First and foremost, the extremely low calorie nature of the PSMF makes medical supervision an
absolute requirement as frequent blood tests must be performed to watch for signs of metabolic
abnormalities (44). Additionally, the excessively low calories will cause a decrease in metabolic
rate making weight regain more likely than if a more moderate approach is used.
Typically the PSMF is only used with cases of morbid obesity, when the risks associated
with the PSMF are lower than the risks associated with remaining severely obese, and where
rapid weight loss is required (44,45). In fact, the PSMF has been shown to be more effective in
individuals who are obese versus those who are lean (43,46).
The ketogenic diet at maintenance calories
A popular belief states that fat can be lost on a ketogenic diet without the creation of a
caloric deficit. This implies that there is an inherent ‘calorie deficit’, or some sort of metabolic
enhancement from the state of ketosis that causes fat to be lost without restriction of calories.
There are several mechanisms that might create such an inherent caloric deficit.
The loss of ketones in the urine and breath represents one mechanism by which calories
are wasted. However, even maximal excretion of ketones only amounts to 100 calories per day
(47). This would amount to slightly less than one pound of extra fat lost per month.
Additionally since ketones have fewer calories per gram (4.5 cal/gram) compared to free
fatty acids (9 cal/gram), it has been suggested that more fat is used to provide the same energy
to the body. To provide 45 calories to the body would require 10 grams of ketones, requiring the
breakdown of 10 grams of free fatty acids in the liver, versus only 5 grams of free fatty acids if
they are used directly. Therefore an additional 5 grams of FFA would be ‘wasted’ to generate
ketones.
However, this wastage would only occur during the first few weeks of a ketogenic diet when
tissues other than the brain are deriving a large portion of their energy from ketones. After this
point, the only tissue which derives a significant amount of energy from ketones is the brain.
Since ketones at 4.5 calories/gram are replacing glucose at 4 calories/gram, it is hard to see how
this would result in a substantially greater fat loss. Anecdotally, many individuals do report that
the greatest fat loss on a ketogenic diet occurs during the first few weeks of the diet, but this
pattern is not found in research.
Only one study has examined a long term ketogenic diet at maintenance calories (17).
Elite cyclists were studied while they maintained their training. Over the span of four weeks
there was a small weight loss, approximately 2.5 kilograms (~5lbs) which was quickly gained
back when carbohydrates were refed. This loss most likely represented water and glycogen loss,
and not true fat loss. Whether this would be different with weight training is unknown. But it
does not appear that a ketogenic diet affects metabolism such that fat can be lost without the
creation of a caloric deficit.
Strangely, some individuals have reported that they can over consume calories on a
ketogenic diet without gaining as much fat as would be expected. While this seems to contradict
basic thermodynamics, it may be that the excess dietary fat is excreted as excess ketones rather
than being stored. Frequently these individuals note that urinary ketone levels as measured by