I just want to point out that it’s usually very hard to change lean or fat mass drastically in a short term. Usually massive weight gain/loss are from water gain/loss — even if you got your fat % reading from one of those cheap body composition scales. I would guess that any thing like “10 lbs lost over a week on soylent” I’ve seen on the forum is more likely the result of balancing Na+ (which is usually a shit load in a regular Joe diet) with K+ (which is usually way suboptimal in a regular Joe diet) and thus losing lots of water over a week, than actually losing that much fat (though of course there can be fat loss, but no way on a 10 lbs magnitude; maybe like .5 lb to 1 lb). This effect can be had if you just add sources of potassium in a regular diet.
Another source of water mass change is from drastically adjusting carbohydrate intake. Lowering carbs could decrease body store of glycogen, which holds a lot of water. For example, this is what one would experience in the first week of a keto/low carb diet.
Of course this is not denying potential fat loss benefits of soylent, but I think there has been some confusion about soylent’s supposedly miraculous fat loss capabilities.
tl;dr: soylent weight loss likely mostly from water loss, from increasing ratio of K+ compared to Na+