Eating for a Marathon: Weight Loss

Last week I talked about some of the techniques I used to encourage my body to use fats as fuel. Most people assume that burning more fat during exercise will result in weight loss.  Sadly, this is not true. To lose weight, you need a calorie deficit – to be taking in less energy than you use. No calorie deficit, no weight loss, as I experienced first hand.

Like most people, I put on a few pounds over Christmas, and on 1 January 2020 I weighed in at 65.7Kg and 30% body fat. I had lost some body fat in January and early February, but I then went to Canada for a couple of weeks for a wedding. When I got back, I weighed 65.4 Kg and 29% body fat – basically back to where I was in early January. As I got back into training, I was hoping the healthier eating and increased running would lead to fat loss.

By the first of April I was down to 64Kg. However, despite an ever increasing training regime, I stayed at 64Kg and 29% body fat. Health and aesthetic benefits aside, I had another reason for wanting to shed some weight. Studies have shown that runners are, on average, two seconds per mile faster for every pound they lose. If I lost 5lbs of fat, I should be a good 4 minutes faster over the course of a marathon.

So I decided to switch up my training and nutrition for a couple of weeks to encourage weight loss. Up to that point, I’d been eating a base of around 1800Kcal per day, then adding exercise calories on top. For example, if I burned 400Kcal on a run, I ate 2200kcal that day. From the 18th of May, I switched to a 1500kcal base. I was still adding the exercise on top, so in reality I was often getting 2000kcal per day.

To ensure I was losing fat and not muscle, I increased by protein intake – at least 1.8g/Kg per day. I did keep carbohydrates in my diet, but was much more strategic in when I had them. I tried to have higher carbohydrate meals after exercise, and lower carbohydrate meals at other times. I also kept an eye on the amount of fat in my diet, to keep the calorie count down. But it wasn’t exactly a low fat diet either. Of course, there was no alcohol and very few refined sugars (the kind you find in chocolate, cake, etc.).

I also switched up my training to further encourage fat burning. On Mondays, Wednesdays and Fridays I went for a 45 minute walk before breakfast, while on Tuesdays and Thursdays, I did an hour long fasted run. I kept my hill sprints on Monday afternoons, and my long run on Saturdays. I was up to an hour and a half on the long run, so I did have breakfast before, but I didn’t take on any carbs during the run. There were also a couple of HIIT/strength training sessions per week (as normal) and I made an effort to get 10,000 steps a day, most days of the week.

And it worked. On June first I weighed in at 61.8Kg and 26.3% body fat. In two weeks I had lost over 2kg and most of that was body fat, not muscle. However, it was not easy. I was often tired and hungry, despite a high protein and high fibre diet. And the quality of my hill reps and HIIT sessions really suffered – I just didn’t have the energy to push myself.

In some ways losing weight to get faster is a short cut. In two weeks I lost enough weight to take 4 minutes off my marathon time. It could take months to get that much from training. And it can be tempting to keep going, especially when you know you have plenty of body fat to lose (26% body fat is high for a runner). But it is a game that catches up with you in the end – as I discovered, your training really suffers.  I decided that I would get better results by hitting all my training targets, rather than chasing a lower body weight.

The good news is that on September second I was still 61.8 kg –  no more, no less. All while fuelling ever increasing amounts of training!

Please note: This is a description of my personal experience of losing body fat to meet a specific performance goal. It is not meant to be taken as a recommendation for anyone else. Using these techniques to lose weight may not be appropriate for you and your circumstances.

Healthy Eating on Budget

Tip 1: Have a Plan

Tip 2: Make your own meals

Tip 3: Make your own snacks

Tip 4: Choose natural, minimally processed foods

Tip 5: Learn to love your freezer

Tip 6: Make big re-usable food

Tip 7: Mind the drinks

Tip 8: Cut down on alcohol

Tip 9: Cut down on junk food

Tip 10: Eat less red and processed meat

Tip 11: Become a part-time vegetarian

Tip 12: Eat what’s in season

Tip 13: You don’t need superfoods to be healthy

Tip 14: Swap big brands for own brands

Tip 15: Avoid the grocery store

Tip 16: Be wary of supermarket offers

Healthy eating on a budget: Bringing it all together

To get lean, you’ve got to exercise

Weight loss is fundamentally a numbers game – If you eat fewer calories than you need, you will lose weight. As much as people go on about the effects of different nutrients on the body’s hormones, weight loss remains primarily a game of energy in and energy out. Diets focus on the energy in part of the equation and most people will lose weight on any diet that causes them to eat less. However, if you want to get lean by losing fat and gaining muscle mass, you need to combine diet with exercise.

