Intermittent fasting continues to be hyped by celebs and influencers but is it an effective strategy for athletes and active people?
Recently, interest in intermittent fasting (alternative periods of eating and abstinence) has rocketed. The 5:2 diet, in which you eat normally for 5 days a week but limit calorie intake on two, was popularised by Michael Mosley after a 2012 documentary. Since then, other approaches have gained traction, including ‘time restricted eating’ (TRE) where you eat in a consistent window of time each day, typically 8 – 12 hours, and fast for 12 – 16 hours.
Potential benefits
The idea behind TRE is to give your digestive system ‘rest’ time – time to switch its priorities away from digestion and onto other functions, such as repair. Advocates claim it can help you lose weight, prevent chronic disease, increase longevity and improve the body’s ability to utilise fat during exercise.
While it is not specifically a weight loss diet, many people find they lose weight. A review of 27 studies published in 2020 found that people who limited their eating window lost 0.8 – 13% of their body weight over 2 – 12 months. This may be explained by the limitation imposed on eating opportunities. Many people find they feel less hungry or have fewer opportunities to eat.
However, there is little evidence to suggest intermittent fasting is more effective for weight loss than other approaches. A systematic review of 40 clinical trials showed that intermittent fasting resulted in equivalent outcomes in terms of weight loss, fat loss and insulin sensitivity (how effectively the body regulates blood sugar) compared to daily caloric restriction.
Most of the studies indicating a health benefit of intermittent fasting have been done with lab animals. There are relatively few human trials to date. Of these, studies point to improvements in insulin sensitivity and blood sugar control, which may help reduce the risk of developing type-2 diabetes, as well as reduced inflammation. It may also benefit gut health. In a study published in the British Journal of Nutrition, people following the 16:8 approach saw a significant increase in their overall microbiome diversity.
Men and women who followed a strength training program had similar gains in muscle mass and strength when practising intermittent fasting compared to a control diet.
A big concern around intermittent fasting for athletes, particularly those undertaking high volume training is that it may result in a significant energy deficit. If the deficit is prolonged this may increase the risk of RED-S, which can have an adverse effect on an athlete’s health and performance.
Fasting and the female body
Fasting together with a strenuous exercise routine can be very stressful for our bodies. According to Dr Stacey Simms, exercise physiologist and author of Next Level, intermittent fasting is not a good idea for women who exercise regularly. “As a woman, your body will fight to preserve energy when you start fasting. When your brain perceives nutrient deficiency, especially a deficiency of carbohydrates, there is a marked reduction in kisspeptin [a neuropeptide] stimulation, which not only increases your appetite but also reduces your sensitivity to insulin. This is why research shows intermittent fasting is more likely to cause impaired glucose intolerance in women than in men.” According to Simms, if you exercise while fasting, it creates a double whammy of stress, raising cortisol levels and depressing thyroid activity, both of which disrupt the menstrual cycle.
Summary
While intermittent fasting may have weight loss and health benefits for the general population and moderately active people, I would not recommend it to highly active individuals or athletes undergoing hard training. This goes especially for female athletes and active older women. Overall, the risks of intermittent fasting outweigh the benefits. Going for long periods without eating can lead to fatigue, under-fuelling and nutrient deficiencies. On the other hand, if you are doing relatively light training and want to give it a go, ensure you eat enough to fuel your day and overall training.
The Complete Guide to Sports Nutrition, ninth edition is the definitive practical handbook for anyone wanting a performance advantage. This fully updated and revised edition includes guidance on:
maximising endurance, strength, performance and recovery
the most popular sports supplements
relative energy deficiency in sport (RED-S)
hydration strategies
nutrition for masters athletes, young athletes and plant-based athletes
nutrition preparation for competition
gut health and how to avoid gut problems during exercise
Under-fuelling can have a serious effect on your health and performance. The problem is many athletes and regular exercisers are eating less than they realise. What starts as a genuine desire to shed a few pounds or get fitter can easily backfire and result in under-fuelling. This means that day after day, week after week, you are not eating enough to meet your energy needs. Instead of getting fitter and faster you can end up fatigued and illness-prone. As many athletes (and here and here) have discovered, under fuelling can also cost you your performance.
Who is at risk?
Athletes most at risk include those involved in sports where havinga high power-to-weight ratio confers an advantage (such as running and cycling), weight category sports (such as boxing), and aesthetic sports (such as bodybuilding, dance and gymnastics). Young athletes can be especially vulnerable to restrictive eating and subsequent under-fuelling.
What is under-fuelling?
Under fuelling or, more accurately, low energy availability (LEA) refers to the situation in which your energy intake does not match the fuel demands of your training and basic body functions. As described in my previous article, LEA can happen either intentionally when you cut your energy intake to lose weight or unintentionally when you increase your training load without adequately increasing your food intake.
