High-fat, low-carb diets: how to try one for yourself

If you haven’t already, we recommend you read part 1 of this series before reading on.

Follow-up lab results

In the first part in this series we presented results from Joe’s lab, showing two athletes’ responses to a incremental step test in which they cycled with an increasing power output. We measured the percentage of energy the athletes got from carbohydrates and the percentage they got from fat.

We repeated the tests 10-15 weeks later — after the athletes had completed their HFLC diets — and compared the two tests, showing just how differently the two athletes responded.

Athlete 1:

Here’s how Athlete 1 sourced energy in the step test before taking on a HFLC diet:

Pre-fat adaptation: Athlete 1

And here’s how Athlete 1 sourced energy in the same test after the HFLC diet:

Post-fat adaptation: Athlete 1

Athlete 1 was able to ride at a much higher intensity using a lot less carbs after the period of reduced carb (and higher fat) eating. While the reduced reliance on carbs occurred across all intensities, from about 74% up to 100% VO₂max this athlete still relied heavily on carbs as the predominate fuel source.

Athlete 2

Here’s how Athlete 2 sourced energy in the step test prior to a HFLC diet:

Pre-fat adaptation: athlete 2

And here’s how Athlete 2 responded to the same test after the HFLC diet:

Post-fat adaptation: athlete 2

Athlete 2 completed a solid 10 weeks training block during this period, and lost significant weight in the process. This combination explains the increased relative power output between tests. The post-HFLC weight was one this athlete last saw in his teenage years, despite being a regular Ironman and half-Ironman podium finisher over the last decade.

As you can see from the graph, the combination of 10 weeks solid training and a HFLC diet significantly increased the proportion of fat contributing to total energy production, all the way up to 100% of VO₂max. The test was stopped at this point, so we don’t have data to show what happened to fat and carbohydrate use at higher intensities, such as during sprints or all-out hill climbing. But being a long course triathlete, such intensities are not so relevant to him anyway.

How to achieve “fat adaptation”

Being “fat adapted” isn’t so much about eating lots of extra fat as it is about changing the amount of fat and carbs available to the body. By restricting the carbohydrate in the diet, the body is forced to utilise fat as its main energy source. At least some of the carbs will need to be replaced with fat though, otherwise you’ll be in a massive energy deficit. A small energy deficit is fine and will result in weight loss if desired, but there’s health risks from very large energy deficits (click here for a previous article on the topic).

So how low do you need to go on the carbs? Because of the fairly limited data on the topic, there’s no particular level recommended as a maximum per day before the affects of fat adaptation are not realised. But suffice to say the level of carbohydrate restriction would need to be fairly tight to achieve the changes seen in the lab results of Athlete 2, probably less than 100 grams of carbs per day.

Failure to restrict carbs enough could land you in no-man’s-land — not getting the changes from fat adaptation, but also being under-fuelled with carbs to tackle harder training sessions, and requiring lots of carbs to prevent bonking on the bike.

Cutting carbs

So what does 100g of carbs look like? Well, a slice of bread is about 15-20g. A banana’s another 20g. A latte is around 10g (add more for sugar). Add in a tub of strawberry yoghurt, an apple and some marinade or barbeque sauce on your steak and that’s your carbs for the day.

Here’s a list of carbohydrate containing foods. Have a look before committing to a HFLC diet; these are the foods you’ll be seeing a lot less of.

• Cereals and grains (including oats)
• Fruit (except berries)
• Biscuits
• Bread, toast and wraps
• Fruit juice
• Cakes
• Pasta
• Yoghurt (small amounts of plain yoghurt are OK)
• Lollies
• Rice
• Sugar as an ingredient in anything
• Potato
• Milk (small amounts are OK)
• Sports drinks
• Corn
• Ice cream
• Soft drinks
• Legumes
• Cordial

But it’s not all doom and gloom. Here are some of the things you can eat on a HFLC diet:

Breakfast: Eggs with bacon or deli meat, spinach, avocado and mushrooms, black coffee OR Specifically formulated low-carb muesli made from nuts and seeds (no grains), with a small amount of plain yoghurt or milk

Morning snack: A handful of nuts (e.g. almonds), or non-sweetened peanut butter with non-starchy vegetables (e.g. celery or capsicum)

Lunch: Meat, fish or chicken cooked in oil, with salad (including avocado and vinaigrette dressing) OR Omelette with cheese, ham and non-starchy vegetables

Afternoon snack: Plain yoghurt with berries

After a ride: Whey protein isolate, egg protein isolate or pea protein isolate OR Tin of tuna, salmon or chicken

Dinner: Meat, fish or chicken cooked in oil, with non-starchy vegetables or salad and oil & vinegar dressing

Dessert: Cheese and olives, vegetables with low-carb dip

Things take time

It can take a few weeks for the body to adjust to exercising using predominately fat as its energy source. Some people have described symptoms of “carbohydrate withdrawl” in the early days of a HFLC diet — headaches, hunger and cravings, fatigue and tiredness on the bike and, not surprisingly, a tendency towards bonking (hypoglycaemia).

These symptoms usually disappear after the initial period, provided the level of carbohydrate restriction is sufficient. We (Joe and Cliff) are currently investigating other novel approaches that may minimise these symptoms, including supplementation of specific types of fats and amino acids.