As discussed last week, simply cutting calories usually results in losing both muscle and fat mass. You may be lighter, but you’ll still have the same proportion of fat to lean tissue. However, regular whole body exercise, particularly resistance exercise, can help preserve and even increase lean mass when dieting. Not only does this improve your health and make you look good, but the higher your muscle mass, the more calories you burn at rest. Muscle takes energy to maintain itself, even when you are not exercising, while fat does not. So, more muscle mass means you need more calories.

Between 60 and 70% of the calories we use each day come from our resting metabolism. These are the processes that are needed just to live, such as keeping our heart beating and brain working. Anything which changes our resting metabolism can have a huge effect on weight loss or gain. Sadly, dieting lowers resting metabolism – our bodies think there is a famine so reduces any unnecessary processes. A lower resting metabolism means you need fewer calories just to maintain your weight, let alone lose any. Plus, eating less also causes us to unconsciously move less, so we burn even fewer calories. All of which makes losing the fat even harder!

However, exercise increases resting metabolism, even when reducing calories. This is both a recovery effect (the body has to work hard to repair the damage caused by exercise), and an effect of gaining muscle mass. While burning more calories though exercise is helpful for fat loss, the big win is the effect of exercise on your metabolism.

Fundamentally, losing the fat is much easier if you get active!

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International society of sports nutrition position stand: diets and body composition

Alan A. Aragon, Brad J. Schoenfeld, Robert Wildman, Susan Kleiner, Trisha VanDusseldorp, Lem Taylor, Conrad P. Earnest, Paul J. Arciero, Colin Wilborn, Douglas S. Kalman, Jeffrey R. Stout, Darryn S. Willoughby, Bill Campbell, Shawn M. Arent, Laurent Bannock, Abbie E. Smith-Ryan, Jose Antonio

Journal of the International Society of Sports Nutrition, volume 14, Article number: 16 (2017)

For long-term fat loss, think slow and steady

If all you want to do is lose weight, then any diet which puts you in a calorie deficit will get the job done. It really doesn’t matter if you go ketogenic, paleo, or on SlimFast meal replacement shakes. However, if your goal is to lose fat mass, rather than losing muscle mass and water, then you need more patience. Several studies1 have shown that a slower rate of weight loss helps to better preserve muscle mass when dieting. This is particularly true of people who are a healthy weight but looking to lose body fat, rather than those who are obese. If long-term fat loss and a lean, toned body is your goal, then slow and steady wins the race.

Being patient is hard. When you are watching what you eat, seeing fast results is very motivating. It can be really disappointing to spend a week planning all your meals and denying yourself treats only to find you’ve lost half a pound. Especially when social media is filled with stories of how people on the latest crash diet are losing 5 pounds a week instead. What they don’t tell you is 1-2 of those 5 pounds will be muscle mass, and at least a pound will be water. Yes, they’ve lost 2 pounds of fat, but at the expense of muscle mass. So actually, the percentage of the body which is fat is the same, just at a lower weight!

The other problem with fast fat loss is that it is completely unsustainable. People will follow these diets for six or eight weeks, lose a stone, and go back to their “normal” diet. Problem is, their normal diet is what got them into this pickle in the first place! Within a year they’ve put all the weight back on and then some. Numerous studies have shown that over 95% of people who lost weight will have put all of it back on (and more) within 5 years. In order to get the benefits of any fat loss plan, you have to maintain it.

