Nutrition

The Dangers of Low Calorie Diets & Side Effects of Dieting

The Dangers of Low Calorie Diets & Side Effects of Dieting

Whether you’ve been enjoying life with no restrictions or just taken some time off – when it comes to losing weight many of us want fast results. Many people choose to turn to low calorie diets for quick and fast results. But did you know about the dangers of low calorie diets?

Low calorie diets involve consuming anything from 500 – 800 calories a day with food or meal replacements. When used short term these diets can offer some health benefits for obese people including lowering chronic disease. But in reality, many of us trying low calorie diets are not obese. If you’re dieting and exercising to lose weight, you NEED to know about the long term effects including:

  1. A Slower Metabolism
  2. Fatigue & Stress
  3. Illness
  4. Loss of Periods
  5. Lower Bones Density
  6. Risks of Eating Disorders & Poor Mental Health
  7. Binge Eating

Find out more about these dangers of low calorie diets in this article.

How Many Calories Do You Need in General?

A low calorie diet involves consuming a low daily food energy intake which is general 800 calories or less. By consuming fewer calories than you burn, you’re in an energy deficit, meaning that you will lose weight.

In general, the calories we burn can be split into 3 areas:

  • Basal Metabolic Rate: The minimum number of calories needed just to live and function. So if you were bed-bound this would be the number of calories just to keep your internal organs functioning.
  • Physical activity and exercise: calories needed during any activity.
  • Digestion: the calories needed to break down the food we eat.

By consuming a low calorie diet you don’t eat enough to even cover your basal metabolic rate. Our bodies are pretty clever, they can adapt to keep us alive! But after long periods of low calorie dieting, the consequences and health effects of keeping our bodies in this deficit can be damaging.

1. You’ll Lower Your Metabolism & Your Metabolic Rate

When we consume too few calories our bodies need to adapt to this to keep us functioning. In simple terms, when you eat fewer calories then you need, you begin to burn fewer calories. This combined with exercise is when many people quickly gain weight as they try to stop their low calorie diets and start eating normally again. One way your body can do this is by wasting muscle. This means although you’re losing weight fast, you much likely to gain weight back in the form of fat.

For example, in the British Journal of Nutrition a study of 35 men and women looked at the change in metabolism during a 15-week weight loss programme. They consumed a 700 calorie deficit and calorie burn was measured at 2 and 8 weeks. The study measured showed resting energy expenditure and the amount of calories being burned fell at both time periods.

Another study from the journal of Clinical Nutrition looked at an 8 week very low calorie diet in 91 men and women. Again, this study found that a process called adaptive thermogenesis took place and metabolism decreased. Not only that, but after the 8 week diet, these changes in metabolism were present even 1 year after the diet!

female running on treadmill

2. You’ll Suffer from Long Term Fatigue & More Stress

At first you may feel fine eating very little, but as time goes on you may struggle with stress and chronic tiredness. This is especially true if you’ve combined a low calorie diet with exercise.

To function and exercise the body needs plenty of energy from carbohydrates and healthy fats. Low calorie diets often promote a high protein intake with low fat and carb intake – meaning you can end up suffering from several hormonal changes that can leave you fatigued. For example, some studies have shown that restricting calories and dieting can increase the likelihood of suffering from chronic stress. This is due to an increase production of the hormone called cortisol. Not only this, but the hunger hormones leptin and ghrelin will also be messed up, meaning your cravings will be wild and you’ll feel hungry at the wrong times.

3. Be Prepared for Nutritional Deficiencies & Poor Immunity

It makes sense if you’re not eating enough food overall – you wont be getting all the vitamins and minerals you need. Sure, some meal replacements have vitamins and minerals added but the best way to absorb these is by getting them from natural sources like fruits and vegetables.

Vitamin and mineral deficiencies can lead to tons of problems from food cravings, fatigue, muscle soreness and poor sleep to serious conditions and risks. Plus you may find yourself always fighting off that common cold or viruses from a poor immune system.

