If you search for how many calories you need, the numbers vary dramatically.
The energy you need to fuel your entire day varies depending on age, weight, sex, muscle mass, metabolic rate, and activity.
But, if you’re trying to lose weight, you’ve probably heard that you have to be in a calorie deficit to lose weight (i.e., calories in < calories out).
However, it’s critical not to eat too few calories—especially for too long—that you compromise body function.
But what is this threshold?
This article will discuss the minimum calories you need to eat to survive. But we’ll also cover why hitting the bare minimum for long might not be optimal, especially if you’re not doing everything you can to get enough of all the critical nutrients you need from your food.
Why Do I Need Energy?
Your body is a complex system of cells that make up your various tissues like your muscle and vital organ.
Each organ has a specific function, like fluid balance, detoxification, waste removal, digestion, cognition, movement, breathing, hormone synthesis or pumping blood. You can liken this to the energy you need to power the various appliances in your home.
When you are not eating as much energy as your body requires to power all of its systems, it begins to burn some stored energy to fuel the difference. It’s like burning through your checking account and having to dip into your savings.
However, your energy bank is not limited to the fat you want to lose. Instead, it includes your metabolically active lean muscle mass, bone, and vital organs.
Because these tissues are the most metabolically expensive, your body looks at using them for fuel in times of energy scarcity, like a computer going into ‘power-save mode’. So, your body conserves resources by breaking down these metabolically expensive tissues and using them for fuel.
Your Basal Metabolic Rate
Your basal metabolic rate BMR (or resting metabolic rate) refers to the absolute bare minimum of energy your body requires to ‘cover all of its bases’ and run organs at a minimal output. It depends on your age, height, weight, muscle mass, and biological sex.
Your BMR does not include your activity. It is just enough to power your system when you lay in a bed, don’t move, and just breathe.
Your BMR is only one part of the calories you require to fully power your energy needs, as it does not include the fuel required to move and do activities!
When inputting all of your biometrics into any run-of-the-mill calorie counter, food tracker, or even our Optimising Nutrition Macro Calculator, your BMR is usually the first calorie calculation you see. Most bioimpedance scales will also estimate your BMR.
But it’s important to mention that this number is based on a range of assumptions and factors, so it should only be taken as a guess.
Your Daily Calorie Intake
In addition to the energy required to fuel your basal metabolic rate, you also need the energy to power your activities. This includes exercising, walking recreationally and going to the bathroom—doing chores, singing, talking, and even thinking. Without enough power, your body will dip into its energy stores, which are your tissues and organs. Most calorie calculators use a multiplier based on your assumed activity levels.
Aside from your activities, your daily calorie intake may change if you have an injury, are sick, or are under a different type of stress. This is because the metabolic rate tends to increase when someone is healing, and they require more energy.
BMR + Activity + Stress Factor = Total Calories
Your total daily calorie intake may vary significantly from someone who is your exact age, sex, weight, height, and age; it is dependent on your ‘stress’ levels and how much activity you do each day. Therefore, your total daily calorie requirements to power your activity today could be different than your total daily calorie requirements tomorrow.
The Problem with Only Eating Your BMR
If you are eating adequate calories—or if your calories in equal your calories out—your body is adequately fuelled, and your energy reserves are left in storage. If you aren’t eating adequate energy, your body takes it from its ‘savings’ account, which includes fat, muscle, organs, bone, and tissues. For more on the calories-in, calories-out discussion, check out Is Counting Calories and Energy Balance a Waste of Time?
When most people start dieting and restricting calories to lose weight, they often download the first calorie and fitness tracker they find, calculate the bare minimum of calories they need to get them to their weight loss goal (i.e., their BMR), and jump in head first, putting their faith in the theoretical calculations.
While this might result in weight loss initially—especially if someone has a lot of weight to lose or has been in a calorie surplus for some time—progress often doesn’t last.
While many believe they will only burn fat, this is not true. There is a limit to the amount of energy we can break down and pull from fat each day. This has been estimated at 69 calories per kilo of body fat per day.
So, the more fat you have, the bigger your deficit can be. But beyond that limit, your body is more prone to breaking down lean body mass (i.e., muscle), bone tissue, and even your organs. When this happens, your appetite kicks into overdrive to avoid further losing your precious lean mass. If you’re interested in learning more, we dive into this more in Secrets of the Nutrient-Dense Protein Sparing Modified Fast (PSMF) Diet.
So, if your deficit is too aggressive, you’ll be breaking down a lot more muscle and other critical tissues that keep your metabolic rate up.
If this happens for a day or two, the effects likely won’t be too noticeable or detrimental. But if you’re eating at a calorie deficit or the ‘bare minimum to keep your body alive’ for a prolonged period and not replenishing your energy and nutrient stores, your body will begin to go into ‘power-save’ mode.
Subsequently, organ function will decrease as your body attempts to save energy. Organ systems—like your reproductive system and thyroid—considered less essential for survival will decrease in function (i.e., output) to prioritise functions needed to keep you alive (i.e., stress hormone production).
