Using a two-sample MR study, we identified two key findings. First, for every 1 SD increase in WBFFM (1 SD = 11.449 kg) and SH (1 SD = 9.276 cm), the likelihood of FL occurrence increased by 1.76- and 1.379-fold, respectively. Second, there was no evidence of a causal association between any of the AIs we studied and DLBCL. Our reverse MR analysis revealed that none of the causal relationships exhibited bidirectionality. Finally, our two-step MR analysis found that the causal relationships between WBFFM and SH and FL incidence were mediated by BMR, with mediated proportions of 69.599% and 54.401%, respectively.
Studies have reported that for the same BMI, individuals of Asian descent have higher body FMs, whereas those of European descent have higher FFMs69,70. FFM, or lean body mass, consists mainly of skeletal muscle and is vital for both mobility and metabolic functions71. It is believed that FFM is a highly heritable characteristic, based on studies in families and twins72, with an estimated heritability rate of 0.52-0.6073,74. Previous observational75–77 and MR studies78 have investigated the association between FFM and the risk of developing NHL. For example, the MR analysis performed by Vithayathil M et al. revealed that NHL risk increased by 1.89-fold (95% CI = 1.21–2.96; p = 0.005) for every 18.6 ± 2.6 kg/m2 increase in FFM index78. However, none of these studies addressed which NHL types are particularly affected by FFM. Our study demonstrated that higher the WBFFM higher the risk of FL, thereby supporting the findings of previous MR-based78 and observational studies75 on the association between WBFFM and NHL. Furthermore, it is plausible that the increased risk of developing FL in the European population is associated with relatively high FFM.
A number of studies have reported that Europeans have been taller than Asians over the last 100 years79, and a link between height and the development of NHL has been demonstrated in both adolescents75,76 and adults80. For instance, Bertrand KA et al.75 found a positive correlation between height and the onset of NHL in adolescents in their observational study (hazard ratio = 1.004, 95% CI = 1.002–1.005; p = 4.29 × 10− 9). A cohort study by Murphy F et al.81 also found that the risk of FL increased by 1.28-fold for every 10 centimeter increase in height. Our study revealed a correlation between increased height and increased susceptibility to FL in adults, which suggested that the differential incidence of FL among European and Asian populations may be linked to differences in height. However, our findings contradict those of the MR study performed by Moore A et al.82, who did not find any association between height and FL prevalence. We hypothesise that two factors contributed to this difference. First, as both outcomes and exposures in Moore A et al.'s study pertained to European populations, there may have been some overlapping exposure and outcome samples. By contrast, in this study, the outcome GWAS data were sourced from the UKB database and the exposure GWAS data originated from the FinnGen project, thereby avoiding the possibility of sample overlap. Second, as our study comprised a smaller number of cases than the study by Moore A et al., we cannot dismiss the possibility that our findings are less conclusive. This implies the need for a larger sample to further investigate the correlation between SH and the development of adult-onset FL.
Our MR study found no evidence of causal associations between BMI, WBFM, WC, and HC and FL, consistent with the findings of Vithayathil M et al.78. We also found that the onset of DLBCL was not associated with any of the AIs. Previous studies have reported no significant correlations between BMI83, height82, WC84, HC85, Wt86, and even obesity85 the onset of DLBCL. One reason for the comparable incidence of DLBCL in European and Asian populations may be that AIs do not have an impact on DLBCL incidence. Contrary to our findings, some observational studies suggested that higher BMI in early adulthood is associated with a higher risk of NHL75,87. For example, one study on Japanese individuals showed that obesity during early adulthood (around 20 years of age) was linked to an increased risk of NHL, particularly DLBCL87. The differences in our results may also be due to the median age of the study population, which was 69.96 years.
Although studies have identified a correlation between FFM and NHL, there is a lack of studies on the potential mediating mechanisms behind this relationship. Since the early 20th century, studies have noted that the BMR of Asian populations is lower than that of their American and European counterparts39. This notion has been supported by a number of studies 40,41. There is strong evidence that FFM, rather than FM, is the main determinant of BMR88. Individuals with high FFM and tall statures usually tailor their eating behaviours to meet the energy needs of their tissues and organs, and these mechanisms are mediated by BMR38,89. However, limited studies have been performed on the potential risks associated with BMR and FL. However, many studies have reported an increased risk of other cancers, including breast cancer, pancreatic adenocarcinoma, postmenopausal breast cancer, and endometrial cancer, associated with a high BMR, even in normal-weight individuals29. Our two-step MR study presented compelling evidence supporting the notion that high WBFFM and SH increase BMR and thus, FL risk as well. The biological mechanisms that underlie this relationship can be explained in three ways. First, individuals with higher BMRs require more cellular energy production to meet their higher energy and metabolic needs, and increased aerobic glycolysis can lead to the production of large amounts of reactive oxygen species, in turn leading to increased protein, lipid, and DNA damage90,91. The accumulation of errors in this process puts pressure on the body's repair system and increases the risk of mutations, thus significantly increasing the risk of cancer29,92. Second, in vivo experimental studies have shown that a high BMR reduces the effectiveness of the immune response and can thus lead to cancer development93,94. Third, a high BMR can lead to a pro-inflammatory state95, which has been observed to be associated with a higher risk of developing NHL86. In a European case-control study, elevated levels of inflammatory biomolecules (IL-2, ICAM, IFN-γ, and TNF-α) were found to be associated with the risk of developing NHL96.
This study has some limitations. First, to minimise statistical bias due to demographic stratification, the population we studied was entirely European; therefore, our findings may not be applicable to other ethnic groups. Second, our study aimed to analyse specific subtypes of lymphoma, and the FL and DLBCL GWAS data included a small number of cases, which may have led to some false-positive results. Third, our MR study evaluated the risk of FL using genetic IVs of whole-body FFM and SH, but it did not consider that the relationship between body composition and FL risk may change over time (although we did implement robust methods to assess the sensitivity and robustness of this hypothesis). Fourth, while certain studies have indicated that BMR is not significantly affected by sex 97, there are notable discrepancies in terms of WBFFM98,99 and SH100 between males and females. As the GWAS data on FL used for this study did not differentiate between the sexes, we could not examine the stratified risk of developing FL in men vs women. Thus, future MR-based research stratified by sex is warranted in this field. Finally, our two-step MR study revealed that BMR had MEs of 66•617% and 53•909% on WBFFM and SH, respectively, thereby indicating the likely presence of other unknown mediating factors.