4. Oh, And Don’t Forget Hypothalamic Amenorrhea: AKA No Periods

For females this is a BIG one and if you’re on the contraceptive pill you may never even realise. Losing your period is one of the biggest dangers of low calorie diets. Our hypothalamus is the part of the brain that controls metabolic processes and is needed to regulate the body. Our BMR (basal metabolic rate) is the minimum number of calories needed for your organs and body to function. In simple terms, when we cut calories too low or over exercise this tells the body there is not enough energy. As a result our hypothalamus tells the body to lower oestrogen levels and stop the menstrual cycle. This means you don’t have periods because the body can’t afford to use any more energy. This can put you at a huge health risk including causing infertility.

female athlete sitting in the dark

5. Lower Bone Density & Weaker Bones is Really Serious

Another effect of losing your periods or consuming too few calories whilst exercising is losing bone density which TRUST me, is SO serious! This is actually known as the female athlete triad and a lowering bone density can result in osteopenia and long term osteoporosis. These conditions greatly affect health in later life and can increase your risk of breaking bones now. If you’ve every suffered from a stress fracture this is a warning sign that something may not be right. Be sure to consume plenty of calcium and enough calories to fuel your body.

6. Eating Disorders, Body Image Disorder & Poor Mental Health Are Life Destroying

What may have started as a simple plan to lose weight could eventually take over your life when dieting for long periods of time. For example, studies have shown that calorie restrictions can lead to long term alterations in metabolism, insulin sensitivity, hormones and psychological health. Whereby people are more likely to develop eating disorder symptoms, have greater feelings of hunger, mood changes and suffer from a lack of concentration.

This means that you may end up with a worse body image that puts your mental and physical health at risk.

female athlete resting between weights7. You’re More Likely to Binge Eat

The effects and habits from depriving ourselves of food has been widely researched in science. In the journal of Addictive behaviours, one study investigated restraining foods and calories from individuals. They found those who were restrained not only under estimated their calorie intakes, but they ate a lot more than those who were not restricted.

By restricting you are more likely to crave foods and binge eat which is not only bad for digestion, but can also cause us to gain weight and create bad habits. Overall, binge eating can result in poor mental health and well being.

female eating in the kitchen

What You SHOULD Do

The take home message here is that you should not follow low calorie diets for fast results. You may lose weight fast but your healthy and psychological well being may be at long term risk.

To avoid the dangers of low calorie diets unfortunately you need to be patient. Try to take your time and create new healthy habits so you can safely lose weight, where it is advised that 0.5-1.5 pounds per week is a good place to start.

How many calories you need is different for everyone. It really depends on so many different factors from how tall you are to your age, weight and how much you exercise. For an average woman 1800 is the recommended calorie intake. A good place to start for weight loss is to track your current calories and how much you are eating. From here to start a small deficit using exercise or lowering your calories, starting with 100-200 calories.

Follow a healthy weight loss routine and diet plan to get you started – for example our free 15 day online training plan comes with 15 days worth of training and a nutrition ebook. If you are dieting getting enough vitamins, minerals and essential fatty acids is vital. You can do this through supplementation. A good protein powder can also help increase your protein intake to maintain your muscle mass and metabolism.


Doucet, E., St-Pierre, S., Alméras, N., Després, J., Bouchard, C., & Tremblay, A. (2001). Evidence for the existence of adaptive thermogenesis during weight loss. British Journal of Nutrition, 85(6), 715-723. doi:10.1079/BJN2001348

Stefan GJA Camps, Sanne PM Verhoef, Klaas R Westerterp; Weight loss, weight maintenance, and adaptive thermogenesis, The American Journal of Clinical Nutrition, Volume 97, Issue 5, 1 May 2013, Pages 990–994, https://doi.org/10.3945/ajcn.112.050310

Tomiyama AJ, Mann T, Vinas D, Hunger JM, DeJager J, Taylor SE. Low Calorie Dieting Increases Cortisol. Psychosomatic medicine. 2010;72(4):357-364. doi:10.1097/PSY.0b013e3181d9523c

Redman LM, Ravussin E. Caloric Restriction in Humans: Impact on Physiological, Psychological, and Behavioral Outcomes. Antioxidants & Redox Signaling. 2011;14(2):275-287. doi:10.1089/ars.2010.3253.

Janet Polivy, Perception of calories and regulation of intake in restrained and unrestrained subjects, Addictive Behaviors, Volume 1, Issue 3,
1976, Pages 237-243, ISSN 0306-4603, https://doi.org/10.1016/0306-4603(76)90016-2.

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Jenny Watt

Jenny Watt

Nutritionist & Personal Trainer Qualified


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