The Consequences of Eating Too Few Calories On Intermittent Fasting

You’re doing intermittent fasting, and finally, you feel that you have control over yourself when it comes to eating. You’ve just stopped relying on food for energy. You’re relying on your own fat storage. And that’s great. You feel great. But you’ve gotten greedy now and you want more. “What would happen if I decrease my caloric intake now even more?” you ask. “I can just stop eating altogether. I’ve become a fat-burning machine!” Not so fast.

You can simply be just feeling full all day, and you’re not hungry even when you’re supposed to break your fast. So, should you eat if you’re not hungry?

That’s what we’ll be diving into today, exploring the dangers you can face when you opt for a huge caloric deficiency, when it is okay to do it and how to do it the healthy way.

We will discuss here two scenarios. The first is for those of you who have recently started intermittent fasting and haven’t yet completely adapted to it. The second is for those who have been following this pattern of eating for some time and now have decided to take it to the next level.

This Is What Happens When You Decide You Want To Do A Huge Daily Caloric Deficiency, When:

A- You’re At The Early Stages of Fat Adaptation With Intermittent Fasting

This is when you’re fairly new to intermittent fasting and haven’t yet become Fata-dapted. It is not that much related to how long you’ve been on the diet, rather whether your body has already gone into the state of ketosis or not. (Some bodies adapt faster, especially when they have gone through Keto or Intermittent Fasting before.) You can know that you’re Fat Adapted when, 1) you no longer feel hungry throughout the day, even when it’s been hours since you have last eaten. 2) You feel more energized throughout the day. 3) You no longer have sugar cravings. And other signs that should drive you to the conclusion that you’re fat-adapted.

If that hasn’t happened yet, then you’re probably in this bucket, and the following are the consequences that are going to happen if you decide to do a huge caloric deficiency.

1. You Might Do Your Metabolism Harm For A Long Time

This is one of the things I am most worried about, for you. If you’re cutting down calories too much too soon in your fat adaptation process, chances are you will slow your metabolism down. And the worse news is that you can have it that way for a long time even after you’ve gone back to normal eating.

The reason for that is that your body has the ability to switch into what we call Starvation Mode. This is when your body is getting too few calories, that it believes you’re in a desert and that there’s no food around. So it’s working so damn hard to keep you alive. That’s the best explanation your body can come up with when you’re restricting your calorie intake that much.

This Can Present As

Having low energy levels throughout the day and feeling a constant hunger.

Here’s How To Prevent It:

  • HIIT exercises. This can boost your metabolism and convince your body that it shouldn’t tap into Starvation Mode that we talked about.
  • Start Keto with 16:8 Intermittent Fasting and gradually increase your fasting window. This also plays a crucial role in increasing you GH levels and significantly lowering insulin.

2. You Might Lose Muscle Mass

As explained earlier, maintaining all the needed macro- and micro nutrients with a very low caloric intake is pretty hard. That’s why many people will find it hard to get enough protein. This results in loss of muscle mass because your body needs glucose, and there are only two macronutrients that can be broken down into glucose: Carbohydrates and Proteins. Your body can’t get glucose from fat. What your body can do, though, is create glucose through a process called Gluconeogenesis.
The problem is, your body will tap into your muscles before it tries to make the glucose on its own, using the aforementioned process. But the good news is that your body will only tap into your muscles for protein if you’re not getting enough protein from your diet.

This Can Present As

Muscle shrinking on the mirror and feeling weaker.

Here’s How To Prevent It:

Add enough protein to your everyday diet. We have talked in this article about how much you averagely need. There are many Keto-friendly protein sources that are low in calories (assuming you want to keep your calorie intake low). This includes chicken breast, white-fleshed fishes, plain Greek yogurt, and low-fat cottage cheese.

3. You Might Get Nutrients Deficiencies

If you’re lowering your food intake to, let’s say, 500 calories, you’re probably not getting enough of the nutrients you need on a daily basis. These are micronutrients like iron, thiamine, potassium, B12, calcium, biotin, iron, and many others. It’s hard to have a very low-calorie intake and still be meeting all the required micronutrients. In fact, a big chunk of people who eat the normal amount of food is still missing key nutrients from their diet every day.

That being said, there are supplements that can help you with that. Although you don’t need them, and although I always advocate aiming to get your nutrients from real food as much as possible, it will be super hard to meet them all if you’re taking it to the extreme with your calorie deficit.

This Can Present As

Hair loss, tiredness, nail abnormalities, and anemia.

Here’s How To Prevent It:

You should observe what micronutrients you’re missing from your diet, and add foods that contain a good amount of it. For example, if you’re lacking Calcium, add more cheese to your diet. If it’s iron, add more eggs or red meat.

Generally, eating more healthy salads helps you eat more of the vitamins and minerals you need.

Another way, and an easier one, to fill these gaps, is to take oral supplements of the nutrients you’re neglecting.

4. It Might Weaken Your Immune System

This extreme version of dieting and cutting down calories too much might, in the long run, result in immunity problems. Thus, making you more susceptible to viruses and other infections. This is worse when you’re exercising heavily, without getting enough calories and nutrients. 

This Can Present As

Fatigue, and getting the common cold and the flu more frequently than usual.

Here’s How To Prevent It:

Look into your diet and try to fill the nutrients gaps you have. Try to combine Keto with Intermittent Fasting, which can promote a process called Autophagy that not only preserves your immune system but also give it a boost.

