Very Low Calorie Intake

Calorie Deficit: The Basic Principle of Weight Loss

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A calorie deficit is a fundamental principle of weight loss. When energy intake falls below energy expenditure, the body uses its reserves to make up the difference. The logic sounds simple, and it is: if the calories entering the body are less than the calories expended, weight loss occurs.

However, there are many factors that influence this equation. Our bodies are biologically adapted organisms that have been accustomed to scarcity for millions of years. Therefore, when we create a calorie deficit, our bodies not only lose weight but also begin to counteract this deficit.

Energy Consumption

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Daily energy expenditure consists of three main components:

• Basal metabolic rate (BMR): The energy that organs expend for vital functions while at rest.

• Energy derived from activity: Exercise, walking, movement, and NEAT (Non-Exercise Activity Thermogenesis).

• Thermal effect: The digestion, absorption, and processing of the food we eat.

  
  

The sum of these three factors is called Total Daily Energy Expenditure (TDEE) . A calorie deficit is energy intake below the TDEE. But there is a critical point here: the amount of the deficit determines the metabolic response.

Magnitude of the Calorie Deficit

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Short-term diets often follow the "the less, the faster you lose weight" philosophy. However, science shows that this is not sustainable.

When the calorie deficit is small (e.g., 15–25%) and a nutritionally balanced diet is followed, the body turns to using fat stores; metabolic balance is largely maintained. However, when the deficit reaches extreme values such as 40–50%, the brain perceives this as a scarcity signal . From that moment on, the metabolism switches to "energy conservation mode".

A 2011 review by Redman and Ravussin highlights the physiological and psychological risks of long-term or extremely low-calorie diets:

• Muscle loss: Lack of energy leads to the loss of muscle tissue along with fat.

• Decreased thyroid hormone levels: Metabolism slows down, fatigue increases.

• Hormonal imbalance: Leptin and testosterone levels decrease, which can lead to menstrual irregularities and decreased libido.

• Behavioral effects: Calorie restriction can trigger episodes of binge eating.

• Psychological effects: Anxiety, irritability, difficulty concentrating, and eating disorders may increase.

  
  

Most & Redman (2020, Experimental Gerontology) also provide comprehensive human data on this subject.

The most important condition of calorie restriction (CR) is maintaining nutritional adequacy while reducing energy intake. This is best seen in the classic Minnesota Starvation Study by Keys et al. (1950). In this study, participants underwent approximately 40% calorie restriction , resulting in a 25% reduction in body weight (70% of the loss was fat mass). However, the diet was nutritionally deficient. As a result, the participants:

• Chronic fatigue, loss of muscle strength, and edema in the legs,

• Serious psychological effects (depression, apathy, hysteria, sexual dysfunction) have been observed.

  
  

In contrast, controlled human trials such as the modern CALERIE studies have implemented a 25% calorie restriction (effectively 12–18%) and provided all participants with vitamin, mineral, and calcium supplementation . The results have shown that this level of CR does not have negative health effects (Rickman, Rochon, Ravussin, 2011–2016).

Metabolic Adaptation

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When energy intake decreases, the body not only loses weight but also reduces energy expenditure. This is an evolutionary "energy conservation mechanism." When the body receives a scarcity signal, it switches to conservation mode to ensure its survival.

Metabolic adaptation was the primary outcome studied in the CALERIE trials. Participants were placed on a planned 25% calorie restriction, and results were monitored for 3, 6, 12, and 24 months .

The results were impressive:

• Energy expenditure during sleep decreased by 8% in month 3, 7 % in month 6 , 6% in month 12, and 5% in month 24.

• Under free-living conditions, the adaptation rate was twice as high. A decrease of 13% was observed in the 3rd month, 7% in the 6th month, 8% in the 12th month, and 9% in the 24th month.

  
  

The study by Most & Redman (2020) shows that calorie restriction not only leads to weight loss but also reshapes the body's energy management system .

There are a few noteworthy points in this process.

Metabolic adaptation becomes less pronounced as the calorie deficit decreases.