LEA underpins a condition called relative energy deficiency in sport (RED-S), which more accurately describes the clinical syndrome previously known as the female athlete triad. Although more common in females, RED-S is also something that affects male athletes.
What does energy availability mean?
Energy availability (EA) the amount of energy left over and available to the body to carry out essential functions after the energy cost of exercise has been subtracted from the energy you take in from food. In other words:
Energy Intake – Exercise Energy Expenditure = Energy Availability
For example, a 70kg athlete with 15% body fat who consumes 2500 kcal a day and burns 1000 kcal training has an energy balance of 1500 kcal. If you divide this by the athlete’s fat-free mass (FFM) (55kg) and you have an EA of 27 kcal/ kg FFM/ day. For optimal health and performance, EA should be about 45 kcal/ kg FFM/ day, although the threshold varies between individuals. Going below this level (particularly below 30 kcal/ kg/ FFM/ day) results in LEA and means you will not have enough energy available to cover basic physiologic demands. Your body will go into ‘energy-saving mode’, reducing its metabolic rate and essential functions.
What are the risks of under-fuelling?
Under-fuelling over weeks or months results in negative health and performance effects. It can impair hormonal function, immune function, gastrointestinal function, cardiovascular function and psychological function. In the longer-term, health problems such as infertility, low bone mineral density, nutrient deficiencies and disordered eating can develop (Fig 1). Even short-term energy deficits, for example during a long workout or not consuming anything following a workout, have been shown to disrupt hormones. Long-term LEA can reduce your performance and increase your risk of illness and injury (Fig 2).
How do you know if you are under-fuelling?
There are several early warning signs to look out for. These include
missed or irregular periods in females (although this can be masked by oral contraception)
lack of morning erection in males
persistent fatigue
a drop in performance or lack of expected improvement in performance
poor recovery
increased perception of effort during exercise and daily activities
recurrent illness and injuries (such as stress fractures)
changes in mood (such as feeling anxious or irritable)
reduced social interaction
problematic relationship with food and/ or training
disrupted sleep patterns.
How to prevent under-fuelling
Contrary to popular belief, lighter does not always mean faster! The weight at which you perform at your best will depend on many factors, including your age, gender, genetics and natural build. Instead of trying to get super lean, aim instead to achieve a healthy weight and body composition that allows you to perform well without compromising your health.
If you really do need to drop a few pounds, then reduce your current calorie intake byapproximately 15% on some training days, but not others. This should produce a relatively modest energy deficit of around 300 calories a day. Any more than this risks under fuelling and LEA. Calculating your personal energy requirements can be a useful starting point and there are a number of food tracking apps that you can use.
Under-fuelling can begin unintentionally so if you have recently increased training volume or if you are doing a hard block of training, you should take steps to avoid it by being more proactive in planning your food. Pay attention to your energy levels, and make a sustained, conscious effort to consume enough energy (calories). You will need to eat more on hard training days. If this proves difficult, then including some lower fibre (e.g. ‘white’ instead of wholemeal pasta) options can make this easier. Include plenty of foods rich in healthy fats, such as nuts, seeds, nut butters, avocado and olive and rapeseed oil in all your meals and snacks. These foods will help boost your energy intake without adding too much extra volume. If you find it difficult to eat larger portions, add high-energy snacks, such as nuts, nut butter on toast, and fruit and nut bars between meals.
If you suspect under-fuelling, then you will need to increase your food intake and/ or reduce your training volume. However, these two things can often be harder than they sound so consider seeking the help of a professional to get you back on track. Your GP, sports doctor, sports dietitian, or a psychologist will be able to provide assessment and advice. My previous article also provides practical advice for those with amenorrhoea.
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Expert advice on how to how to eat well and improve your performance
Do you exercise to lose fat? You’re not alone – getting lean is a major goal for many people who exercise regularly. But attaining a very low body fat percentage – or a level that is unnaturally low for your genetic make-up – carries big health risks. What most people don’t know is that cutting body fat too rapidly or too severely can wreak havoc with your hormones and result in missed periods (amenorrhoea), reduced fertility and weakened bones.
In a study of regular exercisers, half of the women were found to have subtle menstrual hormone disruption. A third had no periods at all. While the idea of not having the hassle of periods each month may sound appealing, it’s important to understand that this is not healthy! All women of reproductive age, whether an athlete or not, should have regular periods, otherwise there are potential serious health and performance consequences.