Successful long-term fat loss is about finding a diet and exercise plan you can happily stick to. Ask yourself: can I see myself still eating this way a year from now? No one can give up all their favourite foods forever, so work in the occasional treat. It is far better to lose body fat slowing while enjoying your food, than alternate periods of serious deprivation with binging. Remember, your mental health is more important than a particular number on the scale!

If you are enjoying this blog, don’t forget to like, share and leave a comment below.



1. International society of sports nutrition position stand: diets and body composition

Alan A. Aragon, Brad J. Schoenfeld, Robert Wildman, Susan Kleiner, Trisha VanDusseldorp, Lem Taylor, Conrad P. Earnest, Paul J. Arciero, Colin Wilborn, Douglas S. Kalman, Jeffrey R. Stout, Darryn S. Willoughby, Bill Campbell, Shawn M. Arent, Laurent Bannock, Abbie E. Smith-Ryan, Jose Antonio

Journal of the International Society of Sports Nutrition, volume 14, Article number: 16 (2017)

Choose nutrient dense fruits and vegetables

If there was such a thing as a magic way to help us lose fat, gain muscle mass, and improve our health, it would be to eat more fruit and vegetables. A recent systematic review found that those who ate more than four servings of vegetables a day were less likely to be obese, and that increasing vegetable intake encouraged weight loss1. Another meta-analysis of several large studies found those who ate the most fruit weighed less and had a lower waist circumference (a measure of body fat)2. If you are looking to get lean, then you need to choose nutrient dense fruits and vegetables!

Fruit and veg can help you control your weight and reduce body fat in several ways. For one, most are high in both fibre and water. As discussed earlier this week, fibre keeps you feeling fuller for longer. The fibre in fruits and vegetables also feeds the good bacteria in your gut, which help regulate your mood, energy levels and immune system. Even more intriguing is the link between the gut microbiome and obesity. It has been found that obese people have a different gut microbiome from lean people. In fact, the composition of the gut microbiome changes with a person’s weight. Whether this is simply a side effect of weight gain, or an actual cause is not yet clear.

However, just as importantly, fruit and veg are very nutrient dense. We get a large portion of our vitamins and minerals from fruit and vegetables. This includes ones vital for energy production, which can keep you from feeling tired even when eating less. Not only that, but a diet high in fruit and vegetables strengthens the immune system, so you are less likely to get sick. It’s pretty hard to feel energetic and ready to hit the gym when you’re fighting off a cold! Plus, muscle is not built on protein alone, so if it’s athletic performance or a toned physique you’re after, nutrient dense fruit and veg can help!

Fruit and veg are also high in a wide range of antioxidants. Antioxidants are compounds that fight the action of free radicals in the body. These highly reactive molecules bounce around the body looking for another molecule to react with. Left unchecked, free radicals can damage the DNA of cells, slowing down healing and recovery from exercise sessions. High levels of free radicals have also been associated with an increased risk of cardiovascular disease, cancer, arthritis and cataracts. The good news is that antioxidants (such as vitamins E and C) bond with the free radicals so they are no longer harmful, and help repair the damage they have done to cells. There is even evidence that high levels of antioxidants can improve skin condition and slow down the ageing process.

To ensure you get all the benefits of fruit and veg, eat a rainbow of different coloured produce every day. The idea is to get at least one serving of red, orange, yellow, green, blue/purple and white fruits or vegetables. A serving of fresh fruit or vegetables is 80g while a serving of dried fruit is 30g. While it may sound daunting, translating this into actual food isn’t as hard as you may think. For example, you could have purple blueberries in porridge, red and yellow peppers in a chicken fajita wrap for lunch, a yellow nectarine as part of an afternoon snack, and orange sweet potato with green broccoli and cauliflower with a pork chop for dinner, with some strawberries for dessert. That’s a day with 3 servings of fruit and 5 servings of vegetables!

I can think of very few situations or health goals where eating more fruit and vegetables would not help!


For a variety of delicious recipes with plenty of vegetables, click here! And if you are enjoying this blog, don’t forget to like, share and leave a comment below.



