Many people from athletes to office workers have been increasingly trialing an intermittent fasting diet. It has quickly become a common topic that most of my clients inquire about, in particular to manage their weight. So what is intermittent fasting? Is this just another fad diet? Can it actually be useful for weight management?
Let’s have a look at the two most popular types of intermittent fasting diets along with the pros and cons for each one.
Calorie-restricted – 5:2
It’s called 5:2 because you eat normally 5 days and fast 2 days per week. The fasting days can be consecutive or split over the week. What makes this intermittent fasting plan difficult is that, for those 2 days, you’re restricted to 500-600 calories per day. That’s equal to less than two avocados!
One fasting day could look like this:
Breakfast: Low-fat yoghurt
Lunch: Vegetable soup broth-style
Dinner: Mixed leaf salad with can of beans or tuna
Snacks: Raw vegetable sticks
Not a lot of food is it? Because it’s so challenging, a new 5:2 Diet actually came out in the last couple of years with an increase to the calorie restriction. Instead of 500-600, you could eat 800 calories on fasting days (so 2.5 avocados).
There are also variations of the 5:2 Diet, including 6:1 (restricting only 1 day per week), 4:3 (restricting 3 days per week) or ADF (alternate day fasting).
- On fasting days, you may choose more nutrient-dense foods such as vegetables, lean meats and low-fat dairy since they’re lower in calories.
- Normal days allow you to have the foods you crave without feeling guilty unlike other diets that are more restrictive.
Time-restricted – 16/8
The 16/8 intermittent fasting plan involves fasting for 16 consecutive hours followed by an 8-hour window for eating. For instance, you can choose to fast from 8:00 pm to 11:00 am and eat between 11:00 am and 7:00 pm. The other time-restricted fasting diets include 15/9, 14/10 and 13/9.
A day for the 16/8 diet could look like this:
Breakfast: Black coffee
Snack: 11:00 am
Lunch: 1:30 pm
Snack: 3:30 pm
Dinner: 6:00 pm
- Encourages eating nutritious foods since the feasting window is small.
- There are no calorie restrictions making it easier to follow compared to the 5:2 diet.
- Suits people with a fixed schedule.
- Practical for people wanting to stop snacking late at night.
Downsides – 5:2 & 16/8
- Over-eating or choosing less nutritious foods on normal days or in the 8-hour window due to feelings of hunger after fasting.
- People may feel fatigued, irritable, have poor concentration and/or low mood during fasting hours or days.
- These diets don’t take into account your age, physical activity level or medical conditions, thus could be detrimental to your overall health.
- For athletes, your performance, muscle recovery and immune system could be negatively impacted and increase the risk of injuries.
- Generally difficult to maintain long-term.
What does the research tell us?
In regards to weight management, research is mixed and inconclusive due to the small sample sizes and animal studies. Any eating plan that reduces calorie consumption, whether through calorie or time restriction, can lead to initial weight loss. However, at this time there is not enough research to know if the 5:2 or 16/8 are safe and effective for weight management in the long term (i.e. months and years).
Having said this, all individuals wanting to implement any intermittent fasting diet are advised to seek guidance from a dietitian. Even though it may seem simple to do, if you don’t plan your meals and snacks properly, it could lead to various health issues including nutritional deficiencies.
Better to be nutrition safe!
De Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medicine. 2019;381(26):2541-51.
Gordon B. What is Intermittent Fasting? 2019 [July 27 2020]. Available from: https://www.eatright.org/health/weight-loss/fad-diets/what-is-intermittent-fasting.
Gabel K, Hoddy KK, Haggerty N, Song J, Kroeger CM, Trepanowski JF, et al. Effects of 8-hour time restricted feeding on body weight and metabolic disease risk factors in obese adults: A pilot study. Nutrition and Healthy Aging. 2018;4:345-53.