January 2025 Issue
The Risks of Undereating for Weight Loss
By Carrie Dennett, MPH, RDN
Today’s Dietitian
Vol. 27 No. 1 P. 24
Why Extreme Restriction Can Lead to Real Health Problems
Traditional weight loss recommendations encourage people to eat less and move more to create a calorie deficit, leading some people to believe that the more they restrict their food intake, the more weight they’ll lose. However, energy regulation in humans is more complicated than “calories in, calories out,” and over restricting poses risks for metabolic health as well as the health of many body systems—whether due to the energy deficit itself or accompanying nutrient inadequacies. But how much restriction is “too much”?
A 2024 study1 used National Health and Nutrition Examination Survey data from 2015–2018 to model the impact of simulated caloric reduction on nutrient adequacy among US adults aged 19 and older with BMIs in the “overweight” or “obese” range. Mean daily calorie intake at baseline was 2,141 kcal. After simulating caloric reductions of 20%, 30%, 40%, and 50%, mean daily calorie intakes decreased to 1,716, 1,501, 1,287, and 1,072 kcal, respectively.
The researchers then proportionately reduced participants’ average intake of protein, total fiber, calcium, folate, iron, magnesium, zinc, potassium and vitamins A, B6, C, D, and E. The most dramatic increase in intakes below the estimated average requirement (EAR) happened at 50% caloric reduction. However, lower calorie reduction levels also resulted in substantial proportions of intakes below the EAR. For example, after a simulated 30% caloric reduction, 25% to 40% of the population had intakes below the EAR for protein, vitamin B6, and zinc, and 75% to 91% of the population had intakes below the EAR for two nutrients important for bone health—calcium and magnesium.
“I work with adults who have spent years dieting and are now facing the consequences of chronic restriction, like osteopenia and osteoporosis,” says Marissa Beck, MS, RDN, owner of REVV Health, a virtual nondiet private practice in the greater Seattle area. “The body requires consistent nourishment over time to build and maintain strong bones, and years of inadequate intake can leave adults at a much higher risk for these conditions.”
While brief bouts of low-calorie consumption may not be overly concerning, extended underconsumption can lead to nutrient deficiency, loss of lean body mass, and disruption of normal metabolic function, says Julie Schwartz, MS, RDN, CSOWM, LD, ACSM-EP, NBC-HWC, an expert in weight management and behavior change and founder of Balanced Nutrition Coach in Safety Harbor, Florida. “Metabolic function may already be compromised in people who are trying to lose or maintain weight and continue to cut calories lower and lower to no avail. My opinion is that ‘too much’ is when calories are too low to meet minimal protein and fiber recommendations. Beyond not nourishing the body, for most people, it leads them away from the health and energy they desire. It’s not sustainable. And leads to a cycle of frustration and further metabolic disruption.”
Katherine Metzelaar, MS, RDN, CD, founder and owner of Bravespace Nutrition in Seattle, says restriction is “too much” when it begins to interfere with someone’s overall physical, emotional, and social wellbeing. In addition to its impacts on metabolic function, muscle, and bone, she points out that undereating can disrupt hormone levels, which in turn can affect sleep, mood, and reproductive health in bodies of any size. “There’s also the mental toll—restricting calories can fuel disordered eating patterns, increase anxiety around food, and ultimately disconnect people from their body’s natural hunger and fullness cues. This cycle can be difficult to break, and it reinforces the false idea that there’s something wrong with their body.”
Los Angeles-based Yasi Ansari, MS, RDN, CSSD, national media spokesperson for the Academy of Nutrition and Dietetics and a specialist in sports nutrition and eating disorders, says the body is remarkable in its ability to adapt temporarily to changes in energy intake and that individuals have different biological responses to inadequate intake. “Some may endure a restrictive eating pattern for a more extended period before experiencing the harmful effects of chronic restriction on their health,” she says, adding that signs suggesting an individual is restricting too much may include fatigue, light-headedness, low blood sugar, changes in digestion, mood fluctuations, irregular eating patterns, and rigidity in food choices and exercise behaviors—but may or may not include significant weight changes. She also notes that isolation during meals or snacks, failure to honor the body’s hunger and fullness cues, hormonal changes, poor mental performance, impaired bone health, sleep disturbances, and abnormal blood markers can also indicate excessive restriction.
