It may be due to appetite and related peptides. Maybe post-prandial ghrelin may have start to decrease as the stomach is filled with water.

Thank you very much for your help. Do you know if these things are possible to test for in a study? If so, is it a difficult and/or expensive test?

You can test the soluble forms of appetite peptides (ex leptin ghrelin) in the human serum/plasma with basic ELISA kits which price depends on the brand. there are really good companies with high prices but you can also find cheaper ones depending on your fund. I recommend a literature search for studies that have measured appetite peptides on human plasma.

One potential strategy to reduce meal energy intake is to modify individuals' perception of fullness prior to eating by consuming a “preload” of water. A systematic review identified only two small laboratory studies that specifically investigated whether water preloading reduced energy intake. Both studies compared participants given a water preload of 500 ml for 30 min before an ad libitum meal with those not given a preload and found that energy intake at the meal was lower for the preload group compared with the no-preload group. Thus, water preloading may improve the effectiveness of weight loss programs.

The only RCTto directly examine the effects of water preloading before meals on weight loss recruited 48 adults with overweight or obesity and allocated them to a hypocaloric diet plus 500 ml of water before meals every day (water preload group) or a hypocaloric diet alone intervention (nonwater group) over 12 weeks. The water group lost about 2 kg more than the nonwater group. Overall diet energy density decreased significantly more in the water preload group than the nonwater group; this study, however, recruited primarily white, older and middle-aged adults and excluded those with common comorbidities. Thus, these results may not be applicable to a general adult population.

The mechanism(s) responsible for the potential effects of water preloading is unclear. Test meal studies have shown that consuming water prior to and during meals increases satiety and changes in subjective sensations of satiety have been associated with a reduced meal energy intake. Whether this reduction results in weight loss is unclear, although the recent RCT by Dennis suggests it might, at least in the short term. However, if changes to satiety do not last until the next scheduled main meal, it may increase snacking between meals and snacking usually involves consumption of high calorie dense food.

In your experiments, make sure to distinguish the temperature of the water. Warm water (and likely, other warm liquids like clear soup broth) is thought to help with weight loss when taken before a meal and also at the end of a meal. It is thought to aid digestion and thus efficiency of nutrient intake. I will try to identify study sources (this is from memory). There is some argument that it is not helpful to consume liquids during your meal. I don't have an opinion on this point. I do think that warm water preloading is helpful for weight loss and prompts signals for satiety. This may possibly be a physical stretch receptor mechanism as well as nutrient entry to blood stream with subsequent chemical release. One thought: if digestion is facilitated, begins 'faster' with preloading, does this ultimately factor into satiety and the ability to stop eating? I agree with Dr. Sarhat that the experience of satiety is short-lived and other behavioral strategies need to be used between meals.