The development of metabolomics in various fields has attracted great attention, and metabolomics is an effective strategy to fully understand kidney disease and its mitochondrial and energy metabolism dynamics[24]. His is the basic unit of protein and one of the essential amino acids, especially for children. At present, many studies have proved the relationship between His, T2DM and DN. Compared with healthy controls, plasma His was significantly lower in patients with T2DM and was negatively correlated with changes in urinary albumin excretion[25]. Carnosine, composed of two amino acids, beta-alanine and l-His, has been shown to have protective effect on the kidney[26]. Researchers have found that patients with chronic kidney disease have lower plasma His, accompanying persistent inflammation and higher mortality[18]. At the same time, dietary supplement of His can reduce oxidation and inflammation, which is expected to treat kidney disease[27]. All the studies are consistent with the results obtained by us, and His supplementation has a certain protective effect on the occurrence of DN.
In addition, our study found that the relationship between His and DN is related to gender. Women with lower plasma His level had a higher risk of developing DN, while His showed no significant protective effect in men. We believe that this phenomenon may be related to the significant activation of mTOR signaling pathway by His supplementation. Some studies have shown that the mTOR pathway can integrate amino acid and insulin signals[28, 29]. And mTOR can sense the availability of nutrients, so it is activated under nutrient-rich conditions, especially high levels of amino acids[30]. Additional studies have found that the addition of His can activate and regulate the mTORC1 pathway[31, 32], which can affect insulin secretion[33, 34]. This is also consistent with our results. And mTOR expression was higher in women, explaining the gender difference in the results[35, 36].
Multiple studies have found that progression of DN leads to changes in serum metabolites[37, 38], Amino acid, as a metabolite, is expected to be a more effective early biomarker. Both His and Trp belong to aromatic and heterocyclic amino acids. Our study found that His not only directly affects the risk of DN in T2DM patients, but also indirectly affects the risk of DN through affecting the concentration of Trp, which constitutes a mediating effect relationship between Trp and DN. Some scholars have proposed that His and Trp biosynthesis is one of the most thoroughly characterized central metabolic pathways[39, 40]. Currently, many studies have linked His and Trp to explore their interaction by comparing their sequence, structure and function[41, 42]. The incidence and severity of DN were significantly correlated with Trp[43–45]. Trp derivatives are considered to be another promising biomarker against DN progression[46]. Metformin, a commonly used drug in diabetes, has been shown to have a significant positive effect on the risk of DN[47, 48]. We found that the role of Metformin in human metabolism is complex, and Metformin may play the opposite role in the intestinal pathway. Studies have shown that the effects of drugs on microorganisms in chronic diseases can confuse conclusions and affect microbial composition and protective effects[49, 50], which is consistent with our conclusion. We believe that its negative effect on DN is produced by affecting the metabolism of amino acids.
Our research has important guiding significance for clinical practice. (1) We proposed that the supplement of His could reduce the risk of T2DM patients complicated with DN, suggesting that His could be used as a biomarker to assist in the early identification of DN risk in T2DM patients. (2) And it was found that the protective effect of His was different between the gender, and the OR value was lower in the female group than general population. (3) The possible pathway of His to DN in female population was proposed, which provided a new idea for further clarifying the role of amino acids on DN. (4) Raising the risk effect of Metformin on the development of DN gives us more perspective to look more carefully when evaluating the effect of Metformin on T2DM and its complications. Our research also has a shortcoming. (1) Due to the nature of the cross-sectional study, we cannot prove the existence of a causal relationship between His and Trp and the occurrence of DN, which needs to be confirmed by more prospective cohort studies. And we adjusted the duration of DN to exclude the influence of DN duration on the results.
In conclusion, we found that His had a higher protective effect on the incidence of DN in the female population than in the general population, and proposed the possible route of its influence. The negative effects of Metformin on the protection of His were also pointed out. Future studies are needed to confirm our findings.