There's a couple of ways in which drinking more water helps you lose weight, some relatively straight forward and some a bit more complicated. The most simple mechanism is replacing sugary drinks thus lowering calories, but more complicated reasons come down to hormones which can functionally confuse hunger and thirst, thus helping people better control their calorie intake as a whole and that processing water in and of itself utilizes calories.
Short answer: It seems to be the case that drinking more water should help with weight loss, a lot of the studies that confirm this however are not great quality. However, some of the mechanisms are self explanatory, such as recucing calories, and filling yourself up before starting a meal. Some of the more complex hormonal mechanisms for linking hydration and weight loss do require further study.
Drinking More Water and Caloric Intake Reduction
The simplest mechanism in which water helps weight loss is the reduction of caloric intake. By drinking water, individuals may feel fuller and more satiated, leading to a decreased desire to consume calorie-dense beverages or food (Popkin, 2005).  It has also been suggested that people who confuse thirst signals with hunger may be less likely to overeat when adequately hydrated (Mattes, 2010). 
In a study by Dennis et al. (2010),  it was observed that middle-aged individuals who consumed water had a 9% lower caloric intake compared to those who did not. This finding suggests that increasing water consumption may help reduce overall calorie intake, contributing to weight loss efforts.
Metabolic Rate and Energy Expenditure
One of the mechanisms through which increased hydration may contribute to weight loss is by promoting lipolysis, the breakdown of fat cells. Studies have shown that increased water intake can lead to an increase in lipolysis in both humans and animals. This increase in fat metabolism is thought to be related to an increase in cell volume due to hydration, which in turn enhances insulin sensitivity and promotes fat breakdown.
Drinking water has also been proposed to affect metabolic rate and energy expenditure. Even a mild dehydration of 1% can cause a drop in metabolic rate (Thornton, 2010).  By staying well-hydrated, individuals may be more disposed to increased physical activity, thereby burning more calories and aiding in weight loss (Shaw, 2009) . This is sometimes described as water induced theromogenesis.
Water-induced thermogenesis is a phenomenon that occurs when the body's metabolic rate increases in response to water consumption. In a study published in The Journal of Clinical Endocrinology and Metabolism , researchers found that drinking 500 ml of water increased metabolic rate by 30% in both men and women. This increase in metabolic rate was observed within 10 minutes after water consumption and reached its peak 30-40 minutes later, lasting for over an hour. The researchers estimated that increasing water intake by 1.5 liters per day could lead to a daily energy expenditure increase of approximately 200 KJ, resulting in the burning of 2.4 kg of adipose tissue over one year.
Stookey et al. (2008)  conducted a study that demonstrated increased fat oxidation in normal-weight individuals who drank water instead of orange juice following a breakfast. Despite consuming more calories than they expended, these individuals still experienced increased fat oxidation. This suggests that water intake may play a role in facilitating the breakdown and metabolism of fat.
Furthermore, hydration has also been linked to improved mitochondrial function, the powerhouse of our cells responsible for energy production. As hydration levels increase, mitochondrial function may improve, leading to an increase in metabolism and ultimately aiding in weight loss and fat burning in general.
The Hormonal Reasons Water Helps Weight Loss
To understand the connection between hydration and weight loss, we need to discuss the role of a hormone called angiotensin II (AngII). AngII is a key player in body fluid regulation and acts on receptors in the body to stimulate thirst, conserve water, and regulate blood pressure. Studies have shown that increased levels of AngII can lead to excessive thirst and sodium appetite, which in turn can affect body weight.
In animal studies, researchers have observed that manipulating the central and peripheral renin-angiotensin system (RAS) can lead to increased drinking and a decrease in body weight. For example, administration of AngII into the brains of rats resulted in increased drinking and a decrease in food intake, leading to weight loss. Similarly, rodents treated with RAS inhibitors, such as angiotensin-converting enzyme (ACE) inhibitors or AT1-specific antagonists, showed increased drinking, reduced food intake, and weight loss, mainly through loss of fat. 
Practical Implications: Drinking More Water for Weight Loss
Understanding the connection between hydration and weight loss can have practical implications for individuals seeking to manage their weight. Incorporating strategies to increase water intake may be a simple yet effective approach to support weight loss efforts. Here are some practical tips:
- Drink water before meals: Consuming a glass of water before meals can help reduce calorie intake and promote feelings of fullness, leading to reduced food consumption.
- Stay hydrated throughout the day: Make a conscious effort to drink water regularly throughout the day, even when you're not feeling thirsty. This can help maintain optimal hydration levels and support metabolic function.
- Monitor water intake: Keep track of your daily water intake to ensure you are meeting recommended hydration goals. There are several smartphone apps and wearable devices available that can help you track and monitor your water consumption.
- Choose water as your primary beverage: Opt for water as your go-to beverage instead of sugary drinks or juices. Water is calorie-free and hydrating, making it an excellent choice for weight management.
- Be mindful of other factors: While hydration is an important aspect of weight management, it is essential to consider other factors such as a balanced diet, regular exercise, and overall lifestyle choices to achieve sustainable weight loss.
Further Research on Hydration and Weight Loss
To fully understand the impact of water consumption on weight loss, it is essential to examine the existing research in this field. Several randomized clinical trials (RCTs) have investigated the effects of water consumption on weight loss outcomes. While many of the studies on the link between hydration and weight loss have been conducted in animal models, there is evidence to support this connection in humans as well.
Several studies have shown that increased water intake is associated with weight loss in obese, overweight, and normal individuals. For example, a study found that adding 500 ml of water before a meal reduced energy intake and promoted weight loss.
Increasing Daily Water Intake - One study, conducted by Muckelbauer et al. (2009),  examined the effect of increasing daily water intake on weight loss. The researchers provided water and nutritional education to 2nd and 3rd graders in selected schools, while control schools did not receive these interventions. The study found that children who were given water and nutritional education had higher water consumption and lower weight compared to those who did not receive these interventions.
Replacement of Caloric Beverages with Water - Another study focused on the replacement of caloric beverages with water as a weight loss strategy. In the CHOICE trial (Piernas et al., 2013),  multiple interventions were implemented, and it was found that substituting high-calorie beverages with water supported weight loss efforts.
Premeal Waterload - In a study by Stookey et al. (2016), the researchers investigated the effect of premeal waterloads on weight loss. The participants were given a water preload (500 ml) 30 minutes before a meal. The study found that this intervention led to a short-term decrease in energy intake and increased fat absorption, contributing to better weight management. 
Furthermore, a high-protein diet, which is known to aid in weight loss, has also been associated with increased water intake. This increase in water consumption may be related to increased urine output needed to excrete the byproducts of protein metabolism. Additionally, studies have shown that a hypocaloric diet, or a calorie-restricted diet, can lead to increased water intake and weight loss.
Conclusion on Hydration and Weight Loss
When considering all the studies, it is evident that water consumption has a positive impact on weight loss. The studies reported weight loss effects ranging from -0.4 kg to -8.8 kg, with a mean percentage of weight loss of 5.15%. The most effective intervention among the studies was the replacement of caloric beverages with water.
However, it is important to note that the quality of evidence for the primary outcome of weight loss was rated as low to moderate. Additionally, the follow-up periods in these studies were relatively short. Therefore, more long-term research is needed to establish the efficacy of water consumption as a sustainable weight loss strategy.
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