Intermittent Fasting - The Science Scoop

Intermittent fasting (IF) has become a popular term in the health and fitness industry and you've probably seen it on your social media feed. While you may have heard good things about this approach from your friends and family I wanted to take a look at some of the research that's been done in this area and see what it can show us. My aim is to discuss whether the results from these studies provide enough support in favour of trying IF and to provide some decision making factors to allow you to decide whether this approach is suitable for YOU.

What is Intermittent Fasting?

Intermittent fasting or IF is a method of energy restriction and is a way of reducing your energy intake through food and drink so that it is below your energy requirements to maintain your bodyweight. The jury is still out on whether certain zero-calorie products like coffee and diet drinks are acceptable whilst IF but provided you don't eat any food during your fast I wouldn't lose sleep over it.

You can create this energy restriction in multiple ways. The traditional approach is through continuous energy restriction (CER), where you are in a negative energy balance every day for a sustained weight loss period . We also have a novel strategy which is intermittent energy restriction (IER). This means you break up the weight loss phase into several smaller chunks and intersperse it with periods of energy balance or energy surplus.

The overall outcome is the same it just means that the weight loss phase is broken up rather than being continuous.

There are three main types of intermittent fasting or IER, these are:

1. 5:2 - Here, the numbers refer to days. So you eat normally for 5 days of the week and reduce your caloric intake for the remaining 2 days.

2. 16/8 - Here, the numbers refer to hours. So you have a 16 hour fasting period and then only eat during a specific 8 hour window. Typically, people will include their fasting period overnight and then skip breakfast. For example, you might eat between 12pm-8pm and fast for the rest of the time.

3. 24 hour fast - A whole day fast, typically from dinner one night to dinner the next night. Normally occurs with a 1 day fast and 1 day where you eat what you want.

What does the science say?

There is a load of research on IF which has shown an amazing number of benefits including:

• Weight loss (Anson et al., 2003)

• Improvement in cardiovascular risk factors (Wan et al., 2003)

• Longer lifespan (Goodrick et al, 1982)

• Reduced body fat and retention of lean mass (Gotthardt et al., 2016)

• Prevention of type-2 diabetes (Nakagawa et al., 2003)

BUT (and it's a big but!) all of these studies have used rodents as participants. It is well known that rodent studies are often poor predictors of human responses. So as promising as it sounds, we need to see human studies before we can really understand the potential benefits and drawbacks of this method.

Studies in humans, although improving, are still very limited. The majority of trials are short in duration (<6 months), have small sample sizes and only include overweight or obese participants. Let's take a closer look...

Study 1 (Antoni et al, 2018)

Study design

41 overweight and obese (BMI > 25) participants took part in this study and were randomly assigned to either an IF or a calorie restriction group. The IF group used a standard 5:2 approach, where they fasted for 2 non-consecutive days at 25% of their caloric needs and ate as normal for the other 5. The calorie restricted group were placed in a deficit of ~600kcal. This continued until they had achieved 5% weight loss.

Main findings

• There was no statistical difference between the groups body composition or how long it took them to reach the desired weight loss goal.

• Drop out rates were higher in the IF group suggesting people may find it more difficult to stick to fasting approaches.

Study 2 (Coutinho et al, 2018)

Study Design

35 obese individuals were split into 2 different groups, IER vs. CER for 12 weeks. The intermittent group used a 3 day fast at 25% of their caloric needs and the continuous group were placed in a deficit for the entire duration of the study.

Main Findings

• There was similar weight losses from both intermittent and continuous energy restriction.

Study 3 (Harvie et al., 2011)

Study Design

107 obese women (BMI ranged between 25-35) were split into two groups, IER vs. CER for 24 weeks. The intermittent group fasted for 2 days on a very low calorie intake (25% of their normal intake) and then ate their normal diet for the remaining 5 days. The continuous group ate 75% of their normal intake for 7 days a week.

