Minimum Dietary Fat Threshold for Effective Ketogenesis and Obesity Control in Mice

Background/Objectives: Ketogenic diets (KDs), defined by very low carbohydrate and high fat content, are widely studied for obesity and metabolic disease. However, KD formulations vary from 60–95% fat, leading to inconsistent induction of ketogenesis and variable outcomes. The fat threshold required for sustained ketosis, and the tissue-specific programs that mediate KD efficacy, remain unclear. Methods: We evaluated multiple KD formulations (80–95% fat) in C57BL/6J wild-type (WT) and diet-induced obese (DIO) mice. Plasma, hepatic, and intestinal β-hydroxybutyrate (BHB) were measured together with expression of ketogenesis and fatty acid oxidation genes. Body weight, adipose distribution, and liver morphology were assessed under both direct feeding and therapeutic settings. Results: In WT mice, only diets exceeding 85% fat induced robust ketogenesis, reflected by elevated BHB and hepatic upregulation of Cd36, Cpt1a, Acat1, and Hmgcs2. Moderate KDs (80–85%) failed to trigger ketosis and resembled high-fat feeding. In obese mice, an 80% KD lowered fasting glucose without reducing body weight, whereas a 90% KD promoted systemic ketosis, weight loss, and adipose reduction. Interestingly, hepatic transcriptional programs for fatty acid oxidation and ketogenesis were suppressed under 90% KD despite elevated BHB, suggesting reliance on substrate availability and peripheral utilization. In contrast, intestinal Hmgcs2 was strongly induced in both WT and DIO mice, with Oxct1 upregulated only in obesity, indicating local ketone production and consumption. Conclusions: These findings identify > 85% dietary fat as a threshold for sustained ketosis and highlight distinct liver–intestine contributions, underscoring ketogenesis as the central driver of KD’s anti-obesity benefits.