1. The Relationship between Vegetable Intake and Weight Outcomes: A Systematic Review of Cohort Studies

Monica Nour, Sarah Alice Lutze, Amanda Grech, and Margaret Allman-Farinelli

Nutrients. 2018 Nov; 10(11): 1626. doi: 10.3390/nu10111626

2. Fruit and Vegetable Consumption and Changes in Anthropometric Variables in Adult Populations: A Systematic Review and Meta-Analysis of Prospective Cohort Studies

Lukas Schwingshackl,  Georg Hoffmann, Tamara Kalle-Uhlmann, Maria Arregui, Brian Buijsse, and Heiner Boeing 

PLoS One. 2015; 10(10): e0140846. doi: 10.1371/journal.pone.0140846

To get lean, don’t forget the fibre!

Beans, lentils, fruit,vegetables, nuts, seeds and whole grains are all high in fibre.

When it comes to getting lean, a lot of websites will tell you it’s protein and fat that count. The low carb/ketogenic diet trend continues, and with it the idea that carbohydrates are stalling your fat loss. And it is true that a diet high in sugars and highly processed carbohydrates (like white bread) is doing no one any favours. However, avoiding all carbohydrates also means you are missing out on essential fibre.

Fibre is the part of plant foods that cannot be digested so passes through. It provides no energy but removes toxins and wastes from the digestive system. Because of this, plenty of fibre is essential for good gut health and prevents certain cancers.

More importantly, fibre also feeds the good bacteria in the gut. There has been an explosion in studies over the past 10 years showing just how important healthy gut bacteria are. Having diverse and healthy gut bacteria has been linked to all kinds of health benefits, including improving gut health (such as reducing symptoms of IBS); lowing cholesterol and the risk of heart disease; reducing the risk of developing diabetes; and improving mood1. However, of particular interest to us, people with a healthy microbiome tend to be a lower weight2. In fact, studies have shown that normal weight people have higher fibre intakes than obese people3.

Another reason why a higher fibre intake helps with fat loss is it allows you to eat more food. Fibre is bulky, making you feel fuller for longer. Your stomach responds to the volume of food eaten, not mass. In other words, you feel full when your stomach is stretched by bulky foods. What’s more, the stretch signal is one of the first to reach the brain and tell you to stop eating. Fibrous foods fill you up faster and help prevent overeating. Unfortunately, a common theme of high fat, high sugar foods is they pack a lot of calories into a very small space. In nutrition circles, we call these foods calorie dense. For example, 100g of raw spinach is a LOT of salad, but it has less than 30 kcal. On the other hand, most people would have no problem eating 100g of milk chocolate, all 500 kcal of it. Of course, your body will realise you’ve just downed a load of calories eventually. But it can take more than 20 minutes, during which time you’re still hungry!

The NHS currently recommends getting at least 30g of fibre a day. Sadly, in my experience, most people are getting less than 20g a day. The best sources are plant foods like fruits, vegetables, beans, lentils, nuts, seeds and whole grains. This is why ketogenic diets, where you avoid most carbohydrates, are usually very low in fibre.

What’s more, these foods tend to be high in vitamins and minerals. They are nutrient dense, but lower in calories. For example, my favourite breakfast of porridge with frozen berries and ground flaxseeds absolutely fills a large bowl once the liquid has been absorbed. Yet it only has 200 kcal. On the other hand, there are several brands of cereal or granola where a standard 40g serving barely fills a third of the bowl and can be 250 kcal, before you add the milk!

Choosing plant based, high fibre foods feeds the good bacteria in your gut and means you can eat “a lot” of food and still keep the calorie count down. A double win for fat loss!


For delicious high fibre recipes, including a Farm Bean Stew with 15g of fibre per portion, click here! And if you enjoyed this post, don’t forget to like, share and subscribe!



1. Slavin J. Fiber and prebiotics: mechanisms and health benefits. Nutrients. 2013 Apr 22;5(4):1417-35. doi: 10.3390/nu5041417.

2. Menni C, Jackson MA, Pallister T, Steves CJ, Spector TD, Valdes AM. Gut microbiome diversity and high-fibre intake are related to lower long-term weight gain. Int J Obes (Lond). 2017 Jul;41(7):1099-1105. doi: 10.1038/ijo.2017.66. Epub 2017 Mar 13.

