Are very low calorie diets a healthy way to lose weight?

When trying to lose weight people usually want to lose it fast, that’s why they often opt for restricting their caloric intake drastically. Of course, calorie deficit is the best way to lose weight; however, following some fad diets or restricting calories severely for a prolonged period of time may lead to a variety of health problems including nutrient deficiencies, slower metabolism, hair loss, menstrual issues and reduced fertility, weaker physical and mental performance, and weaker bones.

This article focuses on seven potentially harmful effects of very low-calorie diets and helps you determine the calorie deficit that’s right for you.

Let’s start first by discussing the reasons your body needs calories throughout the day.

Your body needs calories to sustain its functions and uses them to perform three main processes:

Basal metabolic rate (BMR): which refers to the number of calories needed to perform the basic body functions of your heart, brain, lungs, kidneys and nervous system.

Thermic effect of food (TEF): certain number of calories are used to metabolize and digest the foods you eat, which is known as the TEF or thermic effect of food.

Physical activity: which refers to the number of calories needed to do your workouts and perform your daily tasks.

The majority of calories consumed during the day goes for your Basal metabolic rate or the basic body functions, that’s why it is essential to consume enough calories to sustain body functions; otherwise, these functions would be compromised.

Generally speaking, eating fewer calories than your body needs lead to weight loss while eating more calories than your body requires will cause you to gain weight, mostly body fat.

How Low is Too Low?

The recommended daily caloric intake for women is 2,000 calories a day and for men is 2,500 calories a day depending on activity level[1].

A very low-calorie diet is a diet that restrict your caloric intake to 800 calories or less a day, which is most of the time a lower range than your BMR, this means that the basic body functions would be compromised.

So generally speaking, “too low” calorie restriction is restricting calories below your BMR.

When is a very low-calorie diet prescribed?

Very Low-calorie diets may be useful as a rapid weight loss tool for obese patients with metabolic complications such as diabetes, hypertension and those at risk of coronary heart disease; where such diets would cause rapid weight loss and improvements in obesity complications. However, when it comes to long-term weight loss, lifestyle changes in dietary and activity patterns are needed[2].

Now let’s discuss the effects of excessive caloric restriction on the body.

1. Weaker physical and mental performance

According to studies, 60% to 70% of the energy consumed by an individual over a period of 24 hours is basically required for maintain our lives at rest which is called the resting metabolic rate (BMR)[3]. Regularly consuming low amount of calories below your BMR can have a severe impact on your energy levels and causes physical and mental fatigue, which may impair your daily functioning and make you feel tired even when performing basic daily functions like walking and climbing the stairs and it will most definitely interfere with your exercise routine and performance.

2. Micronutrient deficiencies

Many Vitamins and minerals play a central role in many hormonal and body functions. When you do not consume enough calories, you will be risking having vitamin and mineral deficiencies, to name few: deficiency in iron, folate or vitamin B12 which may cause anemia, and deficiency in vitamin D and Calcium which may decrease bone strength and density and increase the risk of fractures, in addition to many other diseases caused by vitamin and mineral deficiencies.

3. Slower metabolism

The biggest issue with very low-calorie diets is that there’s a limit on the amount of fat a person can burn per day thus the body shifts to breaking down muscles for energy[4]. And since muscles help you burn more calories, it's essential to retain muscle mass when trying to lose weight. That’s why it is very important for people following such diets to be monitored by a dietician to provide them with their exact protein needs and exercise program to prevent muscle breakdown when losing weight.

In addition to that when restricting calories, you will lose weight at first, but after a while, your body will begin to adapt to this low amount of caloric intake by lowering the number of calories it burns and this is called “adaptive thermogenesis”. Your body usually drops your metabolism by up to 500 calories per day as a way to adapt to this increased calorie deficit and rapid weight loss. So, if you are consuming 1,200 calories or less and not losing any weight, this most probably means that your metabolism has slowed down in an attempt to match your low food intake[5].

4. Hair Loss

It is normal to lose around hundred strands of hair daily; however, if you notice that you are losing increasing amount of hair this may be a sign that you are not eating enough calories throughout the day.

Maintaining healthy hair growth requires adequate intake of calories, iron, protein, vitamins, essential fatty acids and other nutrients, and when you do not consume enough of the calories and nutrients needed, your hair’s health will be compromised[6].

5. Constant Hunger

When you decrease your calorie intake too much, your body will send hormonal signals that will drive you to eat more in order to avoid potential starvation. Studies have shown that hunger and food cravings hormones increase and appetite suppressing hormones decrease significantly in response to increased caloric restrictions[7]. In addition to that, low calorie intake has been shown to increase the production of the hormone cortisol, which is a stress hormone that has been linked to increased hunger and the accumulation of belly fat[8].

