Intermittent fasting from a dietitian’s perspective

By Lisa Lovejoy

As a dietitian, I am frequently asked by people — friends, relatives, random people I meet while traveling or at social events — for my opinion on popular issues in nutrition. My response is typically a rather vague, but very honest, “It depends.”

Each person’s nutritional needs are very different depending on that individual’s current health status, lifestyle practices and personal goals and preferences.

When asked for my opinion on intermittent fasting, the answer is most certainly “It depends.” Factors to consider when thinking about trying intermittent fasting include: your goals, your current health status, typical eating habits/preferences and social practices that include eating (business lunches or evening work functions, for example).

What is intermittent fasting?

Let’s take a closer look. Intermittent fasting usually refers to “time-restricted eating,” or periods of fasting and designated eating hours.

Of course, much of the success of this style of eating depends on what is being consumed during the eating hours, but let’s hope that anyone looking this closely at their eating habits will be making at least somewhat healthy choices when they do eat.

Some common variations on timing include alternate-day fasting, where individuals eat normally one day, then fast (consuming 0-500 calories) the next. A variation of this is 5:2 fasting, with unrestricted eating five days a week and fasting on two non-consecutive days each week. Daily time-restricted eating designates a certain number of hours for eating (typically eight or 12), with the remaining 12–16 hours dedicated to fasting. You may see fasting for 16 hours with an eight-hour eating window noted as the 16:8 diet.

Does it work?

When it comes to weight loss, studies support the fact that intermittent fasting does in fact tend to help folks lose weight. By limiting the number of hours spent eating, people tend to eat less overall. It is well known that consistently consuming less energy than you expend over time leads to the utilization of stored fat for energy and weight loss.

But before we consider intermittent fasting to be the cure for our weight loss woes, consider that the weight loss results seen with intermittent fasting tend to be very similar to those for individuals who follow a moderate caloric restriction, focusing on caloric intake rather than timing.

What is more important for both sets of dieters to consider is what they do once they go “off” the diet. If intermittent fasting or moderate caloric restriction become long-term lifestyle habits, both are associated with healthy weight maintenance. If either practice is used as a short-term “fix” and the issues that led an individual to carry excess weight in the first place are not addressed, then neither are truly effective.

What does the science say?

There is much discussion in nutrition literature and social media regarding the metabolic effects and potential health benefits of intermittent fasting — on metabolism, insulin sensitivity and lipid levels. Without getting into too much physiology, when the human body goes for more than about eight hours without eating, its glucose stores begin to be used up and the levels of some hormones, such as insulin, go down.

Lower circulating insulin levels allow more fat to be used for energy (rather than stored) and may be beneficial for weight loss or reducing the risk of some diseases. This is especially true for individuals who carry excess weight and/or already have impaired glucose tolerance or high circulating insulin levels. For these individuals, intermittent fasting may be especially beneficial.

At the moment we have a limited amount of conclusive scientific evidence to support these claims, but the animal research looks promising so far. In animals, fasting for various amounts of time has been linked with weight loss, improved markers of health, reduced risk for chronic disease and improved brain health.

Sounds great, doesn’t it?

Intermittent fasting may turn out to be rather beneficial to many people, but in evaluating the information available right now, we should bear in mind that much of the research in this area has been conducted on fat male rats. Rodents have small bodies and extremely fast metabolic rates, making it possible for them to lose as much as 15 percent of their body weight in just one day. Caution should be used when interpreting study results. For example, a female human body may respond very differently than that of a male rat.

In addition, many of the reported reductions of health risk during intermittent fasting are weight dependent, so the weight loss may be more responsible for changes than the timing of meals. More research, especially in humans and over longer periods of time, is necessary to fully understand the potential risks and benefits of intermittent fasting. But stay tuned, because some of that research is under way now.

As research continues, one area of study that looks promising includes factoring our circadian rhythms into the intermittent fasting protocols. Our bodies are “wired” in a cyclic fashion, with circulating hormone levels changing during the day. A couple of human studies have looked at the effects of intermittent fasting protocols based on circadian rhythms, comparing higher-calorie breakfast versus dinner (with midday intake remaining constant).

The results of these studies support the practice of eating earlier in the day, and then tapering or curtailing intake later in the day. Unfortunately for many Americans, our eating patterns and social habits lean toward eating later in the day, which may prove to be less effective for prompting change. Details such as these may be important in fine-tuning the benefits of intermittent fasting.

Verdict?

The pros of intermittent fasting include the likelihood of weight loss, with the opportunity to focus on when to eat versus what or how much, a refreshing change for dieters. Blood pressure, blood lipid levels and circulating insulin levels may also be reduced.

Potential cons for this style of eating include the social inconvenience of not being able to eat while others are; potential distractors of irritability, moodiness or difficulty concentrating while fasting; and, like most other diets, it might not create sustainable lifestyle changes, so success could be fleeting.

Individuals who are pregnant, breastfeeding or have a history of disordered eating should consult their medical provider before attempting intermittent fasting, as should individuals who are diabetic or taking medications in conjunction with meals or foods. Significantly altering eating patterns could affect the safety or efficacy of certain medications or cause undesirable shifts in blood glucose levels.

For otherwise healthy individuals, intermittent fasting may be worth a try if:

• You are looking to lose weight
• You’d prefer to limit the timing of eating and not the foods or amounts you eat
• Your lifestyle preferