make sure to include healthy fats such as MUFA, Omega 3s and saturated fats. You can follow this guide I have created for fats and oils to make the right choices.

The traditional Fat Fast guidelines advice is to eat 4-5 small meals a day, each about 200-250 kcal. This is totally up to you. If you prefer to have 1-2 regular meals like I do, it's perfectly fine. You can follow the fat fast for no more than 3-5 days.

Tips before you get started

• Ideally, you should take a multivitamin to make up for the missing micronutrients on a fat fast.
• Don't plan to do any extensive exercise on the days of the fat fast. This doesn't mean be inactive. You can always include light cardio (walking) or some strength training at home (squats, press-ups, etc.) but you should take it easy and avoid prolonged exercise. I personally didn't feel lack of energy or hunger and could easily incorporate some exercise every day (either 30-60 minutes walking or 20-30 minutes strength training).
• Get an app to help you track your diet or make sure you track whatever you eat on a piece of paper. My iPad app has been designed specifically for low-carb, ketogenic & paleo diets. This way you can easily keep track of your macros and calorie intake, both of which are critical on a fat fast.

Many Small or Few Regular Meals? It's Up To You...

A traditional fat fast as defined by Dr Atkins in New Diet Revolution says you should be eating 4-5 small meals throughout the day, each of them containing 200-250 kcal. However, what I’ve learnt is that once you get keto-adapted, you don’t need to eat many small meals a day to stay sated. In fact, it may be easier for you to just eat 1-2 regular meals rather than many small meals, especially if you are used to Intermittent Fasting. Eating 1-2 large meals and having a long gap between meals (16-22 hours) is an example of Intermittent Fasting.

I rarely eat breakfast and often have my first meal round 1 pm. When I was doing the fat fast, I ate the same number of meals I’m used to (2 meals a day plus 1-2 cups of coffee with cream). I just increased the amount of fat to 80-90% and ate no more than 1200 kcal. In fact, unless you are hypoglycemic, you don’t need to eat too often. However, those of you who find it easier to have several small meals, you can try all sorts of fat bombs from my blog.

Keep in mind that eating just 1-2 meals instead of 5 small is my approach and it may not be suitable for everyone. I've tried the fat fast twice (with 4-5 meals and 1-2 meals a day) just to make sure all of these can be done and experienced the same results on both plans. However, I was struggling to follow the plan with several small meals. I always wanted to eat more than just a handful and my mind was thinking about food.

Below is an overview of foods you can include on your fasting days. Plus here is a complete list of suitable recipes that is updated regularly, making it easier and less boring for you to follow the fat fast!

3-Day Meal Plan (85-90% fat)

I've tried the fat fast twice and I'll be sharing my progress details as well as my meal plan (Update: you can find my experiment here). For now, here are two sample meal plans I created using the KetoDiet App. These meal plans are designed for those who prefer to eat 2-3 regular meals. The first one is higher in fat (85-90% calories from fat), while the second is lower in fat (75-80% calories from fat) and higher in protein (15-20% calories from protein). If you don't drink coffee, substitute it with tea or a few nuts of your choice.

Day 1

• Coffee with 2 tbsp heavy whipping cream or coconut milk (94% fat, 112 kcal) and 2 oz / 60g of macadamia nuts (93% fat, 406 kcal)
  
• Buttered Spinach with Salmon: ¹/₂ cup spinach (2.8 oz / 78g) cooked in 2 tbsp butter seasoned with salt and topped with a small salmon fillet (4.4 oz / 125g) cooked on 1 tablespoon of butter or coconut oil and topped with 2 tbsp of home-made pesto (Paleo Avocado Pesto or Red Pesto) (79% fat, 639 kcal)

Total daily values: Total carbs: 14.8 g, Fiber: 8.8 g, Net carbs: 6.1 g, Protein: 36.6 g, Fat: 113 g, Calories: 1182 kcal, Magnesium: 307 mg (77% RDA), Potassium: 1347 mg (67% EMR), 86% calories from fat

Day 2

• Ultimate Keto Coffee (85% fat, 474 kcal)
  
• Avocado & salmon salad: 2 oz / 60 g smoked or cooked salmon with 2 oz / 60g cream cheese, 1 tbsp homemade mayo and 1 tbsp finely chopped spring onion or chives on top of one avocado (7.1 oz / 200 g) seasoned with salt (82% fat, 526 kcal)

Total daily values: Total carbs: 23.7 g, Fiber: 14.8 g, Net carbs: 8.9 g, Protein: 33.8 g, Fat: 105 g, Calories: 1114 kcal, Magnesium: 284 mg (71% RDA), Potassium: 1562 mg (78% EMR), 85% calories from fat

Day 3

• — use coconut milk, sour cream or whipped cream (88% fat, 435 kcal)
  
• 1 oz of macadamia nuts (93% fat, 203 kcal)
  
• Keto Vegan "Zoodles" (86% fat, 449 kcal)

Total daily values: Total carbs: 38.3 g, Fiber: 20.5 g, Net carbs: 17.9 g, Protein: 15 g, Fat: 109 g, Calories: 1101 kcal, Magnesium: 198 mg (50% RDA), Potassium: 1924 mg (96% EMR), 88% calories from fat

3-Day Meal Plan (75-80% fat)

Day 1

• Coffee with 2 tbsp heavy whipping cream or coconut milk (94% fat, 112 kcal)
• Keto Mexican Rice (75% fat, 385 kcal)
• Paleo Stuffed Avocado (78% fat, 633 kcal)

Total daily values: Total carbs: 30.4 g, Fiber: 17.7 g, Net carbs: 12.7 g, Protein: 45.6 g, Fat: 95.2 g, Calories: 1132 kcal, Magnesium: 259 mg (65% RDA), Potassium: 2368 mg (118% EMR), 79% calories from fat

Day 2

• 1 oz / 30 g almonds, roasted or raw (78% fat, 172 kcal)
• All Day Keto Breakfast (78% fat, 489 kcal)
  
• Paleo Greek Meatballs aka Soutzoukakia (72% fat, 539 kcal)

Total daily values: Total carbs: 34.8 g, Fiber: 17.9 g, Net carbs: 16.9 g, Protein: 54.5 g, Fat: 98 g, Calories: 1200 kcal, Magnesium: 228 mg (57% RDA), Potassium: 2608 mg (130% EMR), 76% calories from fat

Day 3

• Coffee with 2 tbsp heavy whipping cream or coconut milk (94% fat, 112 kcal)
• Brussels Sprout and Bacon Hash (72% fat, 400 kcal)
• Quick Beef Ragù with "Zoodles" (73% fat, 645 kcal)
  

Total daily values: Total carbs: 20.7 g, Fiber: 7.1 g, Net carbs: 13.6 g, Protein: 58.6 g, Fat: 94.4 g, Calories: 1158 kcal, Magnesium: 249 mg (62% RDA), Potassium: 2195 mg (109% EMR), 75% calories from fat

Fat Fast FAQ

Is fat fast suitable for everyone?

As mentioned above, fat fast is suitable for breaking through a weight loss plateau and those who are already keto-adapted. If you attempt to do a fat fast without being used to a LCHF (low-carb, high-fat) diet, you will make your life difficult. Make sure you follow a low-carb diet for at least 4 weeks before you try a fat fast.

Keep in mind that a fat fast is quite extreme and if you have any medical conditions such as diabetes, you have to inform your doctor and only follow the fat fast under his medical supervision. It is very likely that with any dietary changes, your medication dose may need to be adjusted. This recommendation applies to all dietary changes, especially if you start following a low-carb diet that is very different to what most people have been used to (based on the Standard Dietary Guidelines).

If you are hypoglycemic, you will likely have to eat more small meals a day (traditional Atkins approach) and avoid going for long periods without eating.

Is it safe to follow fat fast for more than 5 days?

Although studies by Kekwick and Pawan and then by Benoit showed no negative side effects on a 10-day fat fast, they have proven to have many flaws and the results are therefore unreliable. The main reason you don't want to do fat fast for longer is the increasing risk of muscle mass and deficiencies during prolonged periods of drastic calorie restriction. It's better to be safe than sorry and for this reason, I'd personally follow the fat fast for up to 5 days, as originally recommended by Dr Atkins. Some resources say that fat fasting for longer (e.g. 10 days) is dangerous but I haven't found any supporting evidence to these claims.

What if I'm hungry?

Fat is calorie-dense and when you eat 1000-1200 kcal from fat, you'll realise it's not a lot of food. It's still better than doing a complete fast but you may feel hungry, at least during the first 1-2 days. If you are not keto-adapted, don't do the fat fast. You will feel hungry with no energy and most likely will develop symptoms of "keto-flu".

Drink loads of water, tea and some coffee and if you have to, avoid all sweeteners, including low-carb. Even low-carb / zero-carb sweeteners may trigger cravings in some people. If you are one of them, avoid using sweet fat bombs during this fast.

Most people find it easier to eat 4-5 smaller meals a day while others (including me) prefer to eat 1-2 regular meals. Make sure you don't exceed the 1200 kcal limit. When it comes to fat percentage per day, it's up to you. As mentioned above in the first part of this post, adding some protein seems to do no harm. Do not add more carbs. If you eat less than 80% fat, make sure it is compensated with protein, not carbs.

Do I gain all the weight back?

After you complete your fat fast, you should go back to your previous way of eating (low-carb, whole foods). Don't expect to keep all the weight off. Although you will probably gain some of the weight back, you should be able to keep a few pounds off. The weight gain doesn't necessarily mean you gain body fat. When you lose weight on a fat fast, part of it is because your digestive system gets "empty" and you also lose water.

If you follow a 3-day fat fast, you should lose 4-8 pounds and gain up to 40-50% back. If you follow a 5-day fat fast, the weight loss could be even larger, up to 6-10 pounds and again gaining up to 40-50% back. The fat loss depends on your current weight and body fat percentage. In general, people with more weight and higher body fat percentage lose more fat than those with just a few extra pounds.

Can I do fat fast without dairy?

Yes, you can. Use coconut milk / creamed coconut milk instead of cream and cheese and ghee, lard or coconut oil instead of butter. You can also use mayonnaise instead of cream cheese in some of the suggested recipes. Although I didn't follow a dairy-free fat fast, I was naturally avoiding too much dairy and included foods rich in micronutrients (avocado, nuts, eggs, spinach, etc.)