Effects of Nut Consumption on Body Composition: A Systematic Review and Meta-Analysis of Randomized Controlled Trials with Subgroup Analyses by Nut Fatty-Acid Profile

Background: Despite their high fat content, nut consumption is suggested to benefit body composition. However, it remains unclear whether nuts with different fatty-acid compositions affect body composition differently. This study aimed to assess the effects of monounsaturated fatty acid (MUFA)-rich and polyunsaturated fatty acid (PUFA)-rich nut consumption on body composition. Methods: A systematic search of PubMed, Embase, CINAHL, Cochrane, and additional sources identified 3118 articles, with 154 meeting the inclusion criteria. Subgroup analyses were conducted by nut fatty-acid composition and control diet type. Nuts were classified as MUFA-rich (MUFA:PUFA ratio ≥ 1.3) or PUFA-rich (<1.3) based on the percentages of total fat content. Results: Although there were no effects on body mass, BMI, or waist circumference, nut consumption significantly reduced % fat mass (WMD: −0.28%, 95% CI: −0.53 to −0.02) and fat mass (WMD: −0.41 kg, 95% CI: −0.58 to −0.27) and increased % lean mass (WMD: 0.71%, 95% CI: 0.32 to 1.10). However, absolute lean mass did not change significantly. Benefits were observed only in the MUFA-rich subgroup, although subgroup differences were not significant. Both MUFA- and PUFA-rich nut consumption showed reduced total cholesterol and low-density lipoprotein cholesterol levels, while MUFA-rich nut consumption further reduced triglyceride levels. Conclusion: Nut consumption, particularly MUFA-rich nuts, can improve body composition by percentage fat mass and absolute fat mass. Consuming either MUFA-rich and PUFA-rich nuts can further improve lipid–lipoprotein profiles.