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You may have seen this trendy eating pattern online, especially for women, where a woman is claiming to eat more than 3000 calories per day, and sometimes, also losing weight. Is this possible? Is 1500 calories per day not enough for even basic bodily functions, like producing hormones and maintaining muscle mass? We’ll get into the research below to see how many calories you really need, and if it truly is possible to consume thousands of calories per day while still losing weight. Please keep in mind, I am writing in reference to recreational, every day athletes (not performance grade) and mostly geared towards women.
How many calories do i need for basic body functions?
Your body needs calories to perform everyday survival functions. Your brain, heart, and breathing throughout the day all contribute to energy usage, and therefore caloric needs, in order to function. Typically, a resting metabolic rate (RMR) calculation will tell you how much is absolute minimum caloric intake in order to exist. This can range from 1000 (in theory, possibly even lower for an elderly lady <5 feet tall) to >2500 calories depending on your age, height, weight, lean body mass, chronic or acute conditions. Add in exercise, and you’ll need to account for more energy expenditure.
I’ve seen a calorie minimum for daily function. What is that about?
Well… I have seen this too. But when I worked at a hospital, and have done research online through scientific databases, the equations for calculating resting metabolic rate (RMR) sometimes come up with <1200 calories per day. (which is likely the number of minimum calories you’ve seen a women needs to eat in a day). In my experience, the lowest RMR I have every calculated was around 980kcal (calories just to exist).
I’ve scoured the internet, and found sources that are NOT cited with ANY evidence. I have found nothing concrete, black and white, in the research, that says explicitly “every single woman on the planet must consume 1200kcal per day in order to survive” with a WHY behind it. I have seen recommendations for different calculations, some of which could yield <1200kcal per day for a small women (12) at rest (RMR). Add in activity factors, thermic effect of food, non-activity thermogenesis, and likely that RMR will be above 1200kcal/day for most women.
There is a lot to do with genetics, and a lot we don’t know about genetics and the influence it has on our metabolism and energy requirements. More information is coming out on this every day.
Indirect calorimetry is the gold standard for determining true RMR (18). In this study (17), the researchers found a RMR of 736kcal/day! That is low, and it is worth noting that this was seen in a women who’s BMI <18.5kg/m2, which is considered underweight.
Indirect calorimetry calculates the heat you produce while resting to determine RMR. It is expensive, and generally cumbersome on the individual to complete as you have to be in a chamber to measure the exchange of respiratory gases.
Are there any benefits to a strict/intense calorie reduction?
Yes! But, it is important to keep in mind that restricting calories intensely is done for only a short period of time, though there are long term benefits (if “normal” calorie re-introduction is done properly). This is a huge topic that deserves more discussion than what I will mention here.
The DiRECT study results (19) showed that 46% of their study participants lost weight and went into type 2 diabetes remission. It is worth noting that these individuals had to follow a ~800kcal diet for 3-5 months and SLOWLY re-introduce calories over another 2 months, then never go back to how many calories they were eating before the study in order to maintain diabetes remission. I read this book by Dr. Roy Taylor who has done incredible and ground breaking research with type 2 diabetes. It’s an interesting read on reversing type 2 diabetes!
Calorie restriction has also been shown to improve lifespan (20), and reduce the risk of age related cardiovascular and metabolic disease (21). If you believe a calorie restriction would benefit you, seek the help of a medical professional and registered dietitian so you can still achieve all micronutrient minimums, and be monitored for signs of malnutrition.
how many calories do I need for exercise?
Exercise demands more energy than baseline body functions since you are doing more and expending more energy.
RMR eats up the most calories per day (14). If you look at metabolic equivalents (METs), which is defined as the amount of oxygen consumed sitting at rest (typically 3.5ml/kg/min) (24), then you can estimate the cost of energy expended. That could be gardening, driving, praying, walking, and much more. Take a look at the Compendium of Physical Activity to see how many METs certain activities utilise. How do you use METs in a caloric equation? You use it to determine how many calories you burn doing an activity.
METs * 3.5 * body weight in kg / 200 = kcal/min.
You just determined how many calories you burn while doing an activity!
There are other equations that can be used as well to determine caloric need, like the Harris-Benedict, Mifflin St Jeor, and the Muller. With some of these equations, you add in an activity factor to account for exercise.
It’s been posited from the International Society of Sports Nutrition that male marines need up to 7000kcal per day, and female military war fighters need 2300kcal per day (accounting for sleep, changes in altitude, weight of typical carry loads, stress, long periods of intense physical demands, etc) (22).
