the brain will not be using glucose for energy, any incoming carbohydrates can be diverted to the
muscles (4). Obviously, if no dietary carbohydrates are consumed following training, ketosis
should be maintained.
Aerobic exercise decreases blood flow to the liver which should decrease the availability of
FFA for ketogenesis (4-6). However, this is offset by an increase in FFA availability and
extraction by the liver (3-7).
If ketone body levels are low at the onset of exercise, there is an increase in ketone
concentrations during exercise. If ketone body levels are high during exercise (above 2-3 mmol),
exercise has little effect on overall ketone body levels simply because they are already high (i.e.
levels of ketosis will not deepen). This reflects one of the many feedback loops to prevent
ketoacidosis during exercise and afterwards (4). High levels of ketones inhibit further fat
breakdown during exercise although insulin levels still decrease. The primary fuel for exercise is
FFA and the body will simply use the FFA already present in the blood for fuel.
Section 2: High-intensity exercise
Very little research has looked at the effects of high-intensity exercise on establishing
ketosis or post-exercise ketosis. However, we can make some educated guesses based on what is
known to occur during high-intensity exercise.
During high-intensity exercise, the same overall hormonal picture described above occurs,
just to a greater degree. Adrenaline and noradrenaline increase during high-intensity activities
(both interval and weight training). The large increase in adrenaline causes the liver to release
liver glycogen faster than it is being used, raising blood glucose (8,9). While this may impair
ketogenesis in the short term, it is ultimately helpful in establishing ketosis. Insulin goes down
during exercise but may increase after training due to increases in blood glucose. Glucagon goes
up also helping to establish ketosis. Probably the biggest difference between high and low-
intensity exercise is that FFA release is inhibited during high-intensity activity, due to increases
in lactic acid (10).
Many individuals report finding a decrease in urinary ketones (or a complete absence)
following the performance of high-intensity exercise. Most likely, this reflects a temporary
decrease in blood FFA concentrations and increase in blood glucose and insulin. Additionally, the
large increase in adrenaline and noradrenaline decreases blood flow to the liver further decreasing
FFA availability for ketone production.
So while high-intensity exercise is arguably the quickest way to establish ketosis (due to
its effects on liver glycogen breakdown), the overall effect of this type of exercise could be
described as temporarily anti-ketogenic. The solution to this dilemma is simple: follow high-
intensity activity (to empty liver glycogen) with low-intensity activity (to provide FFA for ketone
formation). Ten to fifteen minutes of low-intensity aerobics (below lactate threshold) following
intervals or a weight workout should help to reestablish ketosis by lowering blood glucose and
providing FFA for the liver.The impact of different forms of exercise on ketosis appears in table 1.