Effects of exercise on ketosis

Post-exercise ketosis

Simply restricting carbohydrates will establish ketonuria (presence of ketones in the urine) given enough time (typically 3-4 days). The requirements for the establishment of ketosis are discussed in detail in chapter 4 but are briefly reviewed here. Additionally, exercise interacts with carbohydrate restriction and affects ketosis. This chapter discusses the role of exercise, both in helping to establish ketosis, as well as its impact on ketosis once established.

A recap of ketogenesis

Ketosis requires a shift in the liver away from triglyceride synthesis and towards free fatty acid (FFA) oxidation and ketone body formation. Blood glucose must also drop, lowering insulin and increasing glucagon and decreasing the I/G ratio. Along with this hormonal shift, there must be adequate FFA present for the liver to produce ketones.

Exercise is inherently ketogenic and all forms of exercise will increase the rate at which the liver releases its glycogen, helping to establish ketosis. As the rapid establishment of ketosis is important for individuals using the CKD approach (who only have 5-6 days to maximize their time in ketosis), strategies for entering ketosis are discussed. The overall effects of exercise on ketone concentrations is also discussed.

Section 1: Aerobic exercise

It has been known for almost a century that ketones appear in higher concentrations in the blood following aerobic exercise (1). During aerobic exercise, liver glycogen decreases, insulin decreases, glucagon increases and there is an increase in FFA levels in the bloodstream. During aerobic exercise, there is a slight increase in blood glucose uptake which peaks around ten minutes. To maintain blood glucose, the liver will increase liver glycogen breakdown, keeping blood glucose stable for several hours. Thus, the overall effect of aerobic exercise is to increase the production of ketone bodies .The increase in ketone bodies during exercise is smaller in trained versus untrained individuals, due to decreased FFA mobilization during exercise .

Aerobic exercise can quickly induce ketosis following an overnight fast. One hour at 65% of maximum heart rate causes a large increase in ketone body levels. However, ketones do not contribute to energy production to any significant degree . Two hours of exercise at 65% of maximum heart rate will raise ketone levels to 3mM after three hours. High levels of ketonemia (similar to those seen in prolonged fasting) can be achieved five hours post-exercise . 
This increase in ketone bodies post exercise allows for glycogen replenishment in the muscle. Since the brain will not be using glucose for energy, any incoming carbohydrates can be diverted to the muscles . 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 . However, this is offset by an increase in FFA availability and,extraction by the liver .

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 . 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 . 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 lowintensity exercise is that FFA release is inhibited during high-intensity activity, due to increases in lactic acid .

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 highintensity 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 :

Summary

Low-intensity aerobic exercise, below the lactate threshold, is useful for both establishing ketosis following an overnight fast as well as deepening ketosis. High-intensity exercise will more quickly establish ketosis by forcing the liver to release glycogen into the bloodstream. However it can decrease the depth of ketosis by decreasing the availability of FFA. Performing ten minutes or more of low-intensity aerobics following high-intensity activity will help reestablish ketosis after high-intensity activity.

Guidelines for Establishing and Maintaining Ketosis

      1. After a carb-up, if not weight training the following day, perform 45’+ of low-intensity aerobic exercise (~65% of maximum heart rate) to deplete liver glycogen and establish ketosis without depleting muscle glycogen. Interval training will establish ketosis more quickly by depleting liver glycogen but will negatively affect your leg workout.

OR

       2. Perform a high-intensity workout (weight training or intervals) followed by 10-20’ of lowintensity aerobics to provide adequate FFA for the liver to produce ketones.

       3. Perform 10-15’ of low-intensity aerobics after high-intensity training to provide FFA for the liver for ketone body formation.