It’s all down to a concept called low energy availability (EA). This is the amount of energy available to the body to perform all its essential functions after exercise expenditure is subtracted (i.e. energy intake minus exercise energy expenditure). When this drops too low – either by intentionally or unintentionally restricting energy intake below the level required to meet demand or by increasing your training volume without a corresponding increase in fuel intake – then your body goes into deficit and your periods can stop. In healthy adults, a value of 45 kcal/kg fat-free mass /day equates with energy balance and optimum health.
Amenorrhoea is one of the most obvious external symptoms of Relative Energy Deficiency in Sport (RED-S), defined as a mismatch between your food intake and the energy demands of your body. RED-S developed from the concept of the Female Athlete Triad (relationship between three inter-related components: energy availability, menstrual function and bone health) when it became apparent that low EA had multiple effects on health and performance, and also affects men. In men, it can lead to a drop in testosterone levels.
Researchers believe RED-S is caused by low energy availability but a combination of many lifestyle-related factors are often involved, including stress, poor sleep and rapid weight loss. Doctors sometimes call it lifestyle-induced amenorrhoea or functional hypothalamic amenorrhoea.
Why do periods stop? It has been suggested that it’s an energy-conserving adaptation by the body. When there is insufficient energy available, the body goes into an energy saving mode. The body fat threshold that triggers hormonal disruption and amenorrhoea varies from one person to the next – but is thought to be between 17 and 22%.
The body mechanism is as follows: low energy availability increases cortisol production by the adrenal glands, which disrupts the release of gonadotrophin-releasing hormone (GnRH) from the brain. This, in turn, reduces the production of luteinising hormone (LH), follicle-stimulating hormone (FSH), oestrogen and progesterone, and stops you having periods.
One of the most severe health effects of this hormone disruption is the reduction in bone density and increased risk of early osteoporosis and stress fractures. Oestrogen and progesterone both act directly on bone cells to maintain bone turnover, so when levels drop, the breakdown of old bone exceeds the speed of formation of new bone. The result is loss of bone minerals and a loss of bone density. In men, low testosterone has a similar effect on bones and causes bone mineral loss.
RED-S can severely disrupt your fertility (thus reducing your chances of getting pregnant), slow your metabolic rate, impair immunity, increase fatigue and affect cardiovascular and gut health. Thyroid hormone, growth hormone and IGF-1, are also severely reduced.
RED-S also results in many performance-hindering effects. These include an increased risk of soft tissue injuries, stress fractures, decreased strength and endurance, prolonged healing of injuries and reduced ability to recover from hard training sessions.
However, the good news is that once your body fat level increases over a certain threshold and your training volume is reduced, your hormonal balance, periods and fertility generally return to normal. Studies show that when amenorrhoeic athletes improve their diet and restructure their training programme to improve energy balance, normal menstruation resumes within a few months (depending on the severity of the energy deficiency and the duration of amenorrhoea) and performance improves consistently. This is perhaps the most persuasive reason to seek treatment if you have amenorrhoea.
Practical advice
You should definitely seek advice if you have suffered amenorrhoea for longer than 6 months. An initial consultation with your GP will rule out medical causes of amenorrhoea, such as polycystic ovary syndrome. You should then get a referral to a specialist, such as an endocrinologist or sports medicine doctor.
Treatment will centre on increasing energy availability and restoring ‘normal’ body weight and body fat. This usually means increasing your food intake (usually by 300 – 600 kcal a day) and you may be advised to reduce your training frequency, volume and intensity, incorporate periodisation to your current programme or include more recovery.
As well as energy intake, focus on improving the nutritional quality of your food intake to ensure you’re meeting your macronutrient and micronutrient needs. Consult a registered dietitian (RD) or registered nutritionist (RNutr) if you would like more detailed advice.
Avoid any diet that eliminates entire food groups or centres around a set of strict rules – restrictive eating plans are likely to be deficient in nutrients that you need for proper endocrine functioning and may lead to unhealthy obsession with food. If you suspect a food allergy or intolerance, seek a medical diagnosis and professional guidance. Do not self-diagnose or use high street or home test kits.
Focus on your recovery nutrition – replenishing energy and consuming sufficient protein after training sessions are key for the prevention and treatment of RED-S.
If you enjoyed this post and want to find out more about sports nutrition, then check out the new edition of The Complete Guide to Sports Nutrition , the definitive practical handbook for anyone wanting a performance advantage. It’s the 8th edition (the 1st edition came out in 1993!) and it has been fully updated and revised to bring together the latest research and information on sport and exercise nutrition. It covers topics such as
Maximizing endurance, strength and performance
Carbohydrate, protein and fat requirements
Sports supplements
Improving body composition
Eating plans to cut body fat, gain muscle and prepare for competition
The Complete Guide to Sports Nutrition, ninth edition is the definitive practical handbook for anyone wanting a performance advantage. This fully updated and revised edition includes guidance on:
