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Hot Topics: Ketogenic Diets

Everyone is talking about it. Many are following it. But what is actually going on in the body when following a ketogenic diet? Is there a time when it is appropriate vs not?

Ketogenic diets are very low carbohydrate diets when compared to the current recommendation standards.

Current USDA guidelines suggest:

  • Carbohydrates: 45-60% of calories or 225-300 g of carbohydrates for a 2000 calorie diet

  • Protein: 15-30% of calories or a minimum of 0.8g/kg body weight, but with 1.2-2g/kg body weight shown to be the most beneficial for most populations

  • Fat: No more than 35% of calories from total fat, no more than 10% of calories from saturated fat

But a ketogenic diet consists of:

  • Carbohydrates: 10% of calories or <50 g of carbohydrates *primarily coming from leafy greens or non-starchy vegetables

  • Protein: similar to standard recommendations, typically aiming for 1.2-1.5g/kg body weight

  • Fat: Averaging 75-80% of calories

WHY is it so low in carbohydrates and high in fat?

To put it simply: we derive our energy from glucose (like from carbohydrates) and fatty acids (like from dietary fats). Under normal conditions, the body will preferentially use glucose from carbohydrates as the PRIMARY fuel source, especially with increased activity or stress, and use fat as a fuel source during low activity times like while at rest. *Both glucose and fatty acids are used simultaneously throughout the day, but where an increase in activity or stress will increase the use of GLUCOSE only.

Now when it comes to dietary restriction of carbohydrates, the idea is that by limiting glucose intake and availability, that you are forcing your body to primarily utilize fatty acids for fuel instead. [Enter the claims that you are a "fat-burning machine", or "fat-adapted"]

Take note, however, that NOT all tissues can utilize fatty acids (like the brain, red blood cells, etc) to where your body produces its own backups to accommodate for these situations. When glucose availability is low, and the body needs to rely on fatty acids for fuel, some of those intermediates will spill over and be converted into ketone bodies which CAN be utilized directly by those otherwise glucose-dependent cells and tissues like the brain. (Vargas, et al. 2018) In this sense, the body will still be able to function in a fat-adapted state when the preferred fuel source (glucose) is scarce. [This state of ketone formation, or ketosis, is the primary goal when following the diet as it indicates this shift from glucose utilization to primarily fat utilization. Hence, measuring urinary ketones to reflect your state of ketosis.]

Now come the sexy claims that you are a fat-burning machine that is surviving on your own fat stores, right?

Not quite.

Here's the thing:

1. Oxidizing fatty acids for energy is COSTLY...not in the price of foods, but in the cost of efficiency. Oxidizing fatty acids requires MORE oxygen per gram than oxidizing carbohydrates; or another way to look at it: for every liter of oxygen consumed, you can produce more ATP (energy) from glucose compared to fatty acids (Burke, et al. 2017) *This means A LOT when oxygen intake is limited like during exercise above a low-moderate intensity. Now fatty acids comparatively have a high capacity for energy production (a high amount of POTENTIAL energy from the limitless storage capacity in the body, and from the long carbon chain length of each fatty acid). (Burke, et al. 2017) However due to the higher oxygen demands for this fuel source, it is not ideal UNLESS under certain conditions, namely when oxygen intake is sufficient and sympathetic activity is low, like during REST and low intensity activity.

2. Being "fat-adapted" does NOT automatically equate to burning off all of your STORED BODY FAT but rather that you are better at breaking down dietary fat and storing it, and then re-releasing it from storage into your circulation to use it. Your body is so intelligently designed that it will ADAPT to the fuel source that is present and subsequently ADAPT to the demands for fuel by utilizing the most efficient fuel source for the task. (Smith, et al. 2018) This concept is also termed metabolic flexibility where there is an ability to shift metabolism based on supply and demand, and where increased metabolic flexibility can be an indication of good health. (Smith, et al. 2018) You SHOULD be able to adapt to the fuel source that is being provided by breaking down, storing, releasing, and utilizing it without any issues, and particularly when it is the most appropriate fuel source that is needed. (Smith, et al. 2018) [If exercise intensity increases, you should be able to adequately oxidize glucose for fuel quickly and efficiently. Likewise, during extended periods without food, you should be able to release and oxidize fatty acids for fuel to spare your stored glycogen.] In this case with ketogenic diets, you are getting good practice at storing, releasing, and oxidizing fatty acids because you are eating a diet high in fatty acids. i.e. a high SUPPLY of fatty acids coming in compared to glucose.

So then how is weight loss occurring?

