Very low-calorie diets are generally defined as those that have energy intakes below 800kcal per day.

This form of dieting first existed as part of integrated medical interventions or lifestyle modification programs to improve clinical disease states such as morbid obesity and cardiovascular disease.

However, very low-calorie diets are also now part of a new dieting culture in hope to produce fast weight loss results in a short period of time.

Plenty of fad diets are now based on introducing a severely low caloric intake, such as the Detox diet and the Maple Syrup diet. Although these diets are not based on science, they manage to gain popularity through celebrity endorsements and good marketing.

From a nutritional and health perspective, it is very important to concern ourselves with the type of individual that chooses to start this type of diet, and whether the potential benefits outweigh the inevitable and harsh side effects.

The Potential Benefits of a VLCD

Below is a list of every potential benefit that’s backed by credible scientific research:

It’s Helpful in Clinical Settings

Despite the long list of safety issues that such a restrictive approach to dieting leads too, there are certain individuals where the benefits will outweigh the cons.

Specifically, very low-calorie diets have consistently shown to be useful tools in the treatment of obesity to help people regain a normal body weight and rid of life-threatening health complications [1].

Very low-calorie diets are now even advised to morbidly obese people as a healthier option than other nonsurgical treatments – such as medication – and should be considered before opting for surgical treatment.

In general, very low-calorie diets induce significantly greater short-term weight loss compared to less severe caloric reductions, but similar long-term changes (1-2 years) [2]. In non-clinical settings this means that individuals at a more reasonable bodyweight should opt for less severe approaches to achieve the same long-term results, however in clinical settings more restrictive methods can allow life-threatening conditions to be improved at a faster rate.

Just 4-5 months of very low-calorie dieting in morbidly obese patients can reduce blood pressure, total cholesterol, and LDL-cholesterol by ~10%, and lower triglycerides in the blood by ~25% [3]. Improvements in insulin sensitivity and glycaemic control are also noted with rapid weight loss [4].

This style of eating has even been linked to an improved quality of life in obese individuals [5].

The issue becomes trying to define a specific threshold of ‘poor health’ where very low-calorie diets should be advised in medical settings. Currently there are no clear indicators to what exact condition a person has to be in for a reasonable assumption to be made that the positives outweigh the negatives.

It seems best for individuals suffering from obesity (BMI >30) or other serious medical conditions to consult their doctor and get a personalized recommendation based on their condition.

The Potential Negative Effects of a VLCD

There are also things we believe could be viewed as a negative:

Micronutrient Deficiencies

The occurrence of malnutrition is very closely associated with undernourishment, and micronutrient deficiencies are going to be near impossible to avoid when food intake is so low.

This presents a major issue as sub-optimal intake of certain micronutrients is an established factor in a multitude of dangerous health conditions and diseases, including fatigue, memory loss, decreased immunity, impaired wound healing, digestive issues, osteoporosis, and a higher chance of birth defects [5].

Even less restrictive, more balanced diets still note micronutrient deficiencies in 50% of the essential micronutrients for health [6].

In particular, very low-calorie diets increase vitamin B, vitamin C, selenium, iron, calcium, zinc, and lycopene deficiencies and it seems these cannot be effectively corrected by additional supplementation.

Water-soluble vitamins – vitamins B and C – are of particular note as these need to be consumed daily and cannot be stored for later use in body tissues.

Based on this, although micronutrient supplementation may not cure these issues, it is absolutely essential for anyone trying this dietary approach to help minimize the extent of these deficiencies with supplementation.

Psychological Issues

Not only can very low-calorie dieting be physically detrimental, but psychologically disturbances are extremely common at such a low food intake.

It has been consistently documented that extreme diets result in negative emotional consequences such as depression, anxiety, decreased self-esteem, nervousness, and irritability [7].

The large amounts of psychological stress – speculated to be mainly caused by elevated cortisol – can even cause temporary hair loss, known as Telogen Effluvium [8].

Extreme cases of severe mental issues after intense weight loss have even been also been reported, some of which have led to hospitalization.

