Response-to Letter-to-the-editor: “A low-carbohydrate diet induces hepatic insulin resistance and metabolic associated fatty liver disease in mice”

Dear Editors of Molecular Metabolism,

We write in response to the Letter to the Editor critiquing our recent publication, “A low-carbohydrate diet induces hepatic insulin resistance and metabolic associated fatty liver disease in mice” (Long et al., 2023). We are grateful for the opportunity to address the concerns raised and clarify some aspects of our study.

In our original paper, we conducted a literature review, citing studies that both support and challenge our findings. As referenced in our article [[1], [2], [3]], clinical studies have shown that ketogenic diets (KDs) can significantly improve fasting blood glucose and serum lipid profiles, leading to weight loss in obese subjects [[1], [2], [3]]. KDs enhance HbA1c levels and fasting blood glucose compared to low-fat diets in diabetic patients. Short-term KD interventions, ranging from 4 weeks to 6 days, have been reported to reduce triglycerides in obese subjects [4] and decrease hepatic insulin resistance in patients with metabolic-associated fatty liver disease (MAFLD) [[1], [2], [3]]. Conversely, a 3-day KD regimen increased postprandial plasma glucose levels in healthy men [5], and a 4-week KD intervention led to elevated cholesterol and inflammatory markers in obese subjects [4]. These contrasting findings underscore the complex nature of KD's impacts on health. Our study contributes to this ongoing scientific research, adding new perspectives and considerations to understanding KDs, as evidenced by the wide range of studies we reviewed and cited.

We employed a specific ketogenic diet (KD) with high saturated fat content from pork lard, which significantly increased cholesterol levels compared to mice fed with a high-fat diet (HFD). In agreement with our findings, elevated hepatocyte cholesterol can upregulate TAZ, thereby promoting fibrosis and NASH in both mice and humans [6]. High dietary cholesterol has also been linked to the progression of steatosis, NASH, fibrosis, and hepatocellular carcinoma (HCC) in mice [[7], [8], [9]], accompanied by insulin resistance due to gut microbiota dysbiosis [9]. The composition of this KD is different from the KDs typically used in human clinical settings, indicating the relevance of our findings to diets rich in saturated fats, commonly found in Western diets. Importantly, it's essential to distinguish between clinically prescribed KDs and commercially available ones. Clinically prescribed KDs are precisely monitored for nutrient content and have demonstrated beneficial effects in reducing body weight. In contrast, commercially available KDs may not adhere to the standard nutritional guideline characteristic of clinical KDs. Their variable nutritional composition, especially regarding saturated fat content, raises concerns about their long-term health impacts. While these diets might facilitate weight reduction, their uncontrolled nutritional components could potentially lead to adverse outcomes such as fibrosis and MAFLD, similar to the effects observed in our study. This indicates a critical need for cautious evaluation and further research into the long-term metabolic effects of commercially available KDs.

Long-term KD feeding in research, clinically prescribed or commercially available KDS may trigger a complex network of pathogenetic mechanisms, potentially contributing to NASH and MAFLD, independent of IL-6. These mechanisms, including bile acid toxicity, macrophage and mitochondrial dysfunction, hepatic stellate cell activation, and gut microbiota changes, suggest that the metabolic impacts of KDs may extend beyond the established pathways and warrant further investigation.

Our study introduces one of the first metabolic models utilizing a ketogenic diet (KD) under thermoneutral conditions, a crucial step given that thermoneutrality more closely aligns with human living conditions. This approach is supported by data from De Jong et al., 2019 [10], who demonstrated that classical brown adipose tissue (BAT) in humanized middle-aged mice under thermoneutral conditions closely resembles human BAT, suggesting that our model is highly relevant for translating findings to human physiology.

We acknowledge the critique in the Letter to the Editor regarding the general inflammatory responses associated with thermoneutrality, as evidenced by increased macrophage infiltration and cytokine production [11]. While these general responses are indeed a factor at thermoneutrality, our study specifically investigated how a KD high in saturated fats interacts with these conditions, leading to exacerbated metabolic outcomes such as NASH and fibrosis compared to HFD-fed mice. Thus, our study focused on the interplay between a specific dietary composition and the thermoneutral environment rather than overgeneralizing the effects of KDs.

The point raised about caloric restriction is valid. We aimed to investigate the impact of KD's high saturated fat and low carbohydrate content on metabolic health, which differs from studies focusing on the combined effects of KD and caloric restriction. While our study provides valuable insights into this specific dietary composition, we acknowledge that further research is needed to explore the link between different KD compositions, including those with caloric restriction, to understand their metabolic impacts.

As noted in our paper and widely acknowledged in the field, there are inherent differences between human and murine models, and while they are critical, they do not diminish the relevance of our findings to human health. We understand the concern that our study might overgeneralize the negative impacts of KD. Thus, it should be noted that our results are specific to the type of KD used in our research, particularly its high saturated fat and low carbohydrate content. This specificity again underscores the need for a detailed and comprehensive understanding of how various dietary patterns affect metabolic health.

CRediT authorship contribution statement

Christian Wolfrum: Conceptualization, Funding acquisition, Project administration, Supervision, Writing – review & editing. Tenagne D. Challa: Conceptualization, Investigation, Writing – original draft, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

No data was used for the research described in the article.