Low-carb diets have been hugely popular over the years, with the likes of The Atkins Diet and The Dukan Diet seeing millions going low carb in the early 2000s. However, aside from the health implications posed by such diets, sustainability has been a central problem, with dieters falling off the wagon and delving into the bread bin at the weekend. Practicality has proved problematic too, with dieters feeling too fatigued to exercise or even get through the working day. But what if there were a way to regularly switch up your carbohydrate intake, so that you could still lose weight, without feeling sluggish, and even indulge in garlic bread guilt free?
This phenomenon is known as carb cycling – a method of dieting that involves the strategic increase and decrease of carbohydrates to manipulate body composition. Recently, we’ve seen such diets gaining immensely popularity – namely Joe Wicks’ ‘Shift, Shape and Sustain’ plan, which has seen thousands transforming their bodies in just 90 days, and the Instagram star developing a lucrative fan-base in the process. In his plan (specifically phase one and three), carbs are cycled between workout and rest days, with high-carb meals enjoyed after exercise, consisting of HIIT and weight training, and low-carb meals eaten on rest days. The aim is to strip fat and build lean muscle; carbohydrates are eaten after exercise to top up blood glucose levels, prompting an insulin response to transport nutrients to muscles for growth, repair and recovery and, on rest days, carbohhydrates are restricted as a fuel source.
While a recent fad, such diets as a means of manipulating body composition are by no means new. “Carb cycling has been around in the world of bodybuilding for many years and it’s really developed as a way of manipulating your carbohydrate intake in order to achieve a very low body-fat level,” explains Anita Bean, former British bodybuilding champion, nutritionist and best-selling author of The Complete Guide to Sports Nutrition. “There wasn’t a huge amount of science behind it, other than it seemed to work,” she says. “The original idea behind it is actually incorrect. They say that having high-carbohydrate days speeds up the body’s metabolic rate, then when you go to low-carbohydrate days your body is still revved up to burn fat.”
One of the first to write about carb cycling for this purpose was world champion powerlifter Dr. Mauro DiPasquale back in 1995, in his book The Anabolic Diet. DiPasquale developed the diet as an alternative to performance-enhancing drugs, giving bodybuilders the tools to pack on extra muscle and simultaneously burn fat – naturally. Low-carb/high-fat/high-protein meals are eaten in the week (no more than 30g carbs per day) and high-carbohydrate meals are eaten at the weekend.
It can be easy to see the appeal of such diets – particularly if you’re allowed to eat bread at the weekend! And given carbohydrates aren’t completed restricted, you may wonder whether such a diet could work for you as a runner, if you were to simply swap the weights and HIIT sessions for miles, eating low-carbohydrate meals on rest days and enjoying high-carbohydrate meals on the days you train.
Unfortunately, it’s not that straightforward, primarily due to the conflicting purposes of a runner’s diet – the focus of which is performance – and that of someone looking to manipulate body composition for aesthetic reasons. “For runners, eating is all about fuelling your performance and doing things in a safe and effective way,” says Bean. While such diets do integrate exercise in the form of weight and HIIT training, prolonged cardio training, such as running, places very different energy demands on the body, with runners requiring extra fuel – in the most accessible form – to meet these demands. “Carbohydrates are the body’s primary fuel supply, so if your goal is to improve your performance, you absolutely need a sufficient supply,” explains Ross Edgley, a sports scientist, bodybuilder and internationally-renowned stunt runner.
A carbohydrate-restricted diet can have many negative implications for the runner. Speaking from her experience working with elite athletes, coaches and scientists, dietician and sport nutritionist Renee McGregor cites the diet’s negative impact on performance as a central problem. “If you don’t have the energy (carbohydrate) in your system, you can’t hit those paces,” she says. “Yes the body can get that from breaking down fat stores or using other fuel stores to do that, but it’s a lot longer process.” The process of using fat as fuel, in itself, can also become a problem for the runner. “While it [following a low-carbohydrate diet] might train your body to burn fat more effectively, it also hampers your ability to burn carbohydrate during high-intensity exercise,” explains Bean. Therefore, due to this down regulation of carbohydrate-burning enzymes, the runner’s ability to perform at high intensity is inhibited.
The restriction of carbohydrate can also have a negative impact on immune and hormone health. “Carbohydrates are a really important nutrient,” explains McGregor. “If you think about the foods that give you good carbohydrates, most of them give you B vitamins, which are really good for metabolism, and also give you iron and zinc – they’re not just an energy source.”