The psychological stress may be in part related to the large amounts of hunger experienced on such a diet as the body tries to maintain weight homeostasis. Large increases in hunger hormones such as leptin will occur during fast weight loss periods which can cause individuals to battle food cravings from morning until night.

Loss of Lean Muscle Mass

Typical weight loss with very-low calorie diets is around ~45lbs in ~5 months, depending on an individual’s start weight. This may seem appealing to many people, but it should be kept in mind that a significant amount of this weight will be comprised of lean body tissue including muscle mass.

As mentioned, this may be suitable for morbidly obese individuals who are in desperate need to reap the health rewards of such a great amount of weight loss. Additionally, obese individuals are more likely to reduce fat mass at a given energy deficit as they have more fat tissue available to spare, and muscle mass and strength can be preserved in these individuals even during extreme weight loss periods [9] [10] .

However, individuals aiming for more moderate weight loss (<20lbs) will benefit more from gradual, sustainable weight loss as this will preserve strength and muscle tissue, whilst resulting in superior body composition at the end of a weight loss period [11].

In theory and based on real-world results, the smaller the reduction in energy intake, the more an individual will preserve muscle tissue loss when dieting [12].

Will The Weight Lost Be Regained After Stopping The VLCD?

Resting metabolic rate refers to the number of calories required to keep a body functioning at rest – relating to the energy used for internal processes such as maintaining heart function and breathing.

It is very common for people to state that during periods of large energy restriction, the human body will significantly decrease resting metabolic rate, and in turn this will facilitate rapid weight regain when caloric intake normalizes.

This concept is part of a term called “adaptive thermogenesis”, defined as a decrease in energy expenditure (calories burned per day) beyond what can be predicted from changes in body weight alone.

However, it has now been shown that diminished energy expenditure during a dieting phase is mainly as a result of involuntarily reducing non-exercise activity thermogenesis – the energy expended outside of physical activity such as walking, fidgeting, and even the speed at which we talk [13].

In actuality, during periods of rapid weight reduction such as that with very low-calorie diets, resting metabolic rate is relatively well-preserved, and avoids the metabolic adaptation phenomenon [14].

Yes, there may be some extent of decline in resting metabolic rate, but this is not as significant as once thought, and any metabolic alteration can be easily recovered when normal eating habits are restored [15].

This may be an important consideration for individuals thinking about very low-calorie diets (ideally morbidly obese individuals) who are conscious that they may cause permanent damage to their resting metabolic rate and result in massive weight regain.

Instead, after a large weight loss period, individuals should avoid relapsing into old habits that caused the initial weight gain, instead opting for more reasonable and controlled dietary intakes.

Are Very Low-Calorie Diets Safe?

Data shows that very low-calorie diets are safe, well-tolerated, and an acceptable method for weight loss when used as part of a medical intervention weight loss program in suitable obese individuals [16].

However, for people who are not obese this type of dieting approach will cause large reductions in muscle mass, micronutrient deficiencies, psychological issues, and hormonal imbalances.

Although many of these issues will still be present in obese individuals during a medical intervention, the very low-calorie approach may be a safer option to improve overall health when compared to the health issues they experience at their current bodyweight.

A doctor should be consulted before anyone starts a very low-calorie diet to check whether they are suitable for such an extremely restrictive approach.

Conclusion

Very low-calorie diets are generally defined as those that have energy intakes below 800kcal per day.

This form of dieting is beneficial in medical settings for morbidly obese individuals who need rapid weight loss to cure life-threatening conditions.

However, popular crash diets such as this are being increasingly used in healthy populations when there is no need for such an extreme dietary approach.

In non-obese individuals, very low-calorie diets will cause large reductions in muscle mass, micronutrient deficiencies, psychological issues, and hormonal imbalances.

For the vast majority of individuals, a more conservative and less restrictive dietary approach will lead to better outcomes for body composition and health.

A doctor should be consulted before anyone considers starting a very-low calorie diet.