3. Davis JN, Hodges VA, Gillham MB. Normal-weight adults consume more fiber and fruit than their age- and height-matched overweight/obese counterparts.  J Am Diet Assoc. 2006 Jun;106(6):833-40.

To lose fat, avoid processed foods

When it comes to getting lean, not all calories are created equal. There is growing evidence that the more processed foods you eat, the more likely you are to be obese1. Processed foods are ones which have been prepared and packaged by a company. This includes common foods like bread and bacon. Unprocessed foods are ones which are pretty much as harvested – things like apples, eggs, and whole oats. Generally speaking, processed foods have a long list of ingredients, while unprocessed foods only have one ingredient. In other words, unprocessed foods don’t have ingredients, they are ingredients!

A recent study2 showed that people eat, on average, 500 Kcal a day more when given processed foods than when given unprocessed foods. That’s a lot! Part of the problem is food companies know what we like: Lots of fat, salt and sugar. Processed foods often contain far more fat, salt, and sugar than you would ever add if you made the dish yourself. It makes no difference whether it’s something simple like frozen French fries or a complex multi-course Indian ready meal. Processed foods are usually far less healthy for you.

In addition to what’s added, many ultra-processed foods (like cola, pizza pockets, chocolate bars, crisps, etc.) contain very few nutrients. They may be high in fat and simple carbohydrates (such as sugar and white flour), but most are low in protein, fibre, vitamins, minerals and healthy fats. Many vitamins (such as vitamin C) are destroyed by heat and the way companies make these foods vastly reduces the vitamins and minerals in them. Home-cooked foods made from unprocessed ingredients tend to be higher in nutrients and more satiating (keeps you feeling full).

As well, processed foods often contain colourings, preservatives, artificial sweeteners or other additives. While these have all been tested and found safe for human consumption, we know very little about the long term consequences of eating them.

Fundamentally, most of us eat foods that our ancestors would not recognise. We would all do better if we ate things as nature intended. There is solid research showing a diet high in vegetables, wholegrains, pulses, nuts and seeds, fish and lean meats reduces the risk of many diseases, including obesity.

In practice, avoiding processed foods means ditching junk foods like chocolate bars, crisps, biscuits and so on. It also means doing more of your own cooking from scratch, rather than relying on ready meals, sauces in jars or seasoning mixes. When you cook from scratch you can control how much, and what types, of fat and sugar you add. Consider the simple example of switching from a breakfast cereal to porridge you make yourself (not the instant oats with sugar and flavourings added!). Porridge oats are higher in protein and fibre than your typical cereal, plus you can add flavour in the form of fresh, dried or frozen fruit (one of your five a day!). Even if you add honey or maple syrup to sweeten your porridge, odds are you will add far less than the amount of sugar most companies add to cereal.

Cooking from scratch may be more difficult and time consuming, but when it comes to losing the fat, processed foods are not your friend!

For some great unprocessed food recipes, from breakfast to dinner to snacks, click here!


Household availability of ultra-processed foods and obesity in nineteen European countries.

Monteiro CA, Moubarac JC, Levy RB, Canella DS, Louzada MLDC, Cannon G. Public Health Nutr. 2018 Jan;21(1):18-26. doi: 10.1017/S1368980017001379. Epub 2017 Jul 17.

Ultra-Processed Diets Cause Excess Calorie Intake and Weight Gain: An Inpatient Randomized Controlled Trial of Ad Libitum Food Intake.

Hall KD, Ayuketah A, Brychta R, Cai H, Cassimatis T, Chen KY, Chung ST, Costa E, Courville A, Darcey V, Fletcher LA, Forde CG, Gharib AM, Guo J, Howard R, Joseph PV, McGehee S, Ouwerkerk R, Raisinger K, Rozga I, Stagliano M, Walter M, Walter PJ, Yang S, Zhou M.Cell Metab. 2019 Jul 2;30(1):67-77.e3. doi: 10.1016/j.cmet.2019.05.008. Epub 2019 May 16. Erratum in: Cell Metab. 2019 Jul 2;30(1):226.

