Effects of Undereating

Photo of a white plate with a tiny bit of rice and one shrimp at the bottom of the plate and a small stack of 2 cucumber and 2 tomato slices at the top of the plate. There is a fork and spoon on the plate sitting at an angle on the left and right, respectively.

Yep, you read that right.  Despite the word “diet” being in my title, I have no interest in putting everyone on a diet.  In this post, I want to focus on the risks of undereating, not overeating.

I work with people all the time who want to lose weight, but when I start digging into their habits, it’s clear that they are eating way less than their body needs just to maintain basic functions.

Fad diets, social media influencers, and even medical professionals are constantly sending the message that you have to eat less to lose weight.  The truth, though, is that eating too little over extended periods of time could be doing more harm than good.

There are even more health consequences of undereating for women than for our male counterparts, and when you add in regular exercise, the potential for falling short of your needs is much higher.

For most active adults, one of the biggest reasons for athletic training and exercise is for health.  In this article, I’ll break down how undereating can undermine your healthy habits and what signs to look for that you might need to rethink your nutrition strategy.

What do you mean by undereating?

The answer to this question isn’t as simple as you might think. 

Your body has a base number of calories it needs to keep necessary functions going.  Calories are your body’s source of energy, and everything in your body, from your heart to your lungs to your brain, needs energy to function. 

In addition to what you need for just basic bodily functions, you use additional calories for everyday activities like walking around or opening jars, for example.  Exercise is another major energy user.  The food you eat also requires energy for digestion and metabolism.

When you fall short of the calories you need, you are undereating.  But simply not consuming enough calories isn’t the only way you may be undereating.

Your body also requires that you consume certain nutrients in the foods you eat.  Carbohydrates, protein, and fat are the macronutrients that provide calories in food.  Other nutrients, however, such as certain vitamins and minerals, need to come from food as well.

Some people consume plenty of calories, but if the foods they are eating are low in nutrients, they are undereating because they aren’t giving their body everything it needs.

Factors that impact nutrition needs

As mentioned above, your activity level plays a major role in your nutrition needs, but there are many other factors.  This is where things start to get a little tricky, because every single person is unique in their needs.

Factors that can influence not only your overall calorie needs, but also how much of individual nutrients you may need include:

  • Genetics
  • Hormone balance
  • Fitness level
  • Ratio of muscle to fat
  • Composition of your gut microbiome

Long term impact of undereating

If you’ve read this far and you’re thinking, “get to the point, Kathy!”, here it is.  When you undereat for extended periods of time, your body doesn’t have sufficient energy to function optimally.  This can lead to adverse consequences in multiple parts of the body, including: (2)

  • Bones
  • Gastrointestinal system
  • Immune system
  • Endocrine system (hormone imbalances)
  • Nervous system (psychological and mental issues)
  • Cardiovascular system

For active adults who are trying to lose weight, moderate calorie reductions for a limited amount of time can be done safely during times of lower volume exercise.  Moderate calorie restriction means 500-700 calories per day less than your total estimated needs.  For those who have training cycles, the off-season is the ideal time to try to lose weight. (1)(2)

It is also important to keep in mind that higher volume athletic training can suppress appetite.  And, while beneficial to health, high fiber and high protein foods can decrease appetite as well.  Even if you’re not hungry, that does not mean you don’t still need those nutrients!  Be extra mindful of getting enough nutrients when appetite is lower. (2)

Special considerations for women

Female hormones don’t just cause moodiness and menstruation. 

They also impact how our bodies use certain types of nutrients for fuel

That said, the research community has historically generalized nutrition recommendations for women based on research that was largely conducted on men.  Yes, you read that right.

Thankfully, there is a growing body of research focusing on the special needs of women.  There’s still a lot we don’t know, but what we’ve learned so far is eye-opening.

Chart showing the phases of menstrual cycle and fluctuations of estrogen and progesterone throughout.

During the follicular phase of the menstrual cycle, which starts on day 1 of menstruation and ends with ovulation, we have less glycogen (the body’s storage form of glucose).  This is especially important if you’re carb loading during this time.  On carb loading days during the follicular phase, increase carbs to at least 3.6 grams per pound of body weight. And, even if not carb loading, eat carbs as soon as possible post-exercise to replenish glycogen. (3)

During the luteal phase of menstruation, which starts after ovulation and lasts until the beginning of menses, estrogen and progesterone are at their highest levels.  Estrogen and progesterone are the primary female hormones. (3)

Estrogen has been shown to impair the body’s ability to produce glucose from other nutrients, which leads to faster glycogen depletion.  Progesterone promotes the breakdown of protein.  When engaging in intense or prolonged exercise where glycogen is depleted, the mighty estrogen/progesterone combo can cause loss of muscle. (3)

To minimize loss of muscle during the luteal phase, you should consume carbs before exercise.  Then, as soon as possible after exercise, have something with both carbs and protein. Women training for endurance sports should aim for at least 0.74 grams of protein per pound of body weight per day during this phase. (3)

Signs you might be undereating

Since every individual has different nutrition needs, it can sometimes be difficult to know based on general guidelines whether you’re eating enough, especially if you have a new exercise regimen or are ramping up your training.

Sometimes, though, your body will give you signs.  Here are a few things to be on the lookout for:

  • Fatigue: if you are always tired, even after a good night’s sleep, you might not be getting all the nutrients you need.
  • Irritability and/or brain fog: your brain consumes about 20% of the total energy your body uses.  When nutrients are low, the brain is sometimes the first to suffer. (1)
  • Constipation: when you undereat, your body diverts energy away from the digestive system in order to maintain more vital systems, like respiratory and cardiovascular.  That greatly slows digestion and can lead to constipation.
  • Frequent illness: your immune system not only requires energy and nutrients, but it is also largely influenced by your gut microbiome.  Good gut bacteria suffer without proper nutrients.  
  • Injury prone: fatigue from insufficient nutrition can increase risk of injury. (1)

To recap…

I don’t want to sound like a broken record, but I feel the need to reiterate: please do not undereat for long periods of time or during periods of high volume or intense training.

Undereating can mean either not eating enough calories, or not getting the nutrients your body needs regardless of calorie intake.  In other words, what you eat matters, so eat nutritious foods!

When your body doesn’t get the nutrients it needs, it diverts energy away from lower-priority functions in order to maintain high-priority functions, like pumping blood and breathing.  If this goes on for a long time, there can be serious health consequences.

Nutrition needs are dynamic, increasing with exercise volume and intensity, among other factors.  To complicate matters for the ladies, hormone levels at different phases of your menstrual cycle impact how your body uses carbs, fats, and proteins. 

Be aware of the signs of undereating listed above and adjust as soon as you start to see red flags.

If you are unsure what your nutrition needs are or you need help building a strategy to ensure you’re getting all the right nutrients, consider working with a registered dietitian.  If you’re interested in working with me, send me a message!

Now, go eat!

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