Over the past few years, diets like the low carb high fat (LCHF), paleo and ketogenic diets have become popular, especially among endurance and ultra-endurance athletes.

Many athletes claim to have improved their endurance and ultra-endurance performance after switching to a diet primarily fuelled by fat.

While recent evidence has proven low carb high fat (LCHF) can help put type 2 diabetes into remission and manage a range of other chronic diseases, the scientific evidence of its impact in high-performance sports is just beginning.

We’ve come a long way in 30 years

In the 1980s, I co-authored the first Australian sports nutrition book, Food for Sport, with dietitian Karen Inge. The main message of that book was that carbohydrates were the primary fuel for the athlete. And so it stayed for 30 years.

Any serious (or recreational for that matter) athlete was eating a diet full of carbs, loading up on carbs before running a marathon, and recovering with carbs. Carbs were in; fat was out. Sports drinks were in; water was out.

Now, with the benefit of hindsight, we can see the adverse effects of high sugar and carb intake, such as insulin resistance, metabolic syndrome, type 2 diabetes and other chronic diseases.

The traditional high carbohydrate athlete diet

We’ve usually associated poor nutrition with sedentary behaviour, but over the past 20–30 years, we’ve had our finest athletes on a very unhealthy diet.

For decades, athletes have fuelled up with carbs stored as glycogen in the liver and the muscle. The glycogen in our muscles acts as a carbohydrate store and to fuel activity. Leading up to competition, athletes make a concerted attempt to maximise glycogen by ‘carb loading’. However, there’s a limit to the amount of carbohydrates that can be stored. After exercising for some time (about two hours in most cases), the muscle glycogen stores become depleted and need to be restored.

The challenge for the athlete is how to replenish the stores while exercising. The tendency is to use simple, easily digestible carbohydrates such as sports drinks and gels (loaded with sugars).

It will be interesting to follow this generation of carb-loaded athletes and observe the health consequences they carry into middle age. For example, multiple Tour de France winner Chris Froome said on the consequences of consuming lots of carbohydrates on race days: “It’ll be three gels an hour towards the end. Plenty of riders get teeth problems from constant gels and drinks. I’ve had my fair share of fillings.”

Using fat as fuel

While fat can’t provide as much energy as carbs, it has one significant advantage: unlimited fat stores in the body. So even the thinnest athlete has enough fat to use as fuel for many hours of activity.

When the body is deprived of carbohydrates for fuel (as with a low carb or ketogenic diet), it uses other energy (mainly ketone bodies, known as ketones). After some time (usually one to two weeks, but sometimes longer), the body ‘adapts’ to this new fuel and starts to use fat for fuel rather than carbs. During that adaptation period, the athlete will often feel tired and unwell and may struggle to perform at a high level, a phenomenon known as “keto flu”.

One advantage of changing from a carb-dominant diet to a fat-dominant one is weight loss. It’s surprising how many highly trained athletes are still a few kilograms overweight, despite their large exercise loads. Weight loss is particularly appealing to an athlete, as the weight is not lost from muscle but fat, improving their power-to-weight ratio.

So what’s better for sports performance? Carbs or fats?

As with everything in nutrition, it’s created a huge controversy with the fat-as-fuel advocates lined up against the traditional carb-is-king supporters. However, they both have evidence to support their cases, and there’s probably room for both regimes in sport.

There’s no doubt that carbohydrates are the most effective fuel for shorter-term, high-intensity sports and exercise. However, the main concern with the emphasis on carbohydrates for fuel is the health issues we’ve learned to associate with high carb and high sugar intake. Surely that makes these athletes prone to insulin resistance and its complications – obesity, type 2 diabetes and chronic disease – as they reach middle age.

Fat seems to be the preferred fuel for moderate-intensity endurance activity, with less need to refuel for training, less recovery and perhaps even resulting in less inflammation. However, the evidence regarding an LCHF diet and its impact on sports performance is still largely inconclusive.

Many sports experts are now recommending athletes ‘train low, compete high’, meaning an LCHF diet fuels the training phase as the baseline, and on competition day, they’ll fuel with a moderate amount of carbs and revert to their LCHF diet following the game.

What’s it mean for the rest of us?

Suppose you’re not an athlete, and your physical limit is a daily walk, swim or gentle cycle; there’s really no need to ‘carb load’ for a brisk walk around the block. Our bodies have enough fat stores to fuel us for hours of moderate-intensity endurance activity. The latest nutritional and metabolic science points towards the benefits of a low carbohydrate, high fat diet as the key to putting type 2 diabetes into remission, in addition to other metabolic diseases.

Your everyday diet should be low carb, healthy fat, and real food. Avoid sugars, processed foods and seed oils. If you’re transitioning from a traditionally high carbohydrate diet, you’ll need time for your body to adapt to the change of eating patterns.

If you’re on the more active side and run a marathon from time to time, you may find that you need to top up with a moderate amount of carbs before and during higher intensity activity.

More high-quality studies are needed to measure the true impact of an LCHF diet in athletic activities. Watch this space.