Intermittent fasting – it’s a diet that gets people as excited as a climbing trip to their dream crag. It is certainly a diet that promises potentially profound health benefits: weight loss, better metabolic health and longer life. But is it really the silver bullet proponents claim, and is it something that can work for climbers?

When you follow an intermittent fasting diet, you cycle through short periods of fasting and periods of unrestricted eating. The goal of the diet is to reduce your energy intake without having to restrict what you eat every day. Potentially providing an easier approach to following an energy-restriction diet for people.

Research suggests that energy restriction (being in calorie deficit) increases our lifespan by improving our metabolic health. It does this by reducing fat and weight, decreasing blood pressure, cholesterol, triglycerides and inflammation. It also helps us respond to mild stress better and can potentially improve regulation of blood-sugar levels. When people are on an energy-restricted diet long term, however, their bodies can adapt to the restriction and stop losing weight. Intermittent fasting attempts to provide a way to reduce the adaptations that happen when you are in consistent energy restriction, to keep you losing weight or help maintain weight loss.

There are three popular methods to achieve a fast (with a few variations on these):

1. Whole day fasting: for example, the 5:2 diet where two days a week you fast, eating only 500-600 calories (2000-2500kJ) on those days, while on the other five days you can eat a completely unrestricted diet.
2. Alternate day fasting: you alternate between a day of fasting, where you eat just one meal (ideally breakfast or lunch) that provides 25% of your energy needs, for example on a Monday, Wednesday and Friday and on the alternate days you eat with no restriction at all.
3. Time restricted eating: you eat all your meals within a restricted time frame, for example, within an eight-hour window, fasting for the remaining 16 hours – so you’d start eating at 8am and stop by 4pm. Time-restricted eating is based on trying to match our eating to our biological clocks. When we look at the studies that have been done, what has been found is that people tend to eat between 6am and 9pm (a 15 hour time frame, with only nine hours of fasting) and most of their energy after 1pm. Why this is potentially an issue is because researchers have found that the way we eat (eating over to long of a time frame) can negatively affect our circadian rhythms and impact our health. Our circadian rhythm is our (roughly) 24 hour body clock which is controlled by things like hormone production, brain wave activity, cell regeneration, the daylight and darkness cycle and now it seems potentially by the time frame over which we eat. Time restricted eating attempts to reduce our eating window, to match our natural circadian rhythms and protect our health.

So how do these different methods of fasting all measure up?

Firstly, we need to look at fasting and longevity. Many people
who are at a healthy weight are keen to fast for the promise of a longer life. At this stage, it seems that potential increase in length of life is due to improved metabolic health. If you already have a good metabolic profile (healthy weight, good blood pressure and cholesterol, good blood sugar control) and are fit and active, we don’t know if fasting will help you live longer. In addition to this, the evidence for longevity is based on studies in mice. It is extremely difficult to do a long term intermittent fasting study on human longevity, simply because we live so much longer than mice do. Human lives are also far more complex than lab mice in terms of our exposure to stress, environmental hazards, chemicals, sedentary time, exercise, medications, different diets, our moods – just to list a few things that can influence our length of life. So, at this stage it is difficult to work out how much influence intermittent fasting actually has on longevity.

If we then consider fasting in terms of reduced risk of diseases, we need to understand that the evidence for this mostly comes from measuring changes in risk factors e.g. cholesterol levels, blood sugar levels etc rather than measuring whether people actually develop a disease. This is because studies have been short term to date, which makes measuring disease development impossible. For overweight or obese people, reducing their risk factors for developing disease is certainly a positive but we don’t know yet what happens long term.

Finally, when we look at weight loss, researchers have compared the different types of fasting diets with your standard long-term energy-restriction diet and have found that both approaches will reduce weight in the short term. But we need to look a bit deeper to see the longer term implications of what is happening in the body.