Main Findings

• Both approaches achieved similar weight loss

• Both approaches saw similar improvements in risk markers for diabetes such as fasting insulin and insulin resistance

• Intermittent approach was no easier to stick to than the continuous approach

Study 4 (Sundfor et al, 2018)

Study Design

112 obese (BMI ranged between 30-45) participants took part in this year long study. This is one of very few long-term studies and we need to see more research like this before any solid conclusions about the sustainability of intermittent strategies can be confirmed. In this study, the first 6 months were the 'dieting phase' and the final 6 months were a 'maintenance phase'. This was to test out whether participants could lose weight AND keep it off. The intermittent restriction involved 2 non-consecutive day fasts at 20% of normal intake (400kcal for women and 600kcal for men) and 5 days normal eating. The continuous restriction group reduced their daily intake by 26-28%.

Main Findings

• Weight loss and maintenance were similar in both intermittent and continuous restriction groups

• Blood pressure, waist circumference and HDL cholesterol improvements were similar in both groups

• Hunger ratings were higher in the intermittent group

So what do we know?

These are only 4 of the studies on intermittent fasting but they provide a nice picture of what the overall body of research on the topic may look like...

We know it can help us lose weight. However, IF is one of MANY tools to achieve weight loss. These studies demonstrate that it's just as effective as creating a calorie deficit but it is less clear how it compares to other weight loss strategies.

We know it is effective in obese and overweight populations. IF shows promise as an intervention for treating obesity but we don't really know how effective it will be for those in normal weight ranges.

We know it works in the short-term. There is not enough evidence to demonstrate the long-term sustainability of this kind of diet. Weight loss is great but if you can't maintain that weight loss then the strategy is flawed. Too many people are focused on a quick-fix. In most cases, you don't gain the weight in 6 weeks so it's unrealistic to expect to lose it in 6 weeks.

We know there needs to be more research done. The lack of high quality human studies in this area means that over the next 5 to 10 years we are going to see a large increase in knowledge about this dietary tool and the potential benefits it may have other than simple weight loss.

Is intermittent fasting for you? Things to consider...

1. Time - how long do you have to lose weight/change your body composition?

If there isn't a major time restriction on you achieving your goal then you should be favouring moderate over extreme energy restriction. Not only are you likely to feel the benefits psychologically but you are much more likely to keep the weight off once you bring yourself back to energy maintenance.

If you have a set date to achieve your goal e.g. going on holiday at the end of the month, you should first and foremost ask yourself 'is it realistic or safe to reach my target weight/body composition in this time period?' Realistic and safe will vary for each individual and their personal circumstances so if you're unsure what's realistic for you, feel free to get in touch and I can answer any questions you may have. If I can't then I have a referral network of trusted professionals who can help you set a realistic caloric deficit and lose weight safely.

By reducing your calories too much you risk losing muscle mass, messing with your metabolism and depriving your body of the nutrients required to function at even a basic level. Crash dieting can backfire and be extremely dangerous for your health.

2. Stress - What are your current stress levels like?

Fasting creates a stress response in the body so if you are already struggling with how hectic your life is with too much going on, fasting is unlikely to be a good fit for you.

3. Basics - Are you getting adequate nutrition already?

Fasting is one tool of many that can be used to improve your weight and body composition. Yet many people try to run before they can walk. Looking to strategies like 5:2 or Ketogenic diets as if they are magic pills that are suddenly going to help you achieve the body of your dreams without first nailing the basics. The reality is if your diet isn't great already then intermittent fasting will likely do more harm than good. Think of it as a marginal gain. It's only really necessary when you have all the basics in place already. Are you getting enough fruit and veg? Are you getting enough protein? Are you getting 8+ hours sleep a night? Are you hydrated enough? If the answer isn't 100% yes to ALL of these then you'll be better off placing your time and energy into one of those aspects of your life first.

4. Energy output - How high is your training load?

If you are currently heading to the gym in the morning or generally get pretty grumpy when you haven't had food for a while then IF probably won't work for you either. To get the most out of our workouts we need to be adequately fuelled, both during and for recovery. If you hit the gym at 6am before work but don't eat until lunch time you are not giving your body the nutrients it requires to recover and adapt from that workout.

Ultimately, you should be nailing the basics before you even consider an option like intermittent fasting. Nutrition is individual and what works for someone else won't necessarily work for you.

If you want to ask me a question then feel free to reach out to me at nourishnutritionuk@gmail.com using the subject FASTING and I'll be happy to chat.