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6 Long Term Side Effects of Extreme Calorie Deficit That Can Harm Your Life

Learn all about calorie deficit side effects
Healthy Salad Meal
Healthy Salad Meal

By Kyle Sewell, Master Trainer at Camberwell

When it comes to fat loss, no matter the approach taken, a calorie deficit will be at the heart of it. A calorie deficit happens when we consume fewer calories than required to maintain our weight and when we continue over time, weight loss occurs. Are there any side effects beyond fat loss? What does it mean when a calorie deficit becomes ‘extreme’?

What is an Extreme Calorie Deficit? 

This occurs when we eat significantly less food than needed, usually for extended periods of time. While what constitutes ‘extreme’ can vary from person to person, any regime promising rapid weight loss, usually via a one size fits all approach, restricting or demonising complete food groups with hard-line rules around eating could all be considered red flags venturing into extreme territory.

The 6 Side Effects of an Extreme Calorie Deficit

  1. Diet rebound, weight regain and reduced metabolism

It may seem counterintuitive, but long-term calorie deficit and severe calorie restriction can lead to diet rebound and weight gain. Research has shown irregular eating and a history of dieting are two key factors that can accelerate weight gain, rather than preventing it.

In a long-term calorie deficit, our body works to conserve energy through evolved survival mechanisms. Our general day-to-day movement (known as non-exercise activity thermogenesis or NEAT) drops as we subconsciously become less active; activities like walking and fidgeting are reduced to conserve energy. Motivation to exercise drops and when we do exercise, those workouts are less effective.  

As we lose body weight, our energy needs change. Our body’s BMR, the rate at which we burn energy at rest, drops in tandem with weight loss and therefore less energy is required for basic bodily functions. This can actually mean, as energy expenditure declines, we may unknowingly no longer be in a calorie deficit or losing weight. 


  1. Eating disorders and disordered eating

At any given point, around 1M Australians live with a diagnosed eating disorder.  Even more undergo some form of disordered eating that does not meet eating disorder criteria. These encompass a range of mental conditions in which there is a persistent disturbance of eating behaviour. Feeling a constant need to diet could be a sign of disordered eating. Typically, there is an ‘all or nothing’ or negative association with food and eating. Extreme diet protocols with hard-line rules often exacerbate such behaviours.  

Our psychological, mental and social health suffers 

“You’re not you when you’re hungry.” Snickers, 2010. 

Young children go into meltdowns when hungry and denied their favourite treat.  Adults in an extreme calorie deficit are no different, only better at hiding their emotions. Restricting our calories and undereating can leave us drained of energy and more susceptible to mood swings as we run a gauntlet of emotions. Negative emotions can develop when it comes to food and meal times. Worrying about eating habits, calorie intake, body weight and a loss of control in certain eating situations all feed into anxiety and depression. These challenges around food and mealtimes plus the spectrum of moodiness can strain our relationships with others. 


  1. Hormonal disturbances

Calorie restriction has a direct impact on hormone levels as our body prioritises essential body functions by diverting energy away from other systems. Long term calorie restrictions disrupt our appetite regulation, producing less of the hormones responsible particularly the hormones leptin & ghrelin. Leptin, responsible for the regulation of hunger, signals when we are full. Ghrelin is the opposite, released when the stomach is empty or nearing empty, stimulating hunger.  Basically, as low food availability, and disrupted hunger hormones combine, we become overly susceptible to hunger to a point where appetite cannot be satiated. 

Extreme calorie restriction also impacts reproductive hormones. Production of oestrogen and testosterone are affected causing decreased libido. Males can also experience erectile dysfunction, especially when body fat levels are drastically reduced. For females, it can mean an irregular menstrual cycle with abnormal periods leading to Amenorrhea & reducing fertility.

With low body fat and a disruption to our body’s regulatory system, there is the added challenge of regulating bodily temperature, particularly struggling to keep warm. Further disruptions occur to our stress hormones, cortisol and adrenaline, both signalled due to the stress of an extreme calorie deficit. These in turn affect sleep quality, which influences appetite control and can lead to poor decision making around food.

These hormonal issues also lead to not only further irritability and moodiness, but also decreased recoverability, leaving us more susceptible to illness and injury.


  1. RED-S and the Female Athlete Triad

Typically, but not exclusively associated with athletic populations, RED-S (relative energy deficiency in sport), has the potential to impact anyone after an extreme or long-term calorie deficit and active lifestyle; regardless of age, gender and ability. Due to long term calorie deficit paired with the energy demands of an athletic lifestyle, it directly affects athletic performance, hence its titling and association with the sporting community. 

RED-S occurs through decreased intake or increased/ excessive energy expenditure, or both. Basically, a lot of training and not a lot of food. The symptoms presented with RED-S are by no means exclusive to the sporting community and there is carryover to the many other low calorie intake side effects. Fatigue, moodiness, decreased performance, injuries (muscle aches & pains, stress fractures) and menstrual dysfunction are all trademarks of RED-S. A further subset of RED-S, the “Female Athlete Triad”, is characterised by the interrelationship of menstrual dysfunction, low energy availability (with or without an eating disorder), and decreased bone mineral density. 


