The Fast diet FAQs:

Is this diet backed by any research?

“There are loads and loads of animal studies going back over 80 years, human studies now going back over about five or six years. And I’ve counted 11 clinical trials of intermittent fasting. There are also [observational] trials on Muslims [who fast] during Ramadan.”


 Who benefits from intermittent fasting?

 As well as offering a fresh approach for people trying to lose weight, IF has been developed by scientists wanting to help people reduce their risk of developing diabetes, dementia and cancer

  • IF has been most extensively studied in volunteers who are obese or overweight. In a recent study of 115 overweight women, those who restricted their calories two days a week lost more fat and had a greater improvement in biomarkers that relate to breast cancer risk than women doing conventional daily dieting.
  • The benefits in people who are not overweight are less clear because there have been fewer studies. In one experiment, a number of fit young men were asked to practice IF without losing weight for a few weeks. During that time they saw improved insulin sensitivity, a marker for reduced diabetes risk
  • Studies of IF in animals have shown that it reduces risk of dementia. Human studies have just started.
  • Fasting also has a spiritual dimension and has been advocated by most of the great religions 

What are the most common side effects of this diet and how can they be avoided? 

The commonest side effect is feeling hungry, particularly when you first start.

Some people find it hard to sleep on a relatively empty stomach. If so, it is recommend keeping calories aside for a late night glass of milk or snack.

Some people may experience headaches or constipation, particularly at first; these can generally be alleviated by drinking lots of calorie-free liquids and eating foods that are rich in fiber.

Can I exercise on fasting days?

Yes, you can exercise.

We analysed a group of dieters at our research centre, who exercised 3 times a week. We also let people choose whether to exercise on feast or fast days, and they chose equally.

During the first week participants reported low energy, but it bounces back. Some people even report more energy on fast days.

It’s actually advisable to exercise because you lose more weight if you do so – your muscle mass will stay high and, as a result, so will your metabolism.

Participants also tended to lose more weight if they exercised in the morning, before eating. Then when their post-exercise hunger surge came (when the body is rebuilding) they would eat their 500-calorie meal. If you exercise after eating, you still get your hunger surge and are more likely to cheat.

What foods to eat on the fasting days?

Types of food

  • Animal protein (if you want) – beef (e.g. sirloin steak), pork (e.g. lean pork loin, fat-free ham, bacon), chicken breast, fish (e.g. tilapia, salmon, tuna, mackerel, shrimp), eggs (boiled or poached)
  • Vegetables – aim for a wide variety – different colors, textures, tastes, shapes. E.g. asparagus, avocado, broccoli, broccoli rabe, cabbage, carrots, cauliflower, cucumber, edamame, eggplant, fennel, green beans, unlimited leafy greens (spinach, kale, chard, mustard greens, salad leaves, watercress), leeks, mushrooms, bell peppers, scallions, snow peas, a handful of cherry tomatoes, zucchini. Eat a wide range of different-colored plants, No fixed guidelines about eating them raw or cooked – “have both, often”. Be cautious about starchy vegetables as they tend to have a higher GL and calorific value – proceed with caution and don’t add butter. Eat onions in moderation because of their GI value.
  • Fruit – citrus fruits (tangerine or grapefruit in particular), a watermelon slice, an apple (eat the skin, seeds, and core), a handful of berries such as blueberries or a couple of strawberries, a fig, a small banana, a small mango
  • Nuts – almonds, cashews, coconut flakes (unsweetened), pistachios. Eat in moderation as nut calories quickly add up
  • Seeds – sunflower
  • Soup – light broth or miso soup
  • Carbohydrates – whole-grain bread or tortillas, whole-grains (oats, bulgur, couscous, quinoa, brown rice/brown basmati), beans and lentils. As an alternative to pasta or wheat noodles, try shirataki noodles
  • Dairy – low-fat or fat-free milk, cheese, cottage cheese, cream cheese, or yogurt
  • Flavors – including chili flakes, cornichons, cumin, fresh herbs, garlic, ginger, jalapeños, lemon juice, mustard, olives, pepper, soy sauce, Thai fish sauce, vinegar, wasabi. The cookbook refers to these non-carb flavor enhancers as the “fantastic five”: lime juice, soy sauce, fresh ginger, garlic, and Asian fish sauce
  • Use agave as a sweetener if required, as it’s low-GI. Or try a sprinkling of coconut.
  • Choose your own preferred way to consume your 500 or 600 calories – could be one meal a day, breakfast and dinner, two meals with some snacks in between, or your own preference. On purely theoretical grounds, a longer period without food (e.g. breakfast and dinner with a 12-hour break in between) should produce better results than one when you eat smaller amounts more frequently. Aim for as long a fasting window between bouts of eating as possible, as this is where many of the benefits of intermittent fasting lie.

What drinks can be consumed on a fasting day?

You can drink anything that has no calories/low calories. This includes coffee and tea.  Milk is generally a healthier drink than, say, orange juice, being rich in protein and relatively low in carbohydrates. So that would still be a better option than juice. Just keep the calorie count within limits!

Can I fast if I am feeling unwell?

Best not to. Fasting will stress your body, that seems to be one of the ways that it helps (stress provokes repair), but you shouldn’t over stress.

What foods have to be avoided during the fasting days?

  • Stay within your calorie limit
  • Limit or avoid high-glycemic GI or GL foods. Watch out for high-glycemic potatoes and many fruits, as well as dried fruits such as raisins and dates, which can spike your blood sugar and are best left for the days when you are eating freely. Fruit juices also have a high sugar content
  • Avoid starchy white carbohydrates (bread, potatoes, pasta). It isn’t’t expressly discussed in the book, but implied that you should avoid sugar on these days; the cookbook says that as a rule you should avoid white carbs on a fast day
  • There are no guidelines on artificial additives and artificial sweeteners, but in general the book recommends “real” foods for the fast days so we assume these shouldn’t be included
  • Avoid full-fat dairy and butter
  • Remove the skin and fat from meats before cooking
  • Avoid alcohol – it merely provides empty calories
  • Avoid processed foods – they tend to have hidden sugars and not contain much nutritional value
  • Meal replacement shakes – the authors aren’t great fans and think real food is better, but if you find they help you can use them – choose a brand that is low in sugar

 Will it put me into starvation mode?

This is another very common myth. The initial response of your body to a reduction in calories is to increase your metabolic rate. This is because, in our hunter-gatherer past, survival in times of food shortage would have depended on our becoming more active, going out to hunt and look for food. Only under conditions of extreme calorie deprivation, when we have been for weeks without enough food and our body fat has fallen dramatically does the body go into “starvation mode”. IF is not the s ame as crash dieting. Starvation mode does not happen if you cut your calories for a day!!

How do I maintain my weight once I reach my target?

Once you’ve reached your target weight or just a shade below, you can consider adopting the Maintenance Model to stay at your current weight and keep the anti-aging benefits of occasional fasting.

  • Fast on only 1 day a week or once every 8-10 days
  • Or consider adapting your consumption on non-fast days to eat more calorie-dense foods.