6. Menstrual Issues

Decreasing caloric intake too low may cause rapid weight loss and forces the female body to decrease all the non-essential survival functions including the menstrual cycle which causes irregular menstruation and in some cases this will interfere in female’s fertility and her ability to get pregnant[9].

7. Weaker Bones

Consuming too little calories may weaken your bones, and that is mainly because severe calorie restriction can reduce the levels of the two reproductive hormones estrogen and testosterone which is associated with decreased bone formation and increased bone resorption, resulting in weaker bones; moreover, bone loss is a very critical issue since it is often irreversible which in turn increases fractures risk.[10] [11]

How to Eat the Right Number of Calories

Caloric needs vary from person to person depending on different factors such as age, height, weight, sex, and physical activity level. By figuring your caloric needs, you can reduce the likelihood of developing the negative health consequences mentioned above.

Below is a rule which helps you estimate your BMR.

The Mifflin St. Jeor Formula:

Men - 10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) + 5

Women - 10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) – 161

So let’s take as an example a 25 years old female which weighs 60 kgs and is 165 cms, she performs low intensity exercise three times a week. Now let’s apply this rule to her.

BMR = (10 x 60) + (6.25 x 165) – (5 x 25) – 161 = 1,345 calories

So basically, this lady needs 1,345 calories for maintaining her daily body functions. She must never consume below this number. It is important to note that when calculating the daily caloric intake you must account for your daily activity level which varies from a person to person.

Now let’s consider the activity levels and the factor that we are going to multiply the BMR with in order to get the Total Daily Energy Expenditure that a person needs through the day:

Sedentary = little to no exercise in a day (sitting most of the time)- BMR X 1.2

Light activity = light exercise/sports 1-3 days/week - BMR X 1.375

Moderately active = moderate exercise/sports 3-5 days/week - BMR X 1.55

Very active = hard exercise/sports 6-7 days a week - BMR X 1.725

Extra active = very hard exercise/sports and physical job OR 2x training BMR X 1.9

So since this lady is lightly active we multiply her BMR with 1.375.

Total daily energy expenditure is 1,345 x 1.357 = 1,825 kcal

And now let’s assume that this lady wants to lose 2 kgs, she can make a calorie deficit of 300 calories which results in her consuming 1500 calories per day and losing around 0.3 to 0.5 kgs per week and if she increased her workout program a bit she will be able to achieve her target weight within a month without cutting her caloric intake drastically and risking losing muscle mass and deficiencies and ultimately diet failure.

Bottom line, moderation is key in everything, and if you are looking for a way to lose weight and sustain it on the long run, it is reasonable to forget about those restricted low calorie diets and focus instead on a balanced diet that will encourage you to make sustainable lifestyle changes and steady weight loss without depriving your body of the essential energy and nutrients needed for optimal functioning, health and survival.

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[1] What should my daily intake of calories be?

https://www.nhs.uk/common-health-questions/food-and-diet/what-should-my-daily-intake-of-calories-be/

[2] Low and very low calorie diets

https://pubmed.ncbi.nlm.nih.gov/2643004/

[3] Bedside-to-Bench Conference: Research Agenda for Idiopathic Fatigue and Aging

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4540791/

[4] A limit on the energy transfer rate from the human fat store in hypophagia https://www.sciencedirect.com/science/article/abs/pii/S0022519304004175?via%3Dihub

[5] Changes in Energy Expenditure with Weight Gain and Weight Loss in Humans https://link.springer.com/article/10.1007/s13679-016-0237-4

[6] Nutrition and hair: deficiencies and supplements

Nutrition and hair: deficiencies and supplements - PubMed (nih.gov)

[7] Caloric restriction: Impact upon pituitary function and reproductionhttps://www.ncbi.nlm.nih.gov/pmc/articles/PMC2634963/

[8] Low calorie dieting increases cortisol

https://pubmed.ncbi.nlm.nih.gov/20368473/

[9] Magnitude of daily energy deficit predicts frequency but not severity of menstrual disturbances associated with exercise and caloric restriction

https://pubmed.ncbi.nlm.nih.gov/25352438/

[10] Dose-response relationships between energy availability and bone turnover in young exercising women https://pubmed.ncbi.nlm.nih.gov/15231009/

[11] The presence of both an energy deficiency and estrogen deficiency exacerbate alterations of bone metabolism in exercising women

https://pubmed.ncbi.nlm.nih.gov/18486582/