Food is fuel. While quality and quantity matter, quantity is the most straightforward. This article focuses on the symptoms of not eating enough calories in a day.
Typically, the most obvious sign that you’re undereating is difficulty losing weight.
Another common symptom of not eating enough is feeling miserable. It could be mild, such as drowsiness. Or more serious, such as headaches or nosebleeds.
Here’s how you know if you’re experiencing symptoms of not eating enough and what to do to correct the course for healthy weight loss.
what are the symptoms of not eating enough
Calories in, calories out: the general formula for weight loss we’re all familiar with.
The problem is, we see folks every week who undereat because they want the weight loss to happen faster. We get it, but this is not the route to go.
Outside of potential malnutrition, this method may actually result in weight gain or at least a one-way ticket to the weight loss plateau.
Yes, you read that correctly: if you eat too little, you will not lose weight.
How many calories should you be eating?
If there’s one universal law of weight loss, it’s this: it takes 3,500 calories to lose 1 pound.
It can be tempting to reduce your calorie intake to wipe out those 3,500 calories faster, but there are a few important numbers to keep in mind:
1. Calculate your BMR (basal metabolic rate) and never consume fewer calories. Your BMR is basically how many calories your body needs a day to sustain life. If you eat less than your BMR, your metabolism will plunge and your body will hold onto weight and store fat.
2. A rule of thumb is that women should never consume less than 1200 calories a day and men 1600 calories. Even if you’re extremely petite, always follow this law.
Use this helpful calculator to calculate both your basal metabolic rate and daily caloric intake based on your age, sex, exercise level, height, and weight.
If you’re looking to lose weight, don’t automatically jump straight to the 1200-calorie (for women) and 1600-calorie (for men) threshold.
Depending on your starting weight and stature, this may still be too little and you’re at risk of undereating. We promise you that this isn’t doing your body any favors, nor will it propel your weight loss success.
Note: if you’re severely or chronically undereating, visit a doctor to rule out dangerous eating disorders such as anorexia.
Finally, there’s a good chance you have unrealistic expectations about how long it should take you to lose weight. Have you ever thought, “Why am I not losing weight faster?”
If yes, we highly recommend you read our in-depth article (with graphs and charts) titled, ‘How Long Does it Take to Lose 20 Pounds The Healthy Way?‘
Here’s what happens when you eat too little.
What Are the Symptoms of Not Eating Enough?
One of my favorite analogies is you’d never underwater a flower expecting it to grow big and strong. You need calories to build your best and healthiest body.
How can you know if you’ve hit the sweet spot for calorie consumption or if you aren’t eating enough?
Whether you’re potentially undereating due to a weight loss goal, a busy schedule with no time to cook, a restrictive diet, or an aggressive fasting routine (an intermittent or extended fast), here are the side effects of not eating enough.
1. Not Losing Weight
If you’ve been consistently undereating but not losing any weight — or even gaining weight — you may not be eating enough.
As we share in our article on the stubborn weight plateau to lose weight healthily and sustainably, you need to eat enough food.
Sure, calorie-restriction fad diets may help you lose weight fast, but it’s not sustainable. Plus, the weight you lose right off the bat is just water weight — not fat.
Quickly, calorie restriction will create an imbalance of hormones, with your body retreating into “starvation mode.” Your metabolism will slow down to match your low-calorie intake and your body will begin hoarding fat.
How quick is quick? The National Institutes of Health (NIH) ran a study and found that molecular behavior in muscles and insulin resistance can change as soon as five days after a new diet is introduced.
What happens to your body if you don’t eat enough? At least 99 things and weight loss is not one.
2. Feeling Tired
Generally, one of the first symptoms that lack of food causes is fatigue. If you’re consuming less than your basal metabolic rate, your body won’t be able to function optimally and you’ll constantly feel tired.
Studies show that calorie restriction strongly impacts physical fitness and sports performance, but it’s more than that. Mental fatigue, aka brain fog, is also a symptom of eating too few calories.
If you’re feeling lethargic and unfocused, both physically and mentally, you may not be eating enough calories to provide adequate energy throughout the day. Also, if you’re feeling fatigued, make sure you’re drinking enough water.
3. Getting Sick Frequently
Have you caught the cold more times this year than before? Has it lasted longer than it should? You may want to bump up your calorie intake.
A weakened immune system can happen if you’re not consuming enough vitamins and minerals. There is more and more evidence that your gut health is directly related to immunity, and feeding your gut microbiome with an array of nutritional foods is the best way to keep it happy and healthy.
Study after study has displayed this as well. By adding a nutritional shake, one study surveying seniors in a nursing home found that they had fewer fevers, incidences of the flu, and decreased antibiotic use.
What we love about this method is that making a shake is so simple, fast, and nutritionally dense. Here’s a list of our favorite meal replacement shakes, which we view as our “nutritional insurance policy” for this exact reason.