5. It Might Make Your Bones More Fragile

Bones are regenerating parts of the body. Just like the skin, they are always going through both breaking down and building up processes. In childhood, the building process happens at a much faster rate than breaking down. That’s why children have denser, thicker bones. As we get past puberty and hit our 20s, this building process starts to slow down, and continues to do so.
There are many factors that can harm our bodies by speeding up the breakdown of bones or by slowing down the building of new ones. One of those factors is restricting your daily calorie intake. This is because it increases your stress hormone levels and decreases Estrogen and Testosterone, two hormones crucial in building up new bone tissue.

Moreover, one study done on mice shows that not only does low-calorie intake harm the bones and make them more fragile, but also combining the big caloric deficit with exercise can further weaken your bones and make them even more vulnerable, similarly to the case with your immune system.

This Can Present As

Bone breaks\fractures. We must not wait until we get there. Having a low-caloric intake should always be combined with a strategy to help protect your bones. So…

Here’s How To Prevent It:

Get enough amount of vitamin D and Calcium, even if you have to get them as supplements. If you’re a menopausal woman, you’re at a higher risk and you shouldn’t be lowering your calorie intake to an extreme.

6. It Might Harm Your Fertility

As I mentioned above, dramatically reducing your calorie intake causes a drop in estrogen levels, which is a crucial hormone to fertility.

There is also a risk of infertility in men, but that doesn’t actually have enough research to back it up.

The good news is that only happens if you do it for a long time. The bad news is that it might be permanent.

This Can Present As

Irregular menstrual cycle and sometimes amenorrhea.

Here’s How To Prevent It:

If you’re in the reproductive age and plan on getting pregnant in the future, avoid doing these extreme caloric deficits for long periods of time.

B- When You’re Fully Fat-Adapted.

Here you can rest more assured about the consequences and just listen to your body. This is the cool thing about keto. When you tap into it, your food becomes you! Your food becomes your own fat. You’re no longer living to eat, but eating to stay alive. So just as long as you don’t feel hungry, you can cut down a little more on calories. When your body wants food, it’ll let you know.

Your body can now take the required energy from your own fat by breaking it into ketone bodies, but it still needs other nutrients. So even when you’re fat-adapted, you should still be paying attention to what you’re getting into your body, whether you’re having enough protein, enough vitamins and minerals, and not too much carbohydrates in order not to get kicked out of Ketosis.

Another thing to keep in mind is that if you combine Keto with Intermittent Fasting, you will trigger a process called Autophagy. This allows your body to reuse old nutrients even when you’re not eating enough of them on a day-to-day basis.

Calories in Popular Intermittent-Fasting Foods

You should know by now which bucket you fall in based on the many variables we already discussed. But you might be wondering whether or not you ate too few calories in the first place. That’s why I have a compiled multiple tables of the most popular keto-friendly foods, each with its caloric content to give you a better idea of your intakes and to help you plan better for future meals.

FoodServing SizeCalories
Beef (grounded, 85% lean)100g250
Chicken Breast100g165
EggsOne Large, Boiled Egg (50g)78
Cottage Cheese1 ounce (28g)28
Ricotta Cheese100g174
Milk1 cup (240 ml.)103
Almond Milk1 cup (240 ml.)60
Mozzarella Cheese100g280
Black Beans100g132


FoodServing SizeCalories
Almonds1 ounce (28g)165
Almond flour1/4 cup (25g)150
Brazil nuts1 ounce (28g)185
Cashews1 ounce (28g)150
Macadamias1 ounce (28g)207
Hazelnuts1 ounce (28g)220
Pecans1 ounce (28g)201
Pistachios1 ounce (28g)150
Walnuts1 ounce (28g)190


FoodServing SizeCalories
Chia seeds1/4 cup (30g)145
Pumpkin seeds1/4 cup (30g)180
Sesame seeds2 tablespoons (18g)103
Sunflower seeds1/4 cup (30g)160
Hemp seeds1/4 cup (30g)155

Nut and Seed Butter:

FoodServing SizeCalories
Coconut butter1 tablespoon (16g)105
Almond butter1 tablespoon (16g)98
Cashew butter1 tablespoon (16g)94
Macadamia butter1 tablespoon (14g)97
Sunflower seed butter1 tablespoon (16g)99
Tahini1 tablespoon (15g)89

Coconut products:

FoodServing SizeCalories
Coconut Oil1 tablespoon (14g)120
Coconut butter1 tablespoon (16g)105
Shredded Coconut1/4 cup (20g)71


FoodServing SizeCalories
Coconut Oil1 tablespoon (14g)120
Olive oil1 tablespoon (14g)120
Flaxseed oil1 tablespoon (14g)120
Sesame seed oil1 tablespoon (14g)120
MCT oil1 tablespoon (15g)130


Scenario number one: if you’ve just started intermittent fasting and haven’t yet felt that you’re all the way there, you should not try to dramatically decrease your caloric intake. And if, for some reason, you must, then study the six dangers we discussed earlier and learn how to prevent them.

Scenario number two: If you’re already fat-adapted, you’re less likely to suffer consequences. That being said, the dangers mentioned above may still apply to you, so give them a read. Generally, you’re less likely to suffer from them if you 1) pay enough attention to what your body is asking for and 2) combine Keto with Intermittent Fasting.


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Nabeel Kallas

I'm a medical doctor with a lot of passion for Keto & Intermittent Fasting. They have completely changed my life and I am beyond honored to help you experience that same change. I'm ready when you are. Read more here.

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