Much of the observed and reported initial weight lost from following a ketogenic diet is due to decreased glycogen and fluid retention. (Greene, et al. 2018) After that initial loss, continued weight loss is due to a calorie deficit, not because of the type of diet (low carb/high fat diet.) (Aragon, et al. 2017) If reducing carbohydrate intake and increasing dietary fat intake HELPS you to portion control or feel satiated, and therefore you end up eating LESS (i.e. putting you in a calorie deficit), then yes, it will help will continued weight loss. However studies consistently show that it DOES NOT matter whether a diet is low carbohydrate/high fat or low fat/high carbohydrate, it is the calorie deficit that matters for weight loss (assuming protein is accounted for); specifically the diet and lifestyle that is most sustainable for the individual that leads to long term success. (Aragon, et al. 2017)

But that short term weight loss may be relevant in some sports...

...specifically competitive Olympic weightlifting and Powerlifting where there is generally a short term weight reduction prior to a competition. (Greene, et al. 2018) In this study, following a 3 month ketogenic diet resulted in greater losses of body mass, and lean mass, BUT without any changes in performance compared to 3 months of a 'usual diet'. (Greene, et al. 2018) This study suggests that strength and power in competitive weightlifting sports MAY be preserved while following a short term ketogenic diet. (Greene, et al. 2018) HOWEVER...this study did NOT require the athletes to actually 'make weight' for either dietary segment, but rather just follow a change of diet for 3 months [NOT in a calorie deficit or trying to lose weight] and monitor for changes in strength and power when performing 1RM lifts compared to following their usual diet. *The losses in body weight and lean body mass were due to depleted glycogen, water, and amino acid utilization from lean tissues. (Greene, et al. 2018) **This study did NOT imitate actual weight reduction strategies or competition prep, but rather focused on the effects of a diet change during a traditional strength training cycle.

Similarly, just eating a ketogenic diet while following a resistance training program (for non-competitive athletes) saw changes in body mass. In an 8 week study, a group of resistance trained men were randomized to either a ketogenic diet, a non-ketogenic diet, or a control group. (Vargas, et al. 2018) The two diet groups followed a resistance training program and a hyper-caloric diet to assess for changes in body composition including fat mass and muscle mass. (Vargas, et al. 2018) [The idea is that a calorie surplus with resistance training should increase lean body mass.] In the ketogenic diet group, there were significant decreases in fat mass and visceral adiposity (fat around the organs) compared to the non-ketogenic diet and the control group. (Vargas, et al. 2018) However it was ONLY the non-ketogenic group that saw increases in muscle mass and body weight compared to the ketogenic diet and control groups. (Vargas, et al. 2018) *DESPITE being in a calorie surplus, the ketogenic diet group did NOT increase muscle mass in response to resistance training compared to the non-ketogenic diet group.

And what about athletic performance?

When it comes to power and sprint performance, a short-term study randomized participants to either a low-carbohydrate ketogenic diet (< 10% calories from carbs, or < 50g/day) or a high carbohydrate diet (6-10g/kg body wt/day which is ALOT of carbohydrates). (Wroble, et al. 2018) The diets were matched for energy content but differed in their macronutrient compositions. (Wroble, et al. 2018) The study findings suggested a 7% decrease in peak power and 6% decrease in mean power during a maximal effort Wingate test, and a 15% decrease in total distance ran during a sprint yo-yo intermittent recovery test for the ketogenic diet group compared to the high carbohydrate group. (Wroble, et al. 2018) *The ketogenic diet group saw decreased power and anaerobic capacity.

Conversely, it has been suggested that endurance athletes who require a high amount of energy utilization (and fuel oxidation) for their longer duration sports, may benefit from being 'fat-adapted' or preferentially oxidizing fat as a fuel source because of it's relatively unlimited capacity. We can store a seemingly infinite stock of fatty acids in our adipose tissue, and the oxidation of ONE fatty acid has the capacity to produce a greater amount of ATP (energy) compared to ONE molecule of glucose (based solely on carbon chain length). HOWEVER, recall from above that once we take into account the role that oxygen plays in energy production, this process of oxidizing fatty acids is LESS EFFICIENT compared to oxidizing glucose where oxidizing a fatty acid requires MORE oxygen compared to oxidizing glucose.

Now the traditional biology and exercise science teachings emphasize:

  • Power, strength, and sprint actions require primarily ATP, creatine phosphate, and glycogen to fuel performance [TRUE]

  • Longer duration efforts would require a greater amount of energy coming from fatty acid oxidation [KIND OF]

...and with the most often used example ("where longer, slower efforts like marathons rely primarily on fatty acids") makes it seem like RUNNING A MARATHON is an easy, low intensity activity.

Unfortunately this oversight is a bit of an insult to competitive endurance athletes where for example the current world record for the marathon sits at 2:01:39 which is roughly 4:41 mile splits for all 26.2 miles, or turning up the treadmill to 13 mph and running for 2 hours nonstop. Elite endurance athletes are BOOKING it. Not to mention the adrenaline and sympathetic stimulation that goes along with executing such a high intensity, long duration effort. Does this sound like long, slow duration exercise with low to moderate intensity?

**In reality, it is the intensity and duration of the effort that dictate the primary fuel source.

But what if he's fat-adapted?

This idea that shifting your metabolism to burn fat more readily does NOT change the fact that certain conditions that must be present for fat oxidation to occur. Being able to burn fat easily while at rest is certainly to our benefit, but thinking that fat will be a more USEFUL fuel source during exercise is far-reaching. In fact, study findings of elite endurance athletes suggest impaired exercise economy and a lack of training benefits from high intensity training (like an expected increase in aerobic capacity) when following a ketogenic diet vs a carbohydrate-rich diet or even a periodized carbohydrate diet. (Burke, et al. 2017) *Their times did not change significantly, but they required a greater effort, perceived that it was more difficult via RPE rating, and failed to see performance improvements from their training whereas the other groups (that either ate high carbohydrates consistently, or carb-cycled) DID see training benefits and increased their exercise economy.

So WHEN is the ketogenic diet most appropriate?

Up until recently, the research on ketogenic diets for the last century was limited to only its clinical relevance for refractory epilepsy treatment. (D'Andrea, et al. 2019) Research studies suggest that some cases of drug-resistant epilepsy may be associated with a hyper-excitability in the brain, possibly associated with impaired glucose transporter functioning. (D'Andrea, et al. 2019) Study findings suggest that the ketogenic diet can mimic the effects of fasting where instead of glucose, there is an alternate fuel source in the brain (ketone bodies), of which has been shown to provide a temporary decrease in seizure activity. (D'Andrea, et al. 2019) *Most subjects follow the diet for an average of 3-6 months. Discontinuation of the diet is primarily due to its restrictive nature. It is proposed that the decrease in neuronal excitability associated with the ketogenic diet is related to preserved mitochondrial function, increased synaptic stability, and increased GABA synthesis. (D'Andrea, et al. 2019)

Similarly, it is suggested that cerebral glucose metabolism is impaired following a brain injury to where the utility of ketone bodies as an alternate fuel source may help to acutely preserve cerebral metabolic function during this immediate post-injury window of vulnerability. (Prins and Matsumoto, 2014) In this case, the idea is that an acute post-injury period of fasting, ketogenic diet, or calorie restriction beginning immediately after a brain injury could produce endogenous ketone bodies that can help to maintain cerebral metabolic functioning during otherwise impaired glycolytic metabolism. (Prins and Matsumoto, 2014) There are some interesting study findings that looked at the cumulative effects of multiple concussive injuries in rats where the rats that were given a ketogenic diet immediately following the first concussion showed improved cognitive function after the second concussion compared to controls. (Prins and Matsumoto, 2014) [BOTH groups showed declined cognitive function after multiple concussions, but the ketogenic diet group showed greater cognitive functioning compared to the other group, but still below baseline functioning.] However, the research in this area is limited to animal studies which means that the results, and the effects, do NOT directly transfer to humans, but can provide us with some insight and direction for our efforts (for example, a 24-hr fast in a rat may be equivalent to a few days or week-long fast in a human).

Major takeaway points:

  • Fat as a fuel source is best while at REST

  • Initial weight lost (from ketogenic diet) is from depleted glycogen and water

  • Continued weight loss occurs from a calorie deficit, NOT from the type of diet

  • Athletes will likely NOT benefit, and may even impair performance, if following or maintaining a ketogenic diet

  • Producing ketones MAY help preserve cognitive function in circumstances with cerebral trauma


Aragon A, Schoenfeld B, Wildman R, et al. International society of sports nutrition position stand: diets and body composition. Journal of International Society of Sports Nutrition. 2017; 14(16):

Burke L, Ross M, Garvican-Lewis L, et al. Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. Journal of Physiology. 2017; 595(9): 2785-2807.

D'Andrea M, Romao T, Pires De Prado H, et al. Ketogenic Diet and Epilepsy: What We Know So Far. Frontiers in Neuroscience. 2019; 13(5):

Greene D, Varley B, Hartwig T, et al. A Low-Carbohydrate Ketogenic Diet Reduces Body Mass Without Compromising Performance in Powerlifting and Olympic Weightlifting Athletes. The Journal of Strength and Conditioning Research. 2018; 32(12): 3373-3382.

Prins M, Matsumoto J. The collective therapeutic potential of cerebral ketone metabolism in traumatic brain injury. The Journal of Lipid Research. 2014; 55(12):2450-2457.

Smith R, Soeters M, Wust R, et al. Metabolic Flexibility as an Adaptation to Energy Resources and Requirements in Health and Disease. Endocrine Reviews. 2018; 39(4): 489-517.

Vargas S, Romance R, Petro J, et al. Efficacy of ketogenic diet on body composition during resistance training in trained men: a randomized controlled trial. Journal of the International Society of Sports Nutrition. 2018; 15(1): 31.

Wroble K, Trott M, Schweitzer G, et al. Low-carbohydrate, ketogenic diet impairs anaerobic exercise performance in exercise-trained men and women: a randomized-sequence crossover trial. Journal of Sports Medicine and Physical Fitness. 2018;

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