Would I advise every one of my clients to eat 3000 calories per day? No. It depends on your height, age, how much time you actually spend lifting weights, load of the weight, and how often you would be lifting weights. Conversely, if you run marathons or do any other endurance type of sport that requires many hours/kilometres then you may also need close to (women) or above 3000 calories (ultra marathon women runners) (23); BTW, the number of calories required also depends on the speed at which on is running. It is highly dependent on the person!
Does this mean you should go out and eat high calorie foods like burgers and pizza? No. You want to prioritize nutrient dense foods. If you are a professional athlete and have very high calorie requirements, then maybe you have more leeway when it comes to “junk foods”, but those kinds of foods typically don’t make you feel your best in order to perform at your best.
Based on the sports nutrition books I have read, anywhere from 30-50kcal/kg/day is adequate to either maintain weight or increase body mass, depending on your goals, personal physical factors, and type of physical activity. I’ve seen calculations use actual body weight and lean body mass weight.
How many calories should I consume for weight loss?
You have probably heard “calories in = calories out”. As a registered dietitian, I agree with that statement… mostly. While it’s true that on a super basic level, if you reduce your calories, you’ll lose weight, and if you increase your calories, you’ll gain weight. I have also seen the quality (13) of the calories (or food) significantly influence how my clients lose/gain/plateau with weight.
Basal metabolic rate slows in response to calorie restriction or starvation (1, 11, 15). This is a way the body conserves energy knows as adaptive reduction in thermogenesis (16). You lose weight, you lose some metabolically active tissue, so your metabolism goes down a bit. Some studies show, this attributes to resistance of fat loss as subjects lose more weight during calorie restriction (2), sometimes even lowering your metabolism even more (16), in an attempt to restore your “baseline” body weight. Ever have a plateau in your weight loss journey? This may be the reason why.
When individuals want to lose weight, a high protein diet is generally recommended in the setting of a caloric deficit. This is because it’s been shown that increased protein intakes (7) can increase satiety (3), take more energy to metabolise in the body (6), which can lead to weight maintenance after weight loss (5), and in some instances, prevent increased weight re-gain.
So, if you want to lose weight, you need to create some kind of calorie deficit, whether that is in the form of restricting calorie intake, or burning more calories than consumed, or a combination of both. Generally, I’ll look at and calculate a client’s basal metabolic rate, total daily energy expenditure, and factor in any activity factors (which can change daily!), then add in a 200-600 calorie deficit, depending on my client’s goals, personality, lifestyle, and nutrition + health history.
Part of this calculation is height, weight, and age. So, no one calorie program is going to fit every single person. It just depends on the person and what their lifestyle looks like!
Typically, I’ve seen from the sports nutrition books and webinars I have read and attended, 30kcal/kg/day used to determine a good starting point for minimum weight maintenance, and possibly loss for some people, depending on your type of exercise. I’ve seen actual body weight, lean body mass weight, and ideal body weight used.
What about 3000 calories?
I’ve written before about how increased protein intake (and therefore increased calorie intake) can result in loss of fat mass (or even no change at all in body composition), and this has been seen in women who participated in resistance training as well (10) when compared to a “normal” protein intake while resistance training.
So when you see these trending ladies on social media touting 3000 calories a day and losing weight because they are in a deficit somehow, it COULD be possible, especially with higher quality protein intakes (and therefore more calories). ***
There is a fine line to restricting calories too much, or over exercising. Both can lead to disordered eating patterns, or an eating disorder. If you think you’re suffering from or living with either of these, it is important for you to reach out to a medical professional for help in restoring a healthy relationship to food and exercise.
a note on hormonal homeostasis for women
In the setting of metabolic stress, low energy availability, nutrient deficiencies, and excessive physical activity, a women’s period can disappear; termed amenorrhea (8). This happens due to the body prioritizing where it wants to spend its energy – on vital, survival functions like keeping your heart going, your respiration continuing. Estrogen production is reduced, no ovulation occurs, and progesterone is absent, and this whole cascade of hormonal deregulation can disrupt so many body processes, one of which is putting the individual at risk of fractures due to the relationship of estrogen on bone strength (9).
When determining calorie requirements to return to having a regular cycle, it is dependent on lean body mass (or fat free mass), the type of exercise being performed, and lifestyle factors of the individual. Though there are calculators online, it is most helpful to speak with a registered dietitian nutritionist to create custom goals.
references:
(1) Dulloo, Abdul G. “Physiology of weight regain: Lessons from the classic Minnesota Starvation Experiment on human body composition regulation.” Obesity Reviews 22 (2021): e13189.
(2) Tremblay, Angelo, and Jean-Philippe Chaput. “Adaptive reduction in thermogenesis and resistance to lose fat in obese men.” British journal of nutrition 102.4 (2009): 488-492.
(3) Morell, Pere, and Susana Fiszman. “Revisiting the role of protein-induced satiation and satiety.” Food Hydrocolloids 68 (2017): 199-210.
(5) Drummen, Mathijs, et al. “High compared with moderate protein intake reduces adaptive thermogenesis and induces a negative energy balance during long-term weight-loss maintenance in participants with prediabetes in the postobese state: a PREVIEW study.” The Journal of nutrition 150.3 (2020): 458-463.
(6) Bier, Dennis M. “The energy costs of protein metabolism: lean and mean on Uncle Sam’s team.” The role of protein and amino acids in sustaining and enhancing performance (1999): 109-119.
(7) Drummen, Mathijs, et al. “Dietary protein and energy balance in relation to obesity and co-morbidities.” Frontiers in endocrinology (2018): 443.
(8) Ryterska, Karina, Agnieszka Kordek, and Patrycja Załęska. “Has Menstruation Disappeared? Functional Hypothalamic Amenorrhea—What Is This Story about?.” Nutrients 13.8 (2021): 2827.
(9) Gordon, Catherine M., et al. “Functional hypothalamic amenorrhea: an endocrine society clinical practice guideline.” The Journal of Clinical Endocrinology & Metabolism 102.5 (2017): 1413-1439.
(10) Antonio, Jose, et al. “A high protein diet (3.4 g/kg/d) combined with a heavy resistance training program improves body composition in healthy trained men and women–a follow-up investigation.” Journal of the International Society of Sports Nutrition 12.1 (2015): 39.
(11) Most, Jasper, and Leanne Maree Redman. “Impact of calorie restriction on energy metabolism in humans.” Experimental gerontology 133 (2020): 110875.
(12) National Research Council. “Recommended dietary allowances.” (1989).
(13) Osilla, Eva V., Anthony O. Safadi, and Sandeep Sharma. “Calories.” StatPearls [Internet]. StatPearls Publishing, 2021.
(14) Trexler, Eric T., Abbie E. Smith-Ryan, and Layne E. Norton. “Metabolic adaptation to weight loss: implications for the athlete.” Journal of the International Society of Sports Nutrition 11.1 (2014): 7.
(15) Ravussin, Eric, et al. “Energy expenditure before and during energy restriction in obese patients.” The American journal of clinical nutrition 41.4 (1985): 753-759.
(16) Doucet, Eric, et al. “Evidence for the existence of adaptive thermogenesis during weight loss.” British Journal of Nutrition 85.6 (2001): 715-723.
(17) Aliasgharzadeh, Soghra, et al. “Comparison of indirect calorimetry and predictive equations in estimating resting metabolic rate in underweight females.” Iranian journal of public health 44.6 (2015): 822.
(18) Gupta, Riddhi Das, et al. “Indirect calorimetry: from bench to bedside.” Indian journal of endocrinology and metabolism 21.4 (2017): 594.
(19) Lean, Michael EJ, et al. “Primary care-led weight management for remission of type 2 diabetes (DiRECT): an open-label, cluster-randomised trial.” The Lancet 391.10120 (2018): 541-551.
(20) Flanagan, Emily W., et al. “Calorie restriction and aging in humans.” Annual Review of Nutrition 40 (2020): 105-133.
(21) Ravussin, Eric, et al. “A 2-year randomized controlled trial of human caloric restriction: feasibility and effects on predictors of health span and longevity.” The Journals of Gerontology: Series A 70.9 (2015): 1097-1104.
(22) Gonzalez, Drew E., et al. “International society of sports nutrition position stand: tactical athlete nutrition.” Journal of the International Society of Sports Nutrition 19.1 (2022): 267-315.
(23) Tiller, Nicholas B., et al. “International Society of Sports Nutrition Position Stand: Nutritional considerations for single-stage ultra-marathon training and racing.” Journal of the International Society of Sports Nutrition 16.1 (2019): 50.
(24) Jetté, Maurice, Ken Sidney, and G. Blümchen. “Metabolic equivalents (METS) in exercise testing, exercise prescription, and evaluation of functional capacity.” Clinical cardiology 13.8 (1990): 555-565.
