Can a Healthy Individual's Set Point Weight Be Underweight?
No, a healthy individual's set point weight should not be underweight—by definition, a BMI below 18.5 kg/m² is classified as underweight and carries increased health risks, making it incompatible with optimal health status. 1, 2
Understanding Set Point Theory in Context of Healthy Weight
The set point theory proposes that body weight is physiologically regulated at a predetermined level through coordinated adjustments in energy intake and expenditure, similar to how the body regulates core temperature. 3, 4 However, this regulatory mechanism operates within the context of what constitutes healthy versus unhealthy weight ranges.
Official Weight Classifications Define Health Boundaries
The American Heart Association and World Health Organization establish clear BMI thresholds that define healthy weight:
- Underweight: BMI < 18.5 kg/m² - explicitly associated with increased risk of clinical problems 1, 2
- Normal/Healthy Weight: BMI 18.5-24.9 kg/m² - associated with lowest risk of cancer, cardiovascular disease, and overall mortality 1, 2
- Overweight: BMI 25.0-29.9 kg/m² 1, 2
- Obese: BMI ≥ 30.0 kg/m² 1, 2
Why Underweight Cannot Be a "Healthy" Set Point
The American Cancer Society explicitly states that maintaining a BMI between 18.5 and 25.0 kg/m² is necessary to reduce the risk of cancer, cardiovascular disease, diabetes, and premature mortality. 2 This evidence-based recommendation establishes that weights below this range—by definition underweight—are not compatible with optimal health outcomes.
The classification of underweight (BMI < 18.5 kg/m²) as carrying "increased risk of other clinical problems" 1 demonstrates that regulatory mechanisms maintaining weight at this level would represent pathologic rather than healthy regulation. The set point theory describes regulation at a "specified level" 4, but when that level falls below 18.5 kg/m², it represents dysregulation rather than healthy homeostasis.
Set Point Theory and Pathologic Weight Regulation
Research distinguishes between regulated obesity (set point at elevated level) and unregulated obesity (regulatory dysfunction). 3 The same framework applies to underweight states:
- Regulated underweight would represent a set point established at a pathologically low level (e.g., anorexia nervosa, chronic illness)
- Unregulated underweight would represent failure of normal regulatory mechanisms
Neither scenario represents healthy physiology. 3, 4
Clinical Implications
When encountering an individual with BMI < 18.5 kg/m²:
- Do not assume this represents their "natural" or "healthy" set point 1, 2
- Assess for underlying pathology: eating disorders, malabsorption, hyperthyroidism, malignancy, chronic infections 1
- Recognize increased clinical risk: The classification explicitly notes "increased risk of other clinical problems" even without specific comorbidities 1
- Target weight restoration to BMI ≥ 18.5 kg/m² to achieve the healthy weight range associated with optimal outcomes 2
Important Caveat About Metabolic Adaptation
While the body does defend against weight changes through metabolic adaptation (decreased resting metabolic rate with weight loss) 5, 6, this defense mechanism can operate at both healthy and unhealthy weight levels. The existence of metabolic resistance to weight change does not validate an underweight state as "healthy"—it simply reflects the body's resistance to change from its current regulated level, regardless of whether that level is optimal for health. 5, 4