We know that when you are on a long-term energy-restricted diet, changes start to happen in the hypothalamus of your brain. These changes can increase chemicals in your brain that stimulate appetite, reduce the amount of energy you burn and can begin to compromise your musculoskeletal system (muscle and bone). If we then look at the pituitary gland in the body, which controls lots of our hormone systems in our bodies, we see that with long term energy restriction the pituitary gland reduces the concentration of thyroid hormones and increases stress hormones. There are also decreases in things like testosterone, estrogen and progesterone and insulin like growth factor. These changes all contribute to less muscle mass, less strength, decreased bone density and the potential for an increase in storing fat where we don’t want it. When researchers compared intermittent fasting to a long-term energy restriction diet to see if the hormone changes were less, they found that intermittent fasting has the same negative impact on hormones.

So how do we get the potential benefits of some energy restriction, without the negative impact of hormone changes?

The answer is you need to be in energy balance (eating enough to match the energy you use) for your body to recover from period of energy restriction. At this stage, it looks like it takes ten to 14 days for metabolic rate and thyroid function to return to normal. Researchers have used this information to trial an intermittent energy restriction diet. The MATADOR Trial had some pretty promising results. They had people eat an intermittent energy restriction diet, where they cycled through two weeks of a 30% energy reduction diet, then two weeks meeting 100% of their energy needs. They repeated the two weeks on, two weeks off cycle for 32 weeks. At the end of the time, the people that followed this diet had lost more weight and six months later had maintained most of their weight loss. The results also suggested that the negative hormone and metabolic changes were much less with this approach to standard long-term energy restriction.

So how do we consider these diets for climbing and other athletes?

If you are an athlete (or a climber, who is training and climbing a lot), the commonly used intermittent fasting diets (5:2 or alternate day fasting) involve a severe energy restriction for a short period. This kind of energy restriction has been shown to reduce performance and result in more muscle loss than regular energy-restriction diets and is therefore not typically recommend for an athlete. It is also important to remember that most of the intermittent fasting research to date has been in overweight or obese adults, not in athletes or healthy weight people. But based on what we have learned so far, if you are trying to lose a little weight over a longer period of time and want to minimise loss of muscle mass and strength, the diminuation of performance and protect against hormone changes, then following a well-crafted version of the intermittent energy restriction diet would be worth considering. The 2018 ICECAP trial will look at this style of diet, compared to continuous energy restriction in athletes. The results should be pretty exciting and help guide weight management in athletes.

The research on intermittent fasting and intermittent energy restriction is exciting but also in its early days.

• If you are looking at long term intermittent fasting for weight loss, then the evidence does not stack up in its favour due to the potentially negative impact of hormone changes. A short term, intermittent fasting diet may be an option as long as there are long enough periods of normal eating in between fasting cycles.
• For weight loss, the 2 on 2 off, intermittent energy restriction diet looks promising, hopefully reducing weight and the negative hormone changes, while offering an easy to stick to energy restriction diet.
• If you are a healthy weight, and health and longevity are your drivers for wanting to fast, then based on the current evidence, I believe there are other diet-related interventions such as Mediterranean-style eating and eating to maximise gut health that are better for you and more effective than the 5:2 or alternate day fasting diets.
• For people who are still keen to experiment with a fasting style diet, a version of time-restricted eating makes sense if it works for your health, life, climbing and training goals. However, skipping a meal post training because it doesn’t fit into your time restricted eating rules will not be beneficial. If you time restrict your eating to, for example a 12 hour time frame 8am-8pm, you have reduced the opportunity for late night treats and extra alcohol and are giving yourself a framework to plan your daily eating by. You are also making yourself be more mindful about what and when you eat. All of which are a bonus for your health.

At the end of the day, intermittent fasting and time restricted eating research is helping us learn more about how our bodies work but they are not a silver bullet. It is important to remember that no amount of fasting will fix an unhealthy lifestyle or a bad diet, and what goes into our mouth whenever we eat is going to have the biggest impact on our health.