4. Always Cold
Cold hands and feet, anyone?
If your body doesn’t have enough calories, it won’t have the energy to maintain a healthy body temperature.
A study out of the Washington University School of Medicine found that participants who restricted calories had a significantly lower core body temperature (nearly 1 degree Fahrenheit).
While 1 degree may not seem like a lot, it is pretty significant as far as body temperature goes.
If you always feel a bit chilly, this may be the reason.
5. Sleeping Problems
When you undereat, you spend less time in deep sleep.
Deep sleep is known in the sleep cycle as non-REM sleep. Without it, you wouldn’t feel refreshed when you wake up.
It is during this time that your pituitary gland secretes human growth hormone, essential for building muscle, and when your cells regenerate.
Undereating affects the quality of your sleep. Not only are you in deep sleep for a shorter amount of time, but it could also take you longer to fall asleep in the first place. This is, once again, linked to the lack of nutrients when you eat too little, primarily calcium and magnesium.
As a couple who has suffered from sleep deprivation before (due to burnout, not necessarily calorie deprivation), we recommend that you avoid it at all costs. It can really mess you up and gave Ryan an array of bizarre symptoms from allergic reactions to vertigo.
We’ve all experienced hanger, but undereating brings it to the next level.
While it generally takes about 3 weeks for your body to adjust to a new eating routine, like intermittent fasting, it’s a bumpy road ahead.
Irritability is linked to low blood sugar, and once more, a lack of essential nutrients that balance your mood. If you’re easily enraged, touchy, or depressed, you may not be eating enough.
Depending on how much you are undereating, your mood can further suffer from panic attacks, obsessive-compulsive behavior, and withdrawal, according to the National Centre of Eating Disorders.
Next time you’re feeling cranky, take note of how long it has been since you’ve eaten last, and grab a snack.
7. Hair, Nail, and Skin Problems
Several symptoms of not eating enough are hair loss, brittle nails, and bad skin.
Without enough nutrients, particularly Vitamin E, zinc, biotin, and protein, our hair, nails, and skin are more susceptible to inflammation and UV damage.
If your appearance lacks luster and looks drab, your body may be telling you that it needs more calories and better nutrition.
8. Reproductive Health
If you’re trying to get pregnant — and even if you’re not — avoid undereating at all costs.
Not eating enough negatively impacts your hormonal balance. From ghrelin the “hunger hormone” to cortisol the “stress hormone,” staying within your body’s starvation mode is not a good place to be.
In that state, your body focuses on maintaining critical functions, such as breathing, while letting things like sexual desire fall by the wayside. One additional symptom of not eating enough is that your body may skip a menstrual cycle.
A Johns Hopkins School of Medicine study outlines the fine line between healthy intermittent fasting and calorie restriction. If done to an extreme, you may experience reproductive difficulties.
No matter whether or not you’re interested in your reproductive health, this has far-reaching consequences for your body’s hormones and health.
What to do if you have symptoms of not eating enough?
If you suspect that you’re undereating, the good news is that it’s something that can be easily fixed.
As you learned above, it’s not solely about the calories you eat, but the quality of calories. Underconsuming nutrients, vitamins, and minerals have far-reaching effects, from reduced sleep quality to getting sick more often to weight gain.
You can begin by double-checking how many calories you eat a day, such as by logging your meals in a free app such as MyFitnessPal.
Make sure you eat enough calories to cover your basal metabolic rate and daily caloric intake needs, found with this calculator.
If you need to bump up how much you eat, or the quality of your calories, our best resource is the 30-Day Weight Loss Challenge (see the form below). Sign up and we can help you recover from not eating enough.
Are you not eating enough calories but not hungry? The transition to eating more calories in a day needs to happen gradually.
Here are a few more resources to help you do so.
30-Day Weight Loss Challenge
As engineers with a combined twelve years of health coaching experience, we needed to create a data-driven way for our clients to sustain weight loss.
Too many weight loss challenges involve a long list of what you can, cannot, and need to do every day. We’ve reduced the overwhelm and only require you to track five numbers a day — calories, steps, fiber, sleep, and waistline.
You’ll track five numbers daily to give you the highest return for sustainable weight loss. As a bonus, we’ll share our Weight Loss Bundle, which includes strategies, progress tracking tools, and additional weight loss plans.
Hey we're Ryan and Alex
The creators of Ryan and Alex Duo Life. We are a husband-wife duo and “lifestyle engineers.”
After eight years working in the corporate world as engineers, we left our high-powered jobs to tackle our true passion — helping couples engineer their best lives.
The synergy of our engineering minds and ten years of health coaching experience produced Ryan and Alex Duo Life. Our mission is to help you transform your bodies, minds, and relationship as a couple.
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