Why Protein is King for a Lean, Toned Physique

It’s the holy grail of most athletes at every level: How to gain muscle mass while losing body fat. More and more, I’m seeing athletes who are a healthy weight, but have higher levels of body fat than you would expect from their level of training. So this month I’ll be looking at what you should eat to encourage fat loss, without sacrificing muscle mass or performance. Starting with something we’re hearing a lot about lately: Protein.

For all the hype about effects of fats and carbohydrates on fat loss and athletic performance, the first thing to get right is the amount of protein in your diet. Protein is necessary to grow and repair body tissues, including muscle. And when you exercise, you damage your muscles and other tissues. If you are not eating enough protein, your body will be unable to turn all that training you’re doing into increased strength and stamina. What’s more, simply eating 20g of protein causes an increase in muscle protein synthesis (muscle building). If you spread your protein throughout the day, you can stimulate muscle building after every meal and snack. The more muscle you have, the more calories you burn at rest. This is why the muscle building combination of more protein and exercise can really accelerate fat loss.

When you go on a calorie restricted diet, your body breaks down both stored fat and muscle to make up the energy deficit. The more restrictive the diet and the faster the weight loss, the more likely it is that the weight is coming from muscle and water as well as fat (this is why I keep emphasizing that the goal is fat loss, not weight loss!). However, research has shown that consuming a high protein diet can counteract this.  Protein is also the most satiating of all the macronutrients – it makes you feel full, which makes it easier to eat less. It can also reduce cravings for sweets treats. Strangely enough, sometimes when people crave sweets, especially after a workout, what their body actually needs is more protein.

The current guidelines are for 0.6g of protein per kilogram of bodyweight per day for a sedentary person. However, if you are an athlete looking to drop body fat, you need at least 1.5g per kilo per day, and up to 2.4g per kilo if you are cutting calories. For example, if you weigh of 70kg (11 stone), that’s 105 to 168g of protein per day. This may sound like a lot but is quite doable if you include protein with every meal and snack. For example, you could have eggs with breakfast, a tuna sandwich and a glass of milk for lunch, yoghurt and nuts as a snack, and chicken and veg for dinner.

If it’s a lean, toned physique you’re after, plan your meals around protein!

Diet Myth 5: Toxic foods are making you fat

Detox diets are short term programs designed to eliminate toxins from the body. The idea is that a build up of toxic waste in our body is responsible for weight gain, low energy, low mood and dull hair and skin. Most detox diets involve a period of fasting followed by a period of cutting out all “toxic” foods. These vary from diet to diet but usually include alcohol, sugar, caffeine, meat, dairy and wheat. Raw veggies for breakfast, lunch and dinner basically. Often a detox program will include teas, herbs or supplements which are supposed to speed up the elimination of toxins.

All of these programs ignore the simple fact that your liver and kidneys already do a solid job of removing bad things from your body. If they weren’t working efficiently, you would know it, as you’d be very sick! If you want to help out your liver, take a break from the booze and eat more fruit and veg. But a few days of drinking a detox tea makes no difference whatsoever.

What does make a difference is switching from high calorie, highly processed foods to much lower calorie, whole foods. It’s not the lack of toxic red meat or sugar that is helping you drop the pounds, it’s the lack of calories in the vegetables you are eating instead. Eating loads of fruit and veg will also increase the vitamins, minerals and phytonutrients you’re getting. This in turn will help you feel better and probably improve the appearance of your skin and hair (in the short term). But you can certainly eat more vegetables without having to cut out most other foods.

Using detox teas or supplements can also be dangerous. Many brands of detox teas contain Senna, which is classed by the NHS as a laxative. Most detox programmes recommend you drink their tea several times a day, which means several doses of laxatives. Diarrhoea certainly causes weight loss, but mostly from dehydration. As soon as you stop dinking the tea, all the weight you lost will be back. And long term use of such laxatives can damage the liver and bowel.

