High‐fat diet alters serum cytokines before the onset of obesity

Childhood obesity is increasing at an alarming rate. Tall stature and accelerated skeletal maturation are hallmarks of obesity that have detrimental consequences such as limb bowing, slipped epiphyses, and fractures, which rapidly evolve into adult disability. One major treatment obstacle is lack of knowledge of the underlying cause of this growth acceleration. Circulating IGF‐1, the major hormone required for growth, is paradoxically low to normal in obese children. Obesity is characterized by chronic inflammation driven by cytokines such as interleukin‐6 (IL‐6), which suppresses bone elongation by inhibiting IGF‐1. Our lab found that a high‐fat diet accelerates bone elongation in mice before they had excess body fat, indicating that skeletal impacts of a high‐fat diet are well underway before physical signs of obesity emerge. Our goal was to determine whether high‐fat diet alters inflammatory cytokines that could serve as biomarkers for skeletal damage. We tested the HYPOTHESIS that mice on a high‐fat diet have higher levels of inflammatory cytokines before they exhibit overt obesity. METHODS Female and male 3‐week‐old C57BL/6 mice (N=6/diet/age) were put on high‐fat (60% kCal fat) or control (10% kCal fat) diets for 1 or 2 weeks. Serum cytokines (Leptin, TNF‐α, IGF‐1, IL‐6, VEGF, IL‐1α, IL‐1β, MCP‐1) were measured by ELISA, and glucose was measured with a blood glucose monitor. Statistical significance (p<0.05) was determined in SPSS using Mann‐Whitney and t‐tests. RESULTS At 4 weeks, after 1 week on diet, tibial elongation rate was over 20% higher in mice on a high‐fat diet (t=4.4, p<0.001) and body mass was 16% greater (t=5.6, p<0.001), though mice were not yet obese. By 5‐weeks, after 2 weeks on diet, tibial elongation rate was still over 10% higher in the high‐fat diet group (t=2.1, p<0.05) but with no difference in body mass (t=0.5, p=0.62). At 4‐weeks, IGF‐1 (z=2.3, p<0.05), IL‐6 (z=2.4, p<0.05), and IL‐1α (z=2.3, p<0.05) were all significantly decreased in the high‐fat diet mice. At 5‐weeks, TNF‐α (z=2.9, p<0.01) and IL‐6 (z=2.7, p<0.01) were decreased in the high‐fat diet group, while VEGF (z=2.7, p<0.01) was increased. There were no differences in glucose between diets at either age. DISCUSSION Our results support the hypothesis that a high‐fat diet alters inflammatory cytokines before the overt onset of obesity. Unexpectedly, most cytokines were decreased rather than increased as predicted. The most robust finding was the 2‐fold and 1.5‐fold decrease in IL‐6 at the 4‐ and 5‐week age points, respectively. In addition to inflammation, IL‐6 can regulate energy metabolism, with low IL‐6 possibly contributing to human obesity. Decreased IL‐6 is also consistent with our finding of increased bone elongation since high IL‐6 suppresses growth. SIGNIFICANCE These results are relevant for understanding the development of obesity‐related skeletal complications. Reduced inflammatory cytokines, particularly IL‐6, during juvenile growth could be a useful biomarker for assessing potential skeletal damage before a child becomes obese. Support or Funding Information Supported by the National Institute of General Medical Sciences (1P20GM121299‐01) and National Institute of Arthritis and Musculoskeletal and Skin Diseases of the NIH (1R15AR067451‐01).