Meal Timing and Nutrient Distribution
Various forms of intermittent fasting have been used to reduce food intake without continuous calorie restriction, but aggressive fasting approaches such as one-meal-a-
day (OMAD)—when someone fasts for 22 to 23 hours, then consumes the day’s calories within one to two hours, without any nutritional guidelines—are increasing in popularity as a form of calorie restriction. However, claims about the purported benefits for weight loss and health are largely extrapolated from research on other forms of intermittent fasting because the limited research on OMAD itself is mostly based on animal studies.2 The primary human study involved 11 healthy, young, lean adults (aged 29 to 33, average BMI 23.4 to 24.6 kg/m2) who only followed the OMAD diet for 11 days.3
“Even if someone technically could meet their daily calorie needs in that single meal, it is incredibly difficult to get all the necessary macronutrients, especially adequate protein, and micronutrients, in one sitting,” Beck says, noting that trying to meet fiber needs in one meal could be hard on the digestive system.
“I hear from patients that they are hungry and bloated, not energized, and not mentally focused,” Schwartz says. “Some people are not early-in-the-day eaters; however, we know the rhythm of appetite regulation issues more than one peak per day.”
Meeting protein needs on OMAD is a real concern, given current research suggesting that 0.4g/kg/meal offers optimal absorption and muscle protein synthesis.4 Another concern is the potential impact on health. A 2007 study found that participants who ate one meal per day had significant increases in blood pressure and in total and LDL cholesterol,5 and a 2022 study including data from the National Health and Nutrition Examination Survey found eating one meal a day was associated with a 30% increased risk of all-cause mortality and an 83% increased risk of cardiovascular mortality.6
Metzelaar says these aggressive fasting approaches go against the body’s natural need for a balanced distribution of nutrients throughout the day. “This can lead to blood sugar imbalances, mood swings, and poor concentration,” she says. “Additionally, this way of eating encourages a disordered relationship with food, where hunger cues are ignored, and fullness cues may be overridden. Long term, it’s simply unsustainable.”
Restriction and Athletes
Athletes in weight-focused sports can be at risk of excessive calorie restriction. If an athlete doesn’t consume enough calories to support exercise, activities of daily living, and basic physiological functions, this can lead to a state of low energy availability (LEA). LEA may be compulsive in nature, due to disordered eating or eating disorders. It may be unintentional, by simply not increasing caloric intake to match a high volume of exercise. Or it can be misguided but intentional.7
“This is something I encounter in some of my work with young athletes. In many sports, particularly sports that emphasize aesthetics, endurance, or weight classes, such as gymnastics, wrestling, or running, there’s a strong cultural pressure to be leaner and lighter,” Beck says. “Unfortunately, this often leads to restrictive eating behaviors that athletes may see as a path to enhanced performance, but in reality, it can impair both their health and their ability to perform at their best. I often see how insufficient intake can compromise their immune system and increase the risk of injuries.”
Ansari says athletes experiencing LEA may find that their workouts feel harder, and they may experience delayed recovery as well as decreases in endurance, muscle strength, training responses, motivation for their sport, and cognitive performance.8 If LEA is chronic—occurring over many weeks or months—it can not only lead to the development of nutrient deficiencies but also lower immune function, cause unfavorable lipid profiles and endothelial dysfunction, create neuroendocrine abnormalities, adversely affect fertility, and reduce bone mineral density.8-10
Metabolic Adaptation
When someone consumes fewer calories or nutrients than the body needs to sustain healthy metabolic function, the costs to health may outweigh the benefits they’re seeking. The body’s fine-tuned mechanisms for regulating weight and energy storage are designed to ensure survival in the face of variations in energy availability, which includes adapting to starvation by reducing energy expenditure. This ability is known as metabolic flexibility, but these mechanisms can lose their flexibility, resulting in maladaptation to both increased caloric intake and caloric restriction, which can, in turn, lead to the development of several metabolic disturbances.11
“For most people weight regulation is much more than calories in and calories out. For many, continually starving their body creates a maladaptive metabolic system and disrupts appetite regulation,” Schwartz says. “Most dietitians have worked with people who overly restrict and have not seen success in weight loss. Over the years, I’ve heard dietitians comment that they know people must be eating much more than they are telling us, and way back, I may have said similar. We know better now. We know that people restrict and don’t lose weight. The pattern of eating with restriction can lead to further health problems such as higher cholesterol levels when consuming very low fiber and high saturated fat.”