In the study, participants were able to create an average calorie deficit of 21% in the first 3 months. However, this rate gradually decreased in the following period and remained at an average of only 11% from the 6th to the 24th month .

This difference clearly demonstrates that metabolic adaptation is directly dependent on the magnitude of the calorie deficit . Individuals exposed to higher energy deficits in the initial months experienced a greater-than-expected decrease in energy expenditure, while this decrease gradually slowed in the following months when the deficit was more moderate.

In other words, the larger the deficit, the more the body shuts itself down. The body perceives the excessive energy deficiency as a threat and switches to "energy conservation mode." This suggests that, in the long run, large deficits may slow down weight loss rather than accelerate it.

The Decline of NEAT

Most & Redman found an interesting difference when comparing energy measurements in a laboratory setting (room calorimetry) with measurements under real-life conditions: metabolic adaptation was approximately twice as great under free-living conditions.

This difference is not only biological but also behavioral . In individuals experiencing a high calorie deficit, the body begins to move less to conserve energy. This is explained by "Non-Exercise Activity Thermogenesis (NEAT)," meaning a decrease in non-exercise physical activity.

• So, without realizing it, the person walks less and sits more during the day.

• As a result, total energy consumption decreases.

  
  

This shows that the body's energy saving occurs not only at the cellular level, but also at the behavioral level.

In diets with extremely low calorie intake, NEAT (Neural Energy Emission) naturally decreases because the person feels more "sluggish" throughout the day, thus making metabolic adaptation more pronounced.

Another finding supporting this relationship is the lack of metabolic adaptation observed in small energy deficits created by exercise.

For example, in the study by Hopkins et al. (2014), participants created a small energy deficit of about 250 kcal per day and exercised. As a result, although there was weight loss , there was no greater decrease in basal metabolic rate than expected.

The Real Threat: The "Binge Eating" Trap

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Although very low calorie intake can disrupt hormonal balance and reduce metabolism and daily energy expenditure, this only makes a difference of about 10%, which is not enough to halt weight loss.

One major problem with very low-calorie diets is their unsustainability. Prolonged extreme restriction increases psychological and physiological hunger pressures, sometimes leading to binge eating episodes.

There's a crucial point often overlooked here: weight loss depends on the energy deficit created by an overall average , not just a single day.

Real-Life Scenario

Let's say a person burns an average of 2500 kcal per day. To lose weight, they go on a strict diet and reduce their intake to 1500 kcal , creating a deficit of 1000 kcal per day. In the first few days, weight loss is rapid. However, after a few weeks, metabolism slows down and activity decreases: they now burn approximately 2100 kcal per day. In this case, the actual daily deficit drops to 600 kcal . In 10 days, they lose approximately 6000 kcal .

But then the body loses its resilience because hunger, fatigue, and listlessness accumulate. Eventually, the person eats around 3500 kcal for 3-4 days — a classic binge eating phase.

The 6000 kcal deficit burned over 10 days is completely offset by the excess calories consumed in a few days.

Result: The weight problem persists, metabolism remains slow, and motivation disappears.

That's why "eating too little" doesn't lead to weight loss — failing to maintain balance reverses progress.

In short, excessive restriction both slows down metabolism and creates behavioral imbalances , thus hindering progress .

Conclusion

A calorie deficit is key to weight loss. However, a balanced diet is essential for this process to be sustainable. Otherwise, metabolism slows down and behavioral problems may arise. For a healthy life, carefully managing a calorie deficit is important.

References:

• Most J, Redman LM. Impact of calorie restriction on energy metabolism in humans. Exp Gerontol. 2020 May;133:110875. doi: 10.1016/j.exger.2020.110875. Epub 2020 Feb 11. PMID: 32057825; PMCID: PMC9036397.

• Redman LM, Ravussin E. Caloric restriction in humans: impact on physiological, psychological, and behavioral outcomes. Antioxid Redox Signal. 2011 Jan 15;14(2):275-87. doi: 10.1089/ars.2010.3253. Epub 2010 Aug 28. PMID: 20518700; PMCID: PMC3014770.