Fasting is an ancient practice sparking new interest in both pop and scientific culture. Entrepreneurs, venture capitalists and a variety of celebrities have extolled its virtues in helping them lose weight, while medical experts are intrigued by the possibility that it may enhance cognitive functioning and longevity and help treat and prevent some diseases.
As researchers study the different fasting mechanisms to determine the ideal protocol for a variety of outcomes in a variety of populations, however, the general public must proceed by trial and error. That leaves dietitians such as myself concerned that the hype around fasting could encourage people to follow harmful plans that severely restrict nutrients, cause stress or unsuccessfully treat serious conditions.
And there are many plans, because there are many schools of thought about what's considered fasting, what food or drink should be consumed during eating windows, and how long those windows should last. Here are a few of the most common methods:
- Intermittent fasting means eating normally four or five days a week and consuming minimal calories on two or three nonconsecutive days.
- Time-restricted eating requires setting a window for consuming food, such as 7 a.m. to 7 p.m, and having nothing but noncaloric liquids the rest of the time.
- The "fasting-mimicking diet" involves eating a small number of calories daily for a specific period of time - say, five consecutive days every other month.
These protocols all have benefits and drawbacks. One approach may work well for some yet be entirely unsustainable for others.
Although it's possible that fasting helps manage blood sugar and insulin, fight inflammation and ease sleep problems, most people follow a fasting diet to lose weight. Fasting is believed to help in this goal because of the effects that caloric restriction and extended periods without food have on metabolism, or simply because people eat less during the restricted-feeding window. (Some dieters benefit from the structure and clear boundaries that fasting plans offer.)
The problem is that too many people jump on the fasting bandwagon without understanding how to assess its impact. Once a plan has been integrated into your everyday life, you should take time every week or month to analyze how things are going. Even if weight loss is your primary goal, it's important to consider how fasting is affecting other aspects of your health, such as your energy levels, ability to exercise and digestion. Here are nine questions you should ask yourself:
1. How do you feel? Restricting calories can affect mood - for better or worse. Also, changing what foods you're eating can affect energy and behaviors. Keep track in a journal and note mood, energy levels and behavior changes.
2. How's your sleep? Note when you fall asleep and wake, how many times you wake up and whether you feel rested. Many wearables and apps offer to help you with this.
3. How are your labs? Are measures such as your cholesterol and blood-sugar levels moving in the right direction, and all in healthy ranges? Also, many fasting protocols can be low in certain nutrients. Using labs to look at micronutrients - vitamins, minerals, and essential fatty acids and amino acids - is essential to ensure that your intake is sufficient.
4. Are you gaining or losing muscle? Based on a variety of factors, fasting may lead to the loss of muscle mass. Using certain scales can help you assess muscle mass.
5. Are you able to exercise? Many fasting plans don't encourage exercise, because creating a caloric restriction may make it difficult to exercise. But exercise is a long-term healthy habit, so talk with your health professional about whether this is the right long-term plan to support your goals.
6. Are you hungry all the time? A scale of 1 to 10 is helpful in moments like this, with 1 being "ravenous" and 10 "extremely full." What is your number before and after meals? What is your number at moments during the fast? Although hunger may be part of a fast given the caloric restriction, it shouldn't be constant, ongoing and terrible. Also, notice portions when eating: If you haven't created the right balance, you might overeat when breaking a fast, which is hard on your digestion and metabolism. Ideally, once acclimated, you'd stay within the 3 to 7 range at all times, 3 when fasting and 7 after meals.
7. Are you craving foods? Paying attention to your relationship with food during fasting is helpful to assess what's working and what isn't to build longer-term habits. For example, having a sweet tooth may be a major issue for you typically, but avoiding sugar and restricting calories may change things. Or maybe you notice you are craving greens or meat or another food. This may stem from eliminating so many foods that the body is missing certain nutrients.
8. How are your skin, hair and nails? Your skin is your largest organ, so how the body is doing internally is often reflected on the outside. Is your skin breaking or glowing? Are your nails strong or brittle? Is your hair dry or shiny?
9. How's your digestion? Caloric restriction can sometimes also include fiber restriction, which is a key part of digestive health. Note your bowel movements (form and regularity). Talk with your health professional about inconsistencies.
The answers to these questions can help you evaluate whether fasting is contributing to your overall well-being. If it isn't, the answers can also provide clues about where you can tweak your plan - or help you decide that, despite the endorsements from Beyoncé and Hugh Jackman, fasting simply does not work for you.
Berman is a registered dietitian and a personal trainer.