Other detox regimes use activated charcoal. This is the stuff hospitals use when someone comes into the emergency room having ingested an actual poison. The charcoal binds to whatever is in the stomach, preventing it from being absorbed into the bloodstream. The charcoal and harmful substances can then be vomited or pooed out. The problem is that the charcoal doesn’t just bind to harmful substances – it binds to a lot of harmless nutrients as well. People who take activated charcoal for several days are risking serious vitamin and mineral deficiencies. Any weight loss using activated charcoal is because the body can’t absorb all the food you are eating, not because you are eliminating toxins.

Fundamentally, foods like meat, milk and bread are not toxic – they are just foods. Yes, we could all do with eating more fruit and vegetables. Yes, most of us could do with less alcohol, red meat and sugar in our diets. However, any weight lost when detoxing is due to eating (or absorbing) fewer calories, NOT eliminating toxins.

Diet Myth 4: Limiting when you eat means you can eat whatever you want

Over the past decade there has been some exciting new research into the benefits of fasting. Studies have shown periodic fasting can help lower blood pressure and cholesterol, reverse Type II diabetes, and help with weight loss. This research has led to a whole host of intermittent fasting and time restricted eating diets. In intermittent fasting diets (such as the 5:2 diet), you eat a very low number of calories 2 or 3 days a week, then eat normally 4 or 5 days a week. In time restricted eating diets, you increase the amount of time each day where you eat no food at all. These range from ones where you restrict your eating to only 8 hours a day, to an extreme form where you are only allowed one meal a day.

Fasting diets all promise the same thing: So long as you avoid eating during the fasting times, you can eat whatever you want during the feeding times and lose weight. This makes them incredibly attractive, as you are not asked to give up your favourite foods or stop drinking alcohol.

The science behind these diets usually revolves around insulin. Insulin is made by the pancreas and released when there are high concentrations of glucose in the blood (such as after a meal with carbohydrates). Insulin tells cells, particularly muscle cells, to absorb the glucose from the blood. Glucose is then stored in the muscles and liver as glycogen. When glucose levels fall, as happens between meals, glycogen can be released back into the bloodstream.

However, glucose is not our only source of energy; we use fats as well. In fact, free fatty acids are our primary source of energy at rest. High insulin levels after a meal ensure free fatty acids are stored in muscles and fat cells. Falling levels of insulin between meals then causes the release of stored glucose and the breakdown of fat into fatty acids for energy. The problem is for people with chronically high levels of insulin. If insulin levels are always high, either through constant snacking on sweet treats or type II diabetes, then there is never a strong signal to breakdown fat. This makes it very difficult to lose unwanted body fat.

This is rationale behind fasting diets. The idea is to drastically reduce insulin levels to force the body to use fat as fuel.  And they usually work – blood sugar and insulin levels go down, and body fat is reduced.

However, such diets can be hard to maintain long term and can come with several unpleasant side effects. For example, the brain runs solely on glucose so drastically reducing glucose stores can lead to headaches, dizzy spells, mood swings and feeling fuzzy headed. As well, most people find they are very tired during the fasting times and find it harder to exercise. Such symptoms may pass as the body adapts to using fat for fuel, but not always.

Of course, all of these diets rely on the person eating a normal amount of healthy food during the feeding times. Yes, eating 500kcal a day will create a significant calorie deficit for that day. But this only leads to weight loss if the person doesn’t turn around and eat 4000kcal of MacDonald’s every other day of the week. Likewise, limiting food intake to 8 hours only works if the person actually skips a meal, rather than trying to cram the same number of calories into a smaller time frame. Let’s say that the meal I’m skipping is my 250kcal bowl of breakfast cereal. If missing it means I am starving by lunchtime, I may eat much more than an extra 250kcal to compensate. After all, I missed breakfast so I deserve a large slice of chocolate cake with rice cream, right?

Fundamentally, any weight lost on these diets is due to the same reason as on every other diet: eating fewer calories. Yes, there are health benefits to fasting. Periodic fasting may even assist with fat loss. But it is a myth that simply by not eating one day, you can eat whatever you want the next and still lose weight.