A low sugar and a ketogenic diet can both reduce total energy intake but this may be compensated by reduced energy expenditure. 60 healthy adults were randomised to a diet low in free sugars (LOWSUG, 50% carbohydrates of which 5% sugars, 15% protein, 35% fat), a ketogenic diet (KETO, 77% fat, 15% protein, 8% carbohydrates of which 2% sugars) and control diet (MODSUG, 50% carbohydrates of which 20% sugars, 35% fat, 15% protein) for 12 weeks. 45 participants completed the 12 weeks. Both intervention groups significantly reduced energy intake, body mass and fat mass compared to the control group, the reduction was greater in the KETO than the LOWSUG group although it is not reported whether this difference is statistically significant. Energy expenditure was not affected. Compared to the LOWSUG diet, the KETO diet decreased glucose tolerance, increased fat oxidation and altered beta-diversity of the microbiome. The LOWSUG diet had no significant effect on glucose metabolism or microbiome composition but lowered low lipoprotein (LDL) cholesterol. The authors conclude that the LOWSUG diet may be more appropriate than the KETO diet for most people, as the reduction in glucose tolerance and alteration in gut microbiome may have negative effects on cardiometabolic health despite the weight loss achieved with the KETO diet.