term nature of the studies, 9 days or less, as well as the rapid weight loss which occurred in the
first few days of the high fat diets, indicate that the supposed fat loss which was occurring was
coming primarily from changes in water balance (33,34), which can contribute anywhere from 5
to 15 lbs of weight loss within a few days (see next section). Later studies using the same
experimental design, determined that the weight lost and counted as fat was water and that there
was no ‘metabolic advantage’ to low carbohydrate diets in terms of weight loss (35,36).
Water loss on the ketogenic diet
A well established fact is that low-carbohydrate diets tend to cause a rapid loss of water in
the first few days. This occurs for several reasons. First and foremost, glycogen is stored along
with water in a ratio of three grams of water for every gram of stored carbohydrate (37). As
glycogen is depleted, water is lost. For large individuals, this can represent a lot of weight.
Additionally, ketones appear to have a diuretic effect themselves causing the excretion of
water and electrolytes (38). This includes the excretion of sodium, which itself causes water
retention. Electrolyte excretion is discussed in greater detail in the next chapter.
Due to confusions about weight loss and fat loss (see chapter 8), many individuals are
drawn to low-carbohydrate diets specifically for the rapid initial loss of water weight. During the
first few days of a ketogenic diet, water loss has been measured from 4.5 to 15 lbs (17,39-41).
Although transient, this rapid initial weight loss can provide psychological incentive for
dieters, which may mean greater compliance with the diet. In one study of subjects on a very-
low-calorie ketogenic diet adhered to their diet much more than individuals consuming more
carbohydrate, and who lost less weight (8).
Regardless of possible psychological benefits, it should be understood that the initial weight
loss on a ketogenic diet is water. This is especially critical for when individuals come off of a
ketogenic diet, either deliberately or because they ‘cheated’. The rapid weight gain which occurs
when carbohydrates are reintroduced into the diet, which can range from three to five pounds in
one day, can be as psychologically devastating to dieters as the initial weight loss was beneficial.
In the same way that fat cannot be lost extremely rapidly, it is physiologically impossible to gain
three to five pounds of true bodyfat in one day. This is discussed in more detail in chapter 14.
A final thing to note is that this water loss can be misinterpreted as a loss of protein-
containing lean body mass (LBM), depending on the method of measurement. (8). This may be
part of the reason that some studies find report a greater loss of LBM for ketogenic versus non-
ketogenic diets.
Weight loss
The fact that the initial weight loss on a ketogenic diet is from a loss of water weight has
led to a popular belief that the only weight lost on a ketogenic diet is from water, an attitude that
makes little sense. The question then is whether more or less true weight (i.e. non-water) is lost
on a ketogenic diet versus a non-ketogenic diet.