In this blog post, nutritionist Dani Hofstetter debunks some myths surrounding two currently trending diets: the ketogenic diet and intermittent fasting. In a concise yet precise manner, he explains what these diets are all about.

By Dani Hofstetter*. At this time of year, an important phase in their preparation begins for many endurance athletes from Central Europe with their first training camps in the south. During these days, when they can leave their daily routines behind and “train-eat-sleep-repeat” is the order of the day, many turn to the web to find out which new “miracle weapons”—whether in nutrition, training, or equipment—will make the new season even more successful. Ever since a media-savvy U.S. politician told us about “fake news,” we’ve known that not everything found online can be taken at face value.

Ketogenic diet (“Keto”)

Over the past decade, poor carbohydrates have suffered a severe blow to their reputation. Anyone who enjoys a large serving of pasta in their daily life is met with suspicion and skepticism among their friends, and even amateur and professional athletes have sworn them off. An extreme form of this so-called “low-carb” trend is the ketogenic diet.

Our bodies recognize ketosis as an emergency response for times of scarcity when carbohydrate intake is insufficient. “Hallelujah!” shout all the Ironmen and ultratrail runners, “I want to be less dependent on carbohydrates, train my fat-burning ability, and reduce my body fat—so this comes at just the right time.” Unfortunately, this enthusiasm is a bit premature…

Ketosis – the facts: Anyone who consumes less than 50 grams of net carbohydrates per day (equivalent to the amount found in 220 grams of cooked pasta, 180 grams of cooked rice, or 110 grams of whole-grain bread) forces their body into this “starvation mode,” prompting it to produce ketone bodies as an alternative to glucose to supply our brain, heart, lungs, and red blood cells—our muscles can largely rely on aerobic energy production from fat. At the same time, this leads to a significant reduction in our carbohydrate stores (glycogen) and, consequently, increased energy production from body fat.

A side effect of this is very limited performance capacity, a lack of recovery ability, and, not least, the absence of more intense training stimuli—because you don’t have enough carbohydrates to perform in the aerobic/anaerobic zone and largely rely on aerobic fat burning to keep your engine running. Ketosis leads to foul-smelling, sweet-and-sour breath—so the flirt factor is well below zero.

The long-term effects of “permanent ketosis” are controversial, but current scientific evidence advises against it. I definitely cannot recommend it as a targeted measure to improve performance. The extreme form has far more negative and uncertain aspects than benefits overall. What has proven effective at the elite level for several years is the targeted periodization of carbohydrate intake throughout the year. During extensive GA1 phases, reducing carbohydrate intake to at least 30% of total caloric intake can make sense—but that still amounts to well over 50 grams per day. When it comes to competition, the evidence is clear: athletes who can consume more carbohydrates during a race perform better.

Innovative food manufacturers are touting ketone bodies in small bottles as the latest miracle energy boost. Team Sky made headlines because this was supposedly the “secret sauce” behind Chris Froome’s Tour victories. One serving costs about 35 francs, making it arguably the most expensive energy shot in history. Something that expensive must be rocket fuel, right? Far from it: the exogenous intake of ketone bodies is supposed to induce a sort of “instant ketosis” and thus conserve carbohydrates for later in the race. However, scientific studies have so far failed to prove any clear positive effect. Critics claim these keto shots taste so disgusting that anyone who doesn’t immediately throw up after drinking them inevitably benefits from a compensatory positive placebo effect.

Intermittent fasting

Whether it’s a fitness magazine, an endurance blog, or a fashion magazine—we’re living in the age of intermittent fasting. Our daily lives have long been disconnected from the natural day-night cycle, but our bodies have an internal clock that cycles through various processes over approximately 24 hours, and this is exactly where intermittent fasting comes into play.

Facts about intermittent fasting: limiting our calorie intake to a relatively short window of 6–10 hours (out of a total of 24 hours) is said to relieve our metabolism from constant activity, thereby reducing aging and inflammatory processes, using dietary energy more efficiently, and making us much more productive. For completely sedentary people who don’t move (let alone exercise), this works well—initial, reliable studies show exciting effects at the cellular level and surprising differences in how our bodies react to food depending on the time of day or waking hours.

While positive changes can be achieved in people who do not lead a very healthy lifestyle or who already suffer from an existing medical condition, the question we want to address here is: Is there potential for improved performance for the ambitious athlete?

The short answer is clear: No. In this case, the training load spread throughout the day is more important than the circadian rhythm. As athletes, we want to ensure that our energy stores are optimally aligned with the upcoming workout. This might mean doing a moderate-intensity endurance session on an empty stomach, tackling a 5-hour bike ride with many steep climbs while fully fueled, and then immediately promoting recovery with fast-acting carbohydrates—but the timing of the workout is more important than restricting food intake to a specific time of day. When many athletes claim they’ve lost weight through intermittent fasting, it’s primarily because the time-based restriction—especially for working athletes—inevitably leads to an energy deficit. And anyone who consumes significantly less energy will inevitably lose weight—in the form of fat and muscle mass.

For me, the social, practical, and logistical constraints that come with having to eat within a very specific time window are already a major drawback. When I factor in the reduced performance levels for certain workouts, I would advise against it. The method can be useful in cases of injury, recovery after surgery, or extended training breaks when athletes want to maintain their weight as much as possible. However, anyone who wants to specifically reduce their weight while in full training mode—and, above all, lose body fat without sacrificing valuable muscle mass—should aim for a reasonable energy deficit in line with their training plan. Diet must be adjusted accordingly to limit excessive “catabolism” (muscle loss). In addition to professional planning, a high degree of discipline is essential to adhere to both the nutrition and training plans.