What Happens If You Eat Too Few Calories While Dieting?
- Coach Alan
- Dec 18, 2025
- 5 min read
Updated: 4 days ago
Introduction
In the pursuit of a healthier body, many people begin dieting by aggressively cutting calories. While creating a calorie deficit is necessary for weight loss, there is a point where eating too little can backfire.
Consuming too few calories can lead to physical, hormonal, and mental health challenges that ultimately make fat loss harder rather than easier. Understanding where that line is, and how to diet sustainably, is key to long-term results.
Quick Answer: What Happens If You Eat Too Few Calories?
Eating too few calories while dieting can slow your metabolism, reduce energy levels, increase muscle loss, and negatively impact mood, hormones, and overall health. While a calorie deficit is required for weight loss, chronic under-eating can stall progress and lead to fatigue, burnout, and rebound weight gain rather than sustainable results (Rosenbaum & Leibel, 2010; Redman et al., 2018).
Understanding Caloric Needs
Calories are units of energy that the body uses to function. They fuel everything from breathing and digestion to movement, concentration, and recovery.
Daily calorie needs depend on factors such as age, body size, muscle mass, activity level, and lifestyle demands. One of the most common mistakes I see as a coach is people assuming that lower calories always mean faster progress.
In reality, consistently eating below what your body needs to function well often leads to poorer results over time. Estimating your basal metabolic rate (BMR), Total Daily Energy Expenditure (TDEE) and factoring in activity levels can provide a more realistic starting point. Tools such as the Harris-Benedict Equation, or guidance from a qualified professional, can help refine this further.
What Happens If You Eat Too Few Calories While Dieting?
When calorie intake drops too low, the body may perceive this as a threat to survival and adapt accordingly. One of the primary adaptations is a reduction in energy expenditure, often referred to as adaptive thermogenesis (Rosenbaum & Leibel, 2010).
Short-term effects of severe calorie restriction can include:
Low energy and fatigue
Irritability and mood changes
Poor concentration
Disrupted sleep
Reduced training performance
Over time, chronic under-eating can contribute to:
Loss of muscle mass (Weiss et al., 2007)
Hormonal disruption and appetite dysregulation (Dulloo et al., 2011)
Nutrient deficiencies
Increased burnout and difficulty maintaining weight loss
Rather than accelerating progress, extreme restriction often leads to plateaus and frustration.
If you’ve realised you might be under-eating, the next step is finding a balanced way to increase your calorie intake without unwanted weight gain. Check out our guide on how to increase calories without gaining weight for practical steps.
The Role of Nutrients in a Healthy Diet
Calories alone do not tell the full story. The quality of those calories matters just as much.
Macronutrients such as protein, carbohydrates, and fats support muscle maintenance, hormone function, and energy levels. Micronutrients including vitamins and minerals are essential for immune function, bone health, and cognitive performance.
When calories are cut too aggressively, nutrient intake often suffers. Cheaper, "empty" calorie processed foods are often associated with poorer micronutrient adequacy, which can compromise health and recovery over time (Drewnowski & Specter, 2004).
Effects on Physical and Mental Health
A common belief is that pushing harder and eating less will speed up results. In practice, this often leads to the opposite.
Physically, prolonged low energy intake can cause persistent fatigue, frequent illness, poor recovery from exercise, and loss of strength.
Mentally, calorie restriction is strongly associated with irritability, anxiety, low mood, and food obsession, as demonstrated in both classic and modern research on dietary restraint (Keys et al., 1950; Polivy & Herman, 2002).
When both physical and mental resources are depleted, adherence becomes harder and the likelihood of abandoning the diet increases.
How to Ensure a Healthy Calorie Intake While Dieting
A healthy approach to calorie control focuses on consistency rather than extremes.
This includes:
Prioritising nutrient-dense foods such as lean proteins, vegetables, fruits, whole grains, and healthy fats
Eating enough to support training, recovery, and daily energy needs
Paying attention to hunger, fullness, mood, and performance
Setting realistic goals that can be maintained beyond a short dieting phase
In my coaching work, long-term success comes from creating a structure that fits around real life rather than relying on willpower and restriction.
When Extra Support Can Help
If dieting has left you feeling drained, stuck, or frustrated, it may be a sign that your approach needs adjusting rather than tightening further.
Sometimes a short reset focused on nourishment, structure, and consistency can help restore energy and clarity before pursuing fat loss again.
Try the 10-Day Reset Trial
If you want to step away from extreme restriction and rebuild energy, confidence, and consistency around food, the 10-Day Reset Trial is designed to help you do exactly that.
The reset focuses on:
Eating enough to support energy and training
Simple, balanced meals without calorie obsession
Breaking the all-or-nothing dieting cycle
Laying the groundwork for sustainable fat loss
It is ideal if you feel run down, stuck in plateaus, or unsure how much you should really be eating.
Conclusion
Eating too few calories while dieting can undermine both physical and mental health, slow progress, and make weight loss harder to sustain. A successful approach prioritises balance, nourishment, and long-term consistency rather than extreme restriction.
By respecting your body’s needs and using structure rather than deprivation, weight loss becomes more achievable and far more maintainable.
About the Author
Coach Alan is a qualified ITEC Level 3 Personal Trainer with over 9 years of coaching experience, and the founder of Mind-Body Training, where he works as an online personal trainer in Ireland to help clients achieve sustainable fat loss and long-term behaviour change. He is also a psychotherapist-in-training with the Irish Institute of Counselling and Psychotherapy (IICP). His coaching approach is informed by evidence-based principles from psychology, nutrition, and exercise science, with a strong focus on mindful habit formation and realistic lifestyle change. You can learn more about Coach Alan here.
Mind-Body Training provides coaching, education, and personal training services, not personal therapy or clinical counselling. Clients seeking therapeutic support are encouraged to work alongside a different qualified mental health professional where appropriate.
References
Drewnowski, A., & Specter, S. E. (2004). Poverty and obesity: the role of energy density and energy costs. American Journal of Clinical Nutrition, 79(1), 6–16.
Dulloo, A. G., Jacquet, J., Solinas, G., Montani, J. P. (2011). How dieting makes some fatter: from a perspective of human body composition autoregulation. Proceedings of the Nutrition Society, 70(3), 293–306.
Keys, A., Brozek, J., Henschel, A., Mickelsen, O., Taylor, H. L. (1950). The Biology of Human Starvation. University of Minnesota Press.
Polivy, J., & Herman, C. P. (2002). Causes of eating disorders. Annual Review of Psychology, 53, 187–213.
Redman, L. M., Heilbronn, L. K., Martin, C. K., et al. (2018). Metabolic slowing with sustained weight loss is associated with changes in circulating leptin. Obesity, 26(9), 1489–1497.
Rosenbaum, M., & Leibel, R. L. (2010). Adaptive thermogenesis in humans. International Journal of Obesity, 34(Suppl 1), S47–S55.
Weiss, E. P., Racette, S. B., Villareal, D. T., Fontana, L., Steger-May, K., Schechtman, K. B., Klein, S., & Ehsani, A. A. (2007). Lower extremity muscle size and strength and aerobic capacity decrease with caloric restriction but not with exercise-induced weight loss. Journal of Applied Physiology, 102(2), 634–640.
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