  1. Injury and illness

Long term calorie deficits take a physical toll on our bodies.  As our bodies conserve energy for basic functions, our immune system suffers. With our immune system compromised, we become more susceptible to injury and illness; aches, pains and niggles that don’t seem to heal and frequent illnesses and colds. This leaves us susceptible to more serious ongoing injuries. With our bodies unable to maintain muscle mass, muscle and bone wasting increases risk of strains, stress fractures and potential onset of osteoporosis.


  1. Death

The total number of deaths resulting from eating disorders is unknown, however there is a danger of death being the final side effect of an extreme calorie deficit.  

Starvation aside, sudden death has been associated with extreme calorie deficit due to the impact it has on cardiovascular (heart) health.  As our bodies look to conserve energy, our heart rate lowers, causing dizziness and the potential for long-term heart damage.  If left untreated, these irregular heartbeats, known as ventricular arrhythmia may lead to cardiac arrest or death.


How to Recover from an Extreme Calorie Deficit? 

When we’ve found ourselves dieting too hard for too long there are a number of tools to help recover and minimise the negative effects of an extreme calorie deficit. Both physiological and psychological considerations should be taken into account when coming out of an extreme calorie deficit.


  1. Reverse Dieting

A reverse diet protocol involves slowly and gradually increasing calorie intake over a multi week period (3-9 weeks depending on the severity and duration of the calorie deficit), in an effort to raise our metabolism and avoid rapid weight regain. Reverse dieting leads to greater energy expenditure (through increased NEAT), improves exercise performance, improves our adherence and overall helps us stick to our goals. Ideally, reverse dieting should leave us feeling energised and enjoying training while minimising fat, and weight gain.


  1. Diet Breaks & Regular Rests

Diet breaks and regular rests encourage our body to accept weight loss. These can be scheduled to coincide with our lifestyle and would typically last a couple of days to a week.  Think weekends, holidays, or time spent traveling when we might not be able to choose what our typical meals would be. 


  1. Refeeds

Refeeds (I chose the term refeed instead of cheat meals due to the negative connotation associated with ‘cheats’), give a psychological reprieve from the rigors of dieting. This is one meal or one day a week where food intake is (sensibly) increased and allows us to enjoy a favourite food or meal out. When working towards a long-term weight loss goal, there should be scheduled maintenance periods where we work on the lifestyle habits needed to maintain weight.  Most individuals do not have a weight loss problem, they have a weight maintenance problem.


  1. Community Support

Help is available for recovering from the effects of an extreme calorie deficit. The Butterfly Foundation is the national charity that works with those impacted by eating disorders and body image issues, and for the families, friends and communities who support them. If there are concerns around disordered eating or recovery from an extreme calorie deficit, there is help available.

The Vision Difference 

At Vision, we determine calorie and macronutrient recommendations based on individual goals, body type, activity level and fat free mass. We understand this is by no means the only way to determine a calorie recommendation, it is however, the one we feel works best. 


Calorie Deficit Safe Practices

Intelligently managing our calorie intake is key to long term weight loss success. Understand that everyone’s results will be different, even individuals of similar body weights and sizes following the same calories. This is again why a ‘one size fits all’ approach to calories can be detrimental. It is important to take in appropriate calories and nutrition based on our needs.  Although there is no one size fits all and the best diet will be the one that works best for us, there are some things that are generally agreed upon.   

Key things to consider when undergoing a calorie deficit include prioritising protein at meals, ensuring we are getting adequate rest, and making regular physical activity, particularly resistance training, a part of the plan. These can help to minimise the side effects when pursuing a weight loss goal. Check out our article on 5 tips to stay in a calorie deficit for more information.


  • Adnan D, Trinh J, Bishehsari F. Inconsistent eating time is associated with obesity: A prospective study. EXCLI J. 2022 Jan 14;21:300-306. doi: 10.17179/excli2021-4324. PMID: 35368461; PMCID: PMC8971321.
  • Lowe MR, Doshi SD, Katterman SN, Feig EH. Dieting and restrained eating as prospective predictors of weight gain. Front Psychol. 2013 Sep 2;4:577. doi: 10.3389/fpsyg.2013.00577. PMID: 24032024; PMCID: PMC3759019.
  • Cabre HE, Moore SR, Smith-Ryan AE, Hackney AC. Relative Energy Deficiency in Sport (RED-S): Scientific, Clinical, and Practical Implications for the Female Athlete. Dtsch Z Sportmed. 2022;73(7):225-234. doi: 10.5960/dzsm.2022.546. Epub 2022 Nov 1. PMID: 36479178; PMCID: PMC9724109.
  • Nazem TG, Ackerman KE. The female athlete triad. Sports Health. 2012 Jul;4(4):302-11. doi: 10.1177/1941738112439685. PMID: 23016101; PMCID: PMC3435916.
  • van Hoeken D, Hoek HW. Review of the burden of eating disorders: mortality, disability, costs, quality of life, and family burden. Curr Opin Psychiatry. 2020 Nov;33(6):521-527. doi: 10.1097/YCO.0000000000000641. PMID: 32796186; PMCID: PMC7575017.

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