Such nutrients play an important role in supporting the immune system, which athletes may also find compromised on a carbohydrate-restricted diet. Edgely cites one particular study, conducted by scientists from Loma Linda University Medical Center in California, as some of the most valuable evidence demonstrating this. The study found that consuming pre-workout carbohydrates could positively impact cytokine levels in athletes. “Cytokines are substances that carry signals between the cells of the immune system and are believed by researchers to be critical to preventing the body becoming ill and run down from too much exercise,” explains Edgely.
McGregor also cites the importance of a regular intake of carbohydrate – particularly for females – in maintaining healthy hormone levels. “If you reduce your carbohydrate intake too low, it has a massive impact on your oestrogen levels, which can then have an impact on cholesterol levels and bone health,” she says. “I’ve seen in runners, triathletes and even recreational gym users, when they cut carbohydrates out, they don’t menstruate. What people don’t realise is that, if you have low oestrogen, the body produces more cholesterol.”
Given glucose is the brain’s primary fuel source, the restriction of carbohydrate can likewise negatively impact brain function. “Your brain needs 120g of glucose a day just to function – just to keep all of the processes in the body going,” explains McGregor. “If you’ve got carbohydrates broken down to glucose… the brain functions better. You see that in ultra running – if you look at those who don’t take on carbs in a race, compared to those who do, the ones who do tend to make better decisions about directions.”
So we know that carbohydrates are important to our health, and that carb cycling, when used to manipulate body composition, isn’t the right approach for runners, given it fails to accommodate a runner’s performance needs. But for runners with two goals – to lose weight and still maintain or even maximise their performance – carbohydrate manipulation could still have a role to play. McGregor and Bean both recommend a take on carb cycling, in the form of ‘carbohydrate periodisation’, to fulfil both purposes. “What we really talk about now in sports nutrition is carbohydrate periodisation,” explains Bean. “The idea is that you adjust your carbohydrate intake in line with your training volume and it’s all about eating the right amount of carbs at the right time.”
Carbohydrate periodisation can take two forms – one being a very basic yet clever way for the everyday runner to manage their weight; and the other, a much more involved method of manipulating carbohydrate intake, employed by elite athletes to improve performance. “Fuelling your body correctly will mean you benefit from your training, but it depends on what you’re training for,” says McGregor. “If you’re running because you enjoy it and just want to do it three times a week, then having a slightly lower intake of carbs every day is not a bad thing. You can be quite clever. If I’m being very basic, I’d recommend a fist-sized portion of carbs, a palm-sized portion of protein and the rest veg or salad on the days you’re not doing huge amounts and, if you’re training, I’d introduce smaller snacks with carbs in them such as oatcakes with peanut butter.”
For the more experienced athlete, carbohydrates can be manipulated much more strategically. “Elite athletes would certainly look to periodise their nutrition intake, in line with their training plans and goals,” explains Dr Kevin Currell, Head of Performance Nutrition at English Institute of Sport. “Athletes take this approach to maximise training adaptation, rather than weight loss per se.” A very popular way of doing this at present is via an approach aptly named, ‘Train High Sleep Low’. The idea is that athletes increase their carbohydrate intake before tough sessions to maximise their performance, and perform low-intensity sessions with low-glycogen stores. “You do your high-intensity session in the evening and then don’t consume any carbs after that session,” says Bean. “You then perform your low-intensity/low-glycogen run the following morning on an empty stomach. Doing low-intensity sessions on lower carbs enhances training adaption – in other words, you increase the amount of mitochondria in your muscle cells, so increase your body’s ability to burn fat, rather than just carbohydrates. This then has the beneficial side effect of improving your body composition.”
There has been little evidence to show the performance benefits of this approach until only very recently. In April this year, a multi-national project, which investigated the effect of dietary periodisation strategy on endurance performance in trained athletes, showed the diet could be helpful for performing well. Triathletes who performed high-intensity evening sessions with high carbohydrate availability, and low-intensity morning sessions with low-carbohydrate availability for three weeks, improved their cycling efficiency by 11 per cent and their 10K running performance by 2.9 per cent. Athletes also cut their body fat compared with the control group, who consumed the same volume of carbohydrates, but trained with high-carbohydrate availability. “It does suggest that periodising your carbohydrate intake around select training sessions can lead to these sorts of favourable training adaptations and possible improved performance and body composition,” explains Bean.
However, while such a strategy has proven successful for elite athletes, Bean advises that this is “an advanced technique and not something for beginners.” Therefore, a more straightforward approach, as outlined by McGregor, would be more sensible for the recreational runner looking to manage their weight and still maintain performance. And yes, bread is still allowed! Remember, though, that this is just one way of creating a calorie deficit, which is central to weight loss whichever approach you follow. “There’s no single food that makes you fat, it is the overconsumption of everything that makes you put on weight on,” says McGregor. “It’s not about cutting carbs out, it’s about being more mindful about the portions.”