Under Eating

Four Reasons Why Under-Eating Doesn’t Help Weight Loss

October 26, 2020

Many of us have likely heard that we have to burn more calories than we consume in order to lose weight. And while there is some truth to that, chronically under-eating is not always the best approach for long-term weight management or health. Below we outline four reasons why that may be the case.

Four Reasons Under-Eating Doesn’t Work For Weight Loss


Our metabolism goes through normal fed and fasted states; but when we are chronically under-eating that normal metabolism has to compensate. Under normal conditions, the body will rely primarily on glucose (carbohydrate) for fuel.

During periods of fasting, the body will use the stored carbohydrate (glycogen) for fuel first. During extended periods of fasting, the body will start to draw on protein (in the form of muscle) and/or fat (adipose tissue) for energy.

Unfortunately, the process of breaking down protein and converting it to glucose for energy is relatively quicker than breaking down fat and relying on ketone bodies (an alternative source of energy) for fuel. The body can use ketone bodies for energy, but there are certain organs that prefer glucose for energy. Therefore, during extended periods of fasting and chronically under-eating, the body is often breaking down our muscle mass, in addition to fat.

However, that loss of muscle mass is not as easily replaced as fat cells. So inevitably, when an individual starts to eat “normally” again, the body is much more likely store the excess fuel as fat. This is kind of an insurance policy for the body because it is always trying to achieve a state of homeostasis (equilibrium or balance) and unfortunately it doesn’t really like to lose major amounts of weight.

Additionally, the less muscle mass present in the body, the fewer calories the body is able to burn at rest, therefore decreasing the body’s metabolic rate. This is often why yo-yo dieters find that after they lose some weight, they regain what they had lost, but then tend to gain more beyond what they initially lost.


Similarly to what was discussed above, chronically under-eating (especially if coupled with over-exercising) can put the body into starvation mode where the body tries to conserve as many resources as possible in order to prevent drastic, major weight loss.

Unfortunately, the body doesn’t really like to lose major amounts of weight because it’s always aiming for a state of homeostasis. When the body is chronically under-fed, then it starts to prioritize survival and therefore slows down the production of thyroid hormones, sex hormones, and can increase stress hormones such as cortisol.

A decrease in thyroid hormone, specifically a decrease in triiodothyronine (T3), results in a reduction of metabolic rate. As the metabolic rate decreases, the body is unable to burn as many calories as it did previously per the Caloric Restriction in Humans: Impact on Physiological, Psychological, and Behavioral Outcomes. Then, as cortisol rises, it can lead to insulin and leptin resistance.

Chronically elevated levels of insulin tends to promote fat storage and decreases the body’s ability to use stored glycogen or fat for fuel. One study found that as insulin increases, it also caused a subsequent rise in leptin as well.

Leptin is a hormone produced by fat cells to help maintain that state of equilibrium in terms of body weight. Under normal conditions, an increase in leptin would send a signal to the brain to decrease appetite and therefore food intake.

However, in the case of leptin resistance, the brain stops responding to those signals from the fat cells to decrease appetite. When this is coupled with insulin resistance, the body continues to store body fat and doesn’t adjust with the subsequent reduction in appetite.



When constantly monitoring calorie and food intake, one may become overly fixated on one’s diet, and that fixation can increase the body’s stress response. According to one study, “Dieting is likely psychologically stressful.”

Dieting, by definition, is an act of restriction of eating, this deprivation elicits negative emotion. Dieting involves not merely resisting temptation, but also a physically aversive feeling of being hungry. Reviews of dieting studies have documented negative emotional consequences of dieting such as depression, anxiety, decreased self-esteem, nervousness, and irritability. Low-calorie dieting increases cortisol.

In some ways, the body can’t determine if the stress it is experiencing is physical or mental in nature, and therefore responds similarly. So, as one becomes more stressed out by the exact amount of calories being consumed, the body produces more cortisol, and as mentioned previously, elevations in cortisol lead to insulin resistance making it harder to lose weight.



A constant decrease in calorie consumption also can lead to nutritional deficiencies. Several micronutrients are important for the metabolism and breakdown of our food. When we are under-eating, we miss out on vital nutrients that are needed to support our metabolism.

Several micronutrients are needed for proper blood sugar management including glucose metabolism and insulin resistance. We discussed previously that insulin resistance can make it harder to lose weight, so if glucose is not being managed by the body properly due to a vitamin deficiency, insulin resistance can impede our weight loss efforts despite eating fewer calories. Malnutrition of obesity and micronutrient deficiencies promote diabetes.

Some of those nutrients needed for metabolism include many B vitamins and magnesium to name a few. Another reason nutrient deficiencies may not help with weight loss, is that some nutrient deficiencies (particularly iron and B vitamins) can also lead to anemia and fatigue, which can make it more difficult to exercise and be able to maintain the weight you may have lost.

There is also a nutrient deficiency theory of obesity that when an individual is under-nourished, the body will tell them to keep eating until all of their micronutrient needs are met.

As you can see, the body has several mechanisms in place to maintain survival and promote homeostasis. When the body is constantly under-fed, the body has to adapt in many ways to prevent starvation, malnourishment, and death in extreme cases. Therefore, while decreasing calories can lead to weight loss if you’ve been over-consuming, it is still important to fuel the body appropriately to lead to long term weight management and health.

Read more articles by Michelle Bauche, MS, RDN, LD, CSOWM

Pinterest Under Eating
Michelle Bauche


Michelle Bauche, MS, RDN, LD, CSOWM is a registered dietitian working for Missouri Bariatric Services at University of Missouri Healthcare in Columbia, Missouri. Michelle earned her Bachelor’s degree through the University of Missouri’s Coordinated Program in Dietetics and her Master’s degree through the University of Alabama. She is a certified specialist in Obesity and Weight Management and a member of the Weight Management Dietetic Practice Group through the Academy of Nutrition and Dietetics.