Experts warn extreme leanness harms health as body fat is redefined as vital organ

Key points

• Body fat acts as an active organ that stores energy and produces hormones like leptin, which signals the brain to prioritize essential functions like reproduction and immune defense.
• Extreme leanness can trigger a physiological 'starvation mode,' causing the body to break down muscle, stop bone repair, and suppress immune function, increasing fracture risk.
• Healthy body fat percentages vary by individual, but general guidelines suggest 10-20% for men and 20-29% for women; athletes may have lower levels, with the lowest recorded in studies being 10% for men and 16% for women.
• Visceral fat, which wraps around internal organs, is particularly harmful as it releases inflammatory chemicals that raise blood pressure and interfere with insulin, increasing type 2 diabetes risk.
• Measuring body fat accurately is challenging; DXA scans are the most precise but expensive, while smart scales and skinfold tests are often inaccurate due to fluid levels or inability to measure visceral fat.
• Waist-to-height and waist-to-hip ratios are recommended as practical indicators of metabolic health, with healthy waist-to-height ratios ranging from 0.4 to 0.49.

Background

Recent studies have highlighted the limitations of consumer technology in tracking body composition. A 2026 study found that smartwatch calorie estimates become significantly less accurate as body fat percentage increases, with errors often exceeding 50%. Conversely, Google’s PhotoScan technology has demonstrated that AI analysis of selfies can estimate body fat more accurately than wearable bioelectrical impedance analyses, potentially predicting insulin resistance. Additionally, research from the University of Oulu suggested that higher coffee intake is associated with lower total and visceral body fat, though causation remains unproven. These developments underscore the complexity of measuring and managing body fat, reinforcing the BBC’s current emphasis on biological function over arbitrary percentage goals.

Why it matters

The resurgence of extreme thinness trends, often fueled by weight-loss drugs, poses significant public health risks. Misunderstanding body fat as merely a passive energy store ignores its critical role in maintaining hormonal balance and physical resilience. Chasing unrealistic leanness can lead to severe physiological consequences, including organ damage and reproductive failure, while also ignoring the 'obesity paradox' where some individuals with higher body fat survive critical illnesses better than leaner counterparts. Public health messaging needs to shift from demonizing fat to understanding its functional necessity and promoting sustainable, individualized health metrics rather than universal percentage targets.

What to watch

Health professionals are likely to continue advocating for holistic measures of metabolic health, such as waist-to-height ratios, over specific body fat percentages. As consumer technology evolves, more accurate and accessible methods for assessing body composition may emerge, but experts emphasize that obsessing over precise numbers is less important than maintaining a healthy range appropriate for one's sex, ethnicity, and activity level. Future research may further clarify the optimal fat ranges for different populations and the long-term impacts of extreme leanness on chronic disease and longevity.