Recently, while catching up with friends, we were intermittent fasting for weight loss.
I was intrigued to understand more about why they followed this particular diet, and why one friend claimed that this is the best diet ever because he is not restricting any foods. I always find these conversations enlightening. In many of my social situations, I learn about diet trends, how diet information travels and how people interpret what is healthy and what is not.
Healthy is a subjective term, just like defining success. Your doctor can give you a clean bill of health following medical tests, but there is so much that we don’t know, which is why your doctor cannot tell you what diet is best for you. Even amongst dietitians and nutritionists, there is a variance in the interpretation of what a healthy diet looks like. Healthy is relative to each person.
So, back to intermittent fasting for weight loss. When I think of fasting, I think of Yom Kippur, the day of atonement in the Jewish religion. I have been fasting one day a year for the last 36 years, and it gets harder each year. I don’t feel great, I am dehydrated, and I feel ill. When I fast for 24 hours, I learn to appreciate food. The day culminates with a festive meal after the fast, which makes me feel even sicker from overeating. So, why would someone want to subject themselves regularly to fasting?
What is Intermittent Fasting?
Intermittent fasting is simply recurrent or periodic fasting (i.e., periods of time where you don’t eat or drink anything except water). It’s an “eating pattern,” rather than a “diet.” It focuses on regularly reducing food and liquid intake during pre-set times. It’s controlling when you eat and drink, as opposed to what you eat and drink.
Calorie Restricted Diets vs. Intermittent Fasting for Weight Loss
Weight loss diets typically impose restrictions on food intake and portion sizes. By consistently reducing the amount of food eaten by 15-60%, individuals with overweight and obesity lose weight and fat. This is called “continuous” calorie reduction because one repeatedly reduces what is eaten – at every meal and snack, every day. Calorie-reduced diets can include eating smaller servings, using low-calorie substitutions, and/or cutting out some snacks or desserts daily.
Intermittent fasting does not regulate what you eat or how much you eat, but rather the timing of when you eat. Following intermittent fasting allows you to eat whatever and however much you want, but only during certain times.
For people with excess weight to lose, health benefits have been seen with a loss of even as little as 5 to 10% of a person’s body weight. Health benefits included lower blood fats (LDL cholesterol and triglycerides), better blood sugar management (lower blood glucose and insulin), lower blood pressure, and lower levels of inflammation (C-reactive protein).
A review article published in 2022 found that the three main forms of intermittent fasting (alternate day fasting, the 5:2 diet and time-restricted eating) produced weight loss of 3 to 8% from baseline over an 8- to 12-week period. The review also found that the degree of weight loss achieved with intermittent fasting was similar to the weight loss achieved through traditional dieting approaches (i.e., daily calorie restriction). However, it is unclear whether intermittent fasting can manage weight long-term, as studies have typically been of short duration.
Some studies also show that intermittent fasting makes our metabolism more flexible so it can preferentially burn fat while preserving the muscles. However, most of these studies have been conducted in animals, and it remains to be seen whether similar effects are seen in humans.
For my friend, who claimed that “this is the best diet ever”, the benefits of intermittent fasting came after years of trying to fit the mould of typical weight loss diets or counting calories. He much preferred intermittent fasting because he could experience weight loss without guilt.
Numerous studies show what we know already: it’s challenging to sustain a (continuous) calorie-reduced diet for a long time, regardless of the diet followed. This is the reason many people prefer intermittent fasting – it gets similar weight and fat loss results, but it’s easier for some people to stick with.
Who Should NOT try Intermittent Fasting?
Before you try intermittent fasting, I recommend that you check with your healthcare provider to determine if it is suitable for you.
Side Effects of Intermittent Fasting
Before starting, you should consider some of the potential side effects of intermittent fasting:
- Bad temper
- Low mood
- Lack of concentration
- Feeling cold
- Hair loss
- Muscle weakness
- Uric acid in the blood and reduced kidney function
- Menstrual irregularities
- Abnormal liver function tests
- Mild metabolic acidosis
- Preoccupation with food
- Erratic eating patterns
- Hunger pangs.
If done too often or for too many days, intermittent fasting can have more serious effects. Excessive fasting can lead to malnutrition (including vitamin B1 deficiency), decreased bone density, eating disorders, susceptibility to infectious diseases, or moderate damage to organs.
Disordered Eating and Intermittent Fasting
Another issue with intermittent fasting is that it can promote disordered eating behaviours. In fact, a study published in 2022 found that men and women who reported following intermittent fasting also reported engaging in significantly more eating disorder behaviours.
If you’ve struggled with an eating disorder in the past, or have disordered eating tendencies, intermittent fasting likely isn’t a good idea for you.
Irritable Bowel Syndrome and Intermittent Fasting
If you have irritable bowel syndrome (IBS), you may be tempted to try fasting during a flare. However, there is not a lot of scientific evidence to support this, and for some people, fasting could actually make your IBS symptoms worse. This is because you tend to eat larger portions in the “eating window” of intermittent fasting, and large meals could worsen your symptoms.
Also, we don’t yet know the effects of intermittent fasting on the gut microbiome. Since the gut microbiome plays an integral part in our digestive health, we don’t want to do anything that could negatively disrupt it.
If you are struggling with IBS symptoms, a more effective treatment would be the low FODMAP diet. If you’re planning on implementing a low FODMAP diet, it’s best to do so with the guidance of a dietitian, as the low FODMAP diet is very restrictive.
Diabetes and Intermittent Fasting
Finally, intermittent fasting could be dangerous for people with diabetes who are taking insulin or other blood sugar-lowering medications. In these cases, fasting could result in dangerous low blood sugars.
How Long Should I Fast?
There are many ways to intermittently fast. Here are a few different intermittent fasting patterns documented:
- Alternate-day fasting (ADF) – One day of fasting, one day of “feasting.” Continue fasting on alternate days.
- Alternate-day modified fasting (ADMF) – Eat 25 to 40% of your daily needs one day, then eat normally the next. Continue alternating days.
- Periodic fasting (PF) – Each week has 1 or 2 days to eat very few calories per day (e.g., 0 to 880 cal per day). The other five days you eat normally. Example: 5:2 diet, where you eat no more than 500 calories per day for two non-consecutive days each week.
- Time-restricted fasting (TRF) – Fast for 12 to 16 hours every day and eat normally during the other 8 to 12 hours.
- One 24-hour period of fasting each month.
Some researchers suggest that the alternate-day modified (ADMF) fasting is preferable because it is likely the easiest to follow and may cause the least amount of stress on the body and mind.
Keep in mind that reducing your food intake also reduces your nutrient intake. It’s important to ensure you get enough essential nutrients for long-term health.
Is Intermittent Fasting Right for You?
Intermittent fasting is a way to get the benefits of a regular calorie-reduced diet without restricting what you eat. Intermittent fasting reduces both weight and fat (at least in the short term) and can improve blood sugar and blood fats. It has also been shown to lower blood pressure and some markers of inflammation. Losing weight and excess fat reduces the risk of diabetes, improves health, and increases the function of both the body and mind. However, it remains to be seen whether intermittent fasting effectively reduces weight in the long term.
If you’re going to try intermittent fasting, it’s important that you maintain a healthy diet and lifestyle. This includes eating a variety of foods including vegetables, fruit, whole grains, and protein foods; staying hydrated (with water as the drink of choice); and engaging in regular physical activity.