Isocaloric restriction normalizes body weight but not relative adiposity, adipose inflammation, or gut microbial diversity in high-fat diet-fed mice

Background/Objectives: The separate contributions of excess energy intake and diet composition to high-fat diet (HFD)-induced adiposity, adipose inflammation, and gut microbial changes are unclear. We tested whether restricting HFD intake to the energy intake of low-fat (LF)-fed mice (isocaloric restriction) prevents or reverses these outcomes. Subjects/Methods: Male C57BL/6 mice (n = 8/group) received for 12 weeks an LF diet (LF/LF) or HFD (HF/HF) ad libitum, HFD ad libitum then isocaloric HFD from week 7 (HF/IHF), or isocaloric HFD throughout (IHF/IHF). Adiposity, plasma markers, adipose and ileal gene expression, and fecal microbiota (16S rRNA sequencing, n = 4/group) were assessed. Results: Final body weight in IHF/IHF did not differ from LF/LF (difference 0.1 g, 95% CI -6.5 to 6.7; P > 0.99) and was lower than in HF/HF (-8.2 g, -14.7 to -1.6; P = 0.011). Switching to isocaloric feeding reduced weight gain during weeks 6-12 (HF/IHF vs HF/HF, -6.4 g, -9.8 to -3.1; P < 0.001). IHF/IHF mice had lower glucose (P = 0.031) and cholesterol (P = 0.005) than HF/HF mice. However, visceral fat relative to body weight remained higher than in LF/LF in IHF/IHF (+1.5 percentage points, 0.04 to 2.9; P = 0.042) and HF/IHF (+1.8, 0.4 to 3.3; P = 0.008). Adipose Tnf expression remained elevated in HF/IHF (log2 difference 2.2, 0.2 to 4.2; P = 0.025), and fecal phylogenetic diversity was lower in IHF/IHF than in LF/LF (Faith's PD -3.1, -5.9 to -0.4; P = 0.024). Ileal barrier genes and plasma lipopolysaccharide-binding protein were unchanged. Conclusions: Limiting HFD energy intake normalized body weight, glycemia, and plasma lipids but not relative adiposity, adipose Tnf expression, or the loss of gut microbial diversity. Diet composition affects these outcomes independently of energy intake, so normal body weight does not indicate their reversal.