Master-regulatory glands like the hypothalamus and pituitary—two parts of the brain that serve as the ‘control centres’ for almost all other major organs in the body—will also decrease in function. So, similar to a company facing budget cuts, the body will scale down its output.
This has multiple effects on the body. First and foremost, the hypothalamus and pituitary regulate critical organ systems like the adrenals, thyroid, and reproductive systems. Your adrenal, thyroid, and reproductive hormones affect your overall metabolic rate. Thus, if these organs decrease the amount of energy they are using, you will further decrease your BMR.
Although you may be eating less energy, your body is adapting to function at a lower calorie intake. Subsequently, someone who moves from eating at their BMR to an average amount of calories might find their weight rebounding.
As you continue to eat less and less energy, you are also forcing your body to break down the metabolically-active tissues within your body. Your muscles, organs, bone, and other tissues all burn a substantial number of calories at rest. Fat, on the other hand, does not.
So, when you force your body to break down its metabolically active energy reserves, you are essentially slowing your metabolism. While the scale might be going down (initially!), the minimum amount of energy you need is also dropping.
A decreased metabolic rate also lowers the output of various organ systems. As a result, circulation, immunity, cognition, detoxification, and just about any other biological activity may suffer in terms of function.
The Problem with Calorie Counting
Simple dieting—or calorie restriction—comes with some other caveats that most people learn the hard way.
When we talk about nutrition, there are three tiers:
- energy (calories),
- macronutrients (protein, fat, and carbs), and
- micronutrients (vitamins, minerals, essential fatty acids, and amino acids).
Unfortunately, most people simplistically focus on calories and don’t give much thought to the macronutrients and micronutrients in their food. We are told we can eat more low-calorie foods, which often aren’t packed with all the nutrients we need. But nutrients, particularly protein, are really the key to the whole puzzle of maximising satiety and ensuring your body continues to thrive, even when you’re eating less!
Rather than merely worrying about how little food you can survive on; you need to think about how you can pack more of the nutrients you need in with less energy.
Nutrients May Be the Key to Long-Term Fat Loss
First and foremost, protein and the essential micronutrients are critical for just about every biochemical reaction in your body. They also comprise all of your tissues (i.e., muscles) and organs.
We also need minerals and vitamins to power our metabolism and build muscle, which keeps us burning more fuel at rest. The essential nutrients are also critical to support our detoxification and immunity. Minerals like potassium and sodium are critical to moving energy around our body, while the vitamins enable our mitochondria to oxidise the energy in our cells.
Nutrients are also critical for satiety. Like a dog that cries out for food or water when it’s hungry, your appetite revolts against you when you’re not giving it what it needs by motivating you to seek out food. While it needs energy, it needs nutrient-dense energy to power all its metabolic activities!
When you are eating low-energy, low-nutrient-density foods on a calorie deficit, it makes feeling full on fewer calories much more challenging. Not only do you experience poor satiety, but you are also starving your metabolism, and your body is craving the nutrients it needs to thrive and thus, your appetite increases.
So, how can you lose weight and diet without failing?
Getting Enough Essential Nutrients When Dieting
There are probably enough ‘miracle diets’ to create an encyclopedia. But unfortunately, while many work in the short term, these ‘crash diets’ rarely yield meaningful long-term progress.
But people have lost weight and kept it off long term. What’s their secret?
It’s not simply about eating fewer calories. You need to change what you eat. Finding a balance between nutrients and energy is the key to long-term, easy-to-maintain weight loss.
When looking at the vitamins, minerals, amino acids, and fatty acids in your food, it’s critical to consume nutrient-dense foods that provide all the essential nutrients with less energy.
While some low-energy density foods—like vegetables—are good for bulk, consuming them as the majority of your diet will leave you full in the short-term but hungry not long after. Instead, increasing your overall per cent of total calories from protein, or your protein per cent (protein %), will provide you with the high-satiety protein you need to feel fuller on fewer calories.
Eating complete protein from whole-food sources like meat, seafood, and dairy will give you the raw ingredients required to build metabolically-active lean body mass. Combining protein with fibre will improve your satiety even further, as foods that naturally contain fibre, like green and leafy vegetables, will provide you with even more nutrients.
Our analysis and Simpson and Raubenheimer’s work on the Protein Leverage Hypothesis show that protein is the most satiating macronutrient.
Increasing satiety is a process of dialling up your protein and fibre intake while scaling back your carb and fat intake until you start seeing a sustainable rate of progress. This is the process guide our Optimisers through in our four-week Macros Masterclass.
To help Optimisers get the nutrients they need with less energy, we created this chart that shows nutrients (per serving) vs protein (%). The colouring is based on our Satiety Index Score.
- Foods at the top of this chart will give you more protein and less energy.
- Foods towards the right of this chart will give you plenty of all the essential nutrients in the foods that make up the foundation of your diet.
- Foods shown in green will help you feel full with fewer calories than those in red.
To dive into the detail, you can check out the interactive Tabluea version of this chart here. You will also find separate charts for plant-based foods, animal-based foods and seafood.
You can also download a wide range of food lists tailored for various goals and preferences, including maximum satiety and nutrient density.
We have also analysed over a thousand recipes to help Optimisers with the meals that provide the nutrients they need. The chart shows our NutriBooster recipes in terms of diet quality score vs protein (%). Again, the colouring is based on our Satiety Score.
You can dive into the interactive Tabluea version of this chart here or learn more about our suite of NutriBooster recipe books here.
So, How Many Calories Do I Need?
As we’ve highlighted above, in your enthusiasm, it’s ideal if you don’t avoid losing too much weight too quickly. But unfortunately, when you lose too much weight too quickly, your body perceives starvation. While it might be self-inflicted, your primitive instincts don’t know the difference. As a result, you might experience binge episodes and uncontrolled thoughts about food. For more details, see How Much Weight Should I Lose Per Week?
While you might want to get the weight off as soon as possible, it’s often not optimal for long-term progress. Hence, we recommend a weekly weight loss rate of 0.5-1% of starting body weight. This is a critical principle we emphasise in all our Masterclasses.
In the end, the best way to determine how many calories you require is to track your typical intake for a week using a calorie-tracking app like Cronometer.
In our Macros Masterclass, Nutrient Optimiser provides weekly guidance to dial back their energy from carbs and/or fat if they’re not seeing at least 0.5% weight loss per week. However, if they’re losing more than 1.0% per week, Nutrient Optimiser will add back a few calories to your goal for the coming week to prevent excessive hunger and rebound binging.
If you’re not interested in tracking your food, you can also try our Data-Driven Fasting app. Here, we walk our Optimisers through using their blood glucose as a fuel gauge to determine the best time to refuel and what to eat based on their blood glucose. In a short time, many can lose a considerable amount of body weight, lower their blood glucose and improve their metabolic health.
Finally, it’s important to remember that you’ll have excessive cravings over the long term without nutrients. Hence, prioritising the most nutrient-dense foods per calorie will make the weight loss process much easier.
To increase your micronutrient intake to ensure you’re filling in all your gaps, you might check out our four-week Micros Masterclass. Here, we walk our Optimisers through determining the nutrients they’re currently not getting enough of and which ones they could scale back on. From there, we work on incorporating foods that contain more of the ones they need and vice versa.
- Your basal metabolic rate (BMR) or resting metabolic rate (RMR) is the number of calories your body requires to fuel its baseline activities (i.e., breathing, organ function, heart rate, etc.) at complete rest. It does not include the calories required to fuel your daily activities like exercise, walking, or chores.
- Your daily calorie requirements refer to the fuel required for your BMR, activity output and stress levels (i.e., healing or illness). Thus, this is the number of calories that will usually allow you to maintain your weight, providing you with as many calories as you’re burning.
- Eating less than your daily calorie requirements will allow you to lose weight. Consuming your BMR or less than your BMR will often result in rapid weight loss, as it will create a massive calorie deficit—especially if you’re active!
- Consuming at or below your BMR for a prolonged period can have adverse metabolic consequences. As your body is pushed to dive into its energy stores to fuel your day-to-day activities, it may diminish your metabolic rate when it forces your body to break down lean mass for fuel. Additionally, it can create stress on organ systems that regulate metabolism.
- Focusing on protein and nutrients is critical to losing weight, keeping it off over the long term, and preserving your lean mass. This will allow you to feel full on fewer calories while maintaining your muscle to optimise satiety and give your body the nutrients it needs to focus on releasing fat for energy.
- The Most Nutrient-Dense Foods per Calorie – Tailored to Your Goals and Preferences
- Is Counting Calories and Caloric Balance a Waste of Time?
- How to Maximise Satiety Per Calorie
- Secrets of the Nutrient-Dense Protein Sparing Modified Fast (PSMF) Diet
- Nutrient-Dense Meals and Recipes
- What Are Macros in Your Diet (and How to Manage Them)?
1 thought on “The Surprising Science Behind Thriving with Less Food”
Hi Marty, very insightful analysis on the boundaries of energy conversion. This deserves much more study. In the case you are presenting, people are reaching a depletion state by removing ingested energy. I think this needs to be calibrated against the depletion state from repeat long distance runners. There is a theoretical work limit on how far in distance an average human can traverse over a weeks long or months long effort. I believe the distance was roughly 50 miles per day.
I think was is missing is how the body defines “essential” tissues. Obviously lean muscle mass will not be consumed if it is needed the next day. One would assume that protein synthesis is prioritized over energy conversion. This may be a simple mass balance where muscle cells are pulling excess amino acids from the blood, thereby leaving less available for energy conversion. If someone is at their BMR day after day, the muscle cells are not extracting amino acids so these “excess” amino acids in the blood get used for energy.