Beck says earlier in her career, she worked with patients to help them lose significant amounts of weight, often using traditional MNT approaches. “While the initial weight loss that my patients experienced might seem like a win, their body’s natural response to chronic underfueling slowed down their metabolic rate. This usually led to a “weight plateau,” making it harder to keep losing weight. A lot of people end up restricting even more, which only worsens the cycle, leading to nutrient deficiencies, muscle loss, and long-term metabolic damage. This approach can set patients up to fail because it pulls them further away from their body’s natural cues and internal wisdom. It often becomes a vicious cycle.”
Disordered Eating and Eating Disorders
The connection between calorie restriction and eating disorders is complicated, with some research suggesting restriction may precede an eating disorder and other research finding that the restriction co-occurs with eating disorders.12 But for individuals who are predisposed to an eating disorder, such as anorexia nervosa, bulimia nervosa, or binge eating disorder, undereating could be a tipping point.
Beck says she finds undereating is usually the triggering event. “It sets off all the thought patterns that lay the foundation for further disordered eating and full-blown eating disorders.” She notes that some of her clients have been caught in a cycle of chronic dieting or restrictive behaviors for years, which she feels can blur the line between trying to lose weight and disordered eating. “Restriction can look different from one person to the next, so assessing whether it’s ‘too much’ is going to require a bit of detective work so that we aren’t inadvertently assuming anything based on body size alone.”
Ansari agrees that over restriction can impact anyone, especially someone predisposed to an eating disorder. High-risk groups include those with a family history of an eating disorder or mental health concerns such as depression, anxiety, substance use disorders, perfectionism, stress, trauma, and a history of weight-bullying. “There’s a continuum, where taking part in disordered eating habits long enough can lead to a diagnosis of an eating disorder. Disordered eating may start with healthy dieting, then become more restrictive and extreme.”
Metzelaar says for some individuals, the initial phase of restriction may feel like control or even empowerment, but over time, it can spiral into obsessive behaviors and a fear of certain foods or weight gain. “In the case of binge eating disorder, restriction often triggers a cycle of deprivation and subsequent binge episodes, which further fuels feelings of guilt and shame.”
Schwartz says she’s had patients with an array of eating disorders, including binge eating disorder in people with a “normal” BMI and anorexia in people with a “high” BMI. “I believe it’s vital for dietitians to evaluate a person’s food pattern, listen for restrictive eating or bouts of binge eating. Listen for shame in eating patterns or choices. We need to listen for the person’s relationship with food and judgments about food and themselves. It’s important to have a curious conversation to uncover meaningful information.”
Nutrition Counseling Approaches
Undereating may be due to misinformation, improper guidance, or lack of guidance. It could also be a sign of disordered eating. With thoughtful and attuned nutrition counseling, dietitians can uncover the roots of the behavior.
Metzelaar says preoccupation with food or obsessive thoughts about when, what, and how much to eat can be clues that a client may be undereating, as can physical signs such as chronic fatigue, dizziness, or digestive issues such as bloating or constipation. “Dietitians can spot these issues by asking clients about their energy levels, menstrual health (for those who menstruate), their relationship to food, and emotional responses to eating. Educating clients on the importance of adequate intake for metabolic health, alongside addressing their relationship with food, is key in preventing long-term harm from restriction.” She also asks open-ended questions such as, “How do you feel after a meal? What are your thoughts about certain foods? What foods are fear foods? What food feels safe to eat? How long do you go without eating in between meals?”
Beck says she often sees clients who are influenced by advice from friends or family and by social media, which tends to oversimplify or misrepresent nutrition. “In my initial consultations, I like to ask open-ended questions such as: ‘Where do you get your nutrition information from?’ or ‘Have you tried any diets or eating plans in the past?’ which helps me understand the source of their beliefs. I also ask about their relationship with food, including questions like: ‘How do you feel after eating certain foods?’ or ‘Do you ever avoid foods because of fear or guilt?’”
When Schwartz finds that a patient is following misinformation or poor guidance, she says it’s powerful to ask in a neutral tone, “How is that working for you?” Follow-up questions could be, “Is this a sustainable routine for you?” and “What about it is sustainable, and what would you change?” she says. “Be curious and see where their knowledge and guidance (when there is some) comes from. Help them discover discrepancies in their knowledge. Guide them to an eating plan that meets their health goals, is energizing, enjoyable, and tasty. As nutrition experts, we can guide people to healthy eating and a positive relationship with food and their bodies.”
— Carrie Dennett, MPH, RDN, is the nutrition columnist for The Seattle Times, owner of Nutrition By Carrie, and author of Healthy For Your Life: A Non-Diet Approach to Optimal Well-being.
References
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