Tag: low carb diets

Can a low carb diet help or hinder your performance?

With so many diet trends out there, it’s hard to know which ones help or hinder your performance. One trend that’s recently become popular among endurance athletes is the low-carbohydrate high-fat (LCHF) diet. The idea behind it is that training with low glycogen (carbohydrate) stores trains your body to burn fat more efficiently and increases muscle adaptations to endurance exercise. Theoretically, this should allow you to keep going longer before reaching fatigue and may even help you avoid ‘bonking’ or ‘hitting the wall’ in a marathon (running out of stored carbohydrate).

Indeed, studies have found that ‘training low’ (i.e. with low glycogen stores) increases the number of mitochondria – the fat-burning powerhouses – in the muscle cells. Others have also shown that ‘training low’ enhances cell signalling proteins, gene expression and fat burning enzyme activity.

However, despite these cellular changes, there’s a lack of evidence that chronically training on a LCHF diet improves performance. What’s more, it has been shown to hamper the muscles’ ability to break down glycogen during high intensity exercise and hinder power output during sprinting. You literally lose your top gear.

The most definitive evidence comes from the ‘Supernova’ study of 21 elite race walkers who followed either a high carbohydrate, a periodised carbohydrate or a LCHF diet for 3 weeks. The researchers found that

• Although the LCHF diet increased the body’s ability to burn greater amounts of fat during exercise, it also reduced exercise efficiency i.e. the athletes required MORE oxygen to exercise at the same speed
• the LCHF diet impaired race performance
• High carb and periodised carb diets improved exercise economy and race performance

In practice, many athletes find that training low feels harder, especially when they want to pick up the pace. Think hills, intervals, tempo runs, breakaways or sprinting for the finish line in a race. The truth is high intensity exercise (such as running fast) feels easier when you eat carbohydrate (such as potatoes, rice, pasta and bananas).

Another downside of ‘training low’ is that it can deplete your immune function and increase the risk of infection. The key to preventing this is to avoid high intensity sessions when your glycogen stores are low.

Body fat may be available in abundance (compared with carbohydrate) but it is a ‘slow’ fuel, which means it converts to energy comparatively very slowly. You can produce up to 25–30 kcal per minute from carbohydrate but only 6 kcal per minute from fat. Train above 85% VO2max, and you’ll be burning almost exclusively carbs.

A LCHF diet may suit those doing mostly low intensity workouts, like long slow runs or ultras. It means you won’t need to carry as many high carb snacks with you! But if you want to train hard and fast, then you’re better off with carbs.

If you want to get the best of both worlds – maximum fat-burning plus maximum performance – then ‘flexible fuelling’ could be the way to go. It’s backed by cutting edge science and is used by some top endurance athletes to give them a competitive edge. It involves matching your fuel (carbs) to your workout. The advantage of this approach is that you get the dual benefits of ‘training low’ – namely an increased ability to burn fat – as well as the performance benefits of high intensity training.

Here’s how it works:

1. Before your easy or low intensity workouts (e.g. long, easy runs), minimise carbohydrate beforehand. This will encourage the muscles to become more efficient at using fat as fuel. Suitable pre-workout meals or snacks include an omelette or poached eggs, an avocado salad, or hummus with vegetables.
2. Before harder or high intensity workouts, eat carbohydrate-based meals or snacks. This will help you train at a higher intensity for longer. Good options include porridge, rice and beans, or potatoes with cheese.
3. The simplest way to ‘train low’ is to run before breakfast but other protocols may be used.
4. If you struggle with ‘training low’, try taking caffeine approximately 45 – 60 min before your session. The optimal ergogenic dose is around 3mg/ kg body weight. This can reduce perceived effort and fatigue.

What’s the evidence? The best evidence for this flexible fuelling comes from two recent studies. One study with 21 triathletes , found that those who cut carbs around selected training sessions (a ‘sleeping low’ protocol) for three weeks improved their cycling economy (power output per calorie) by 11 per cent, 10km running performance by 2.9 per cent, time to exhaustion during high intensity exercise by 12.5 per cent, compared with those who did all their training with high glycogen stores.

The second study with 11 cyclists found that the same ‘sleeping low’ protocol followed for just 6 days resulted in a similar (3.2%) performance improvement.

In summary, there may be small advantages to gained from ‘training low’ before specific workouts. The research is fairly limited and there’s no definitive proof that it leads to big performance improvements. If you do decide to try it, pick just one or two low-intensity sessions a week alongside your normal training when you are fully fuelled. Keep a training log and see whether it helps your performance or not. If you suffer gut problems when consuming food during running, then ‘training low’ may suit you. Also, not having to carry bars and gels during long runs may be helpful from a practical point of view.

If you enjoyed this post and want to find out more about sports nutrition, then check out my brand new book, The Runner’s Cookbook. It features more than 100 delicious recipes to fuel your running. With a foreword from five-time Olympian Jo Pavey, the book also provides practical nutrition advice on fuelling before, during and after running, guidance on hydration and supplements, weight loss tips, how to recover from injury, and how to prepare for 5k, 10k, half marathons, marathons and ultra races.