What did the two athletes tested do to achieve “fat adaptation”?

By his own admission Athlete 1 had a ‘sweet tooth’ and would often consume high amounts of refined sugary foods prior to his HFLC intervention. He continues to eat some higher carb meals (e.g. oats for breakfast) and quite a few pieces of fruit, but has cut bread, rice, pasta and potatoes back to only once or twice a week. He also minimises all sources of sugar.

He’s reduced the amount of food he takes on the bike to an occasional banana plus water with no bonking issues (although it wasn’t an issue beforehand either). Athlete 1 certainly doesn’t eat a super-low carb diet, but it’s much lower than what he previously ate.

Athlete 2 began trying to follow a strict HFLC diet with less than 50g of carbs a day, but quickly found it too restrictive for his liking. Instead he removed all grain-based foods and fruit. This suited his personal taste preferences and was far more sustainable over the testing period.

Our opinion – Is the low carb, high fat approach the way to go?

Cliff Harvey:

I believe that for most people, most of the time, a ‘lower-carb’ approach is the way to go. By this ‘lower-carb’ I don’t necessarily mean a typical keto-diet (restricting carbohydrate to very low levels), but instead a nutrition strategy that allows for someone to become more highly fat-adapted, and thus able to use fat for fuel, and also alternate fat fuels at higher thresholds of exercise intensity.

This strategy also allows the athlete to take in enough carbohydrate (especially during exercise and after training) to replenish muscle glycogen and allow for efficient, effective high-threshold activity bursts.

Joe McQuillan:

I believe athletes will report positive and negative outcomes on this type of dietary intervention, be they professional or recreational cyclists. Moreover, there is a ‘metabolic flexibility’ in which someone who is on a HFLC diet has the option to eat a reasonable carb intake but still remain ‘fat adapted’; we do this as part of preparing for, assisting with or recovering from high-intensity training bouts.

Would I encourage you to try the HFLC approach? Absolutely! This would be for both cycling and as an alternative lifestyle choice, just as I would encourage further informed reading to assist your education.

While I’m not anti-carbs it is a necessity to take a healthy dose of scepticism when intending to consume gels, carb based drinks and other high GI products that we are encouraged to feast on as endurance athletes all the time. Aside from the competitive side of sport, a lot of us participate for the associated health benefits. The health aspect of a HFLC diet is a much larger part of the overall picture than what the sporting aspect is.

In summary, if you want to be competitive in a sport such as road cycling there is an absolute need for some level of carbohydrate supplementation. This can be employed in the lead up to and during the event. Likewise in times of high intensity training bouts some carbs before, during and after is recommended.

Similarly, steady state ultra-endurance athletes will still stand to benefit from carbohydrate supplementation, but by being fat adapted will have less carbs required during exercise, and reduce the chances of gastric upset.

Alan McCubbin:

Asking if HFLC is the answer for cyclists is a bit like asking “what type of bike should I buy?”. The answer is going to be different for different people, in this case depending on their level of training, racing, body composition and health.

I’m not convinced that the HFLC approach will provide a benefit to competitive road cyclists. In road cycling inevitably the outcome of the race is determined by short periods of very high intensity riding (e.g. establishing a break, sprinting, dragging off an opponent on a climb, etc). It’s likely that the HFLC approach will probably reduce the ability to put out maximum power when the race is up for grabs.

It’s not a surprise to me that the professional peloton hasn’t embraced the HFLC concept — if there was a benefit to it they would’ve been doing it years ago (after the research of the early 2000s).

But it’s not about lots of carbs all the time either. I would suggest that for competitive cyclists a more useful approach is to adjust carbohydrate intake according to training and competition needs. Here’s a previous article I’ve written on the topic.

Recreational cyclists are a group that may stand to benefit more from the HFLC approach. If you ride for fitness, commuting, enjoyment and a sense of personal achievement, then absolute top-end power probably isn’t that crucial to you. If that’s the case then the HFLC strategy may have benefits. There’s less risk of bonking on long rides, it’s a method of reducing body fat, and potentially leads to a reduced risk of chronic disease.

Perhaps the most interesting group is those who participate in long, slow, distance events such as 24-hour mountain biking, ultra-running and adventure racing. Without the super-high intensity efforts required for victory, there is at least a theoretical potential that the HFLC approach may be beneficial to this group of athletes.

There are examples of ultra-endurance athletes who have succeeded on reduced carbs, but caution should be taken when interpreting the hype surrounding them.

In 2012 Timothy Olsen broke the Western States 100-mile ultramarathon record. But even as the poster-boy for HFLC diets, Olsen himself conceded in an interview that he still eats extra carbs (mainly sweet potato) the night before a high intensity training session, and his blog account of the race suggests that he consumed at least 30-40 grams an hour of carbs during his record-breaking attempt.

In summary

It’s worth carefully considering whether a strict HFLC strategy is right for you or not. The journey to becoming “fat adapted” can be a rocky one for some, particularly the first couple of weeks. You also need to consider whether you’re be prepared to give up your favourite high-carb foods, and whether you can maintain a low enough carb intake to achieve the changes described.

Some recreational and amateur cyclists rave about HFLC diets and clearly it’s helping them achieve their goals, but we’re yet to see any of the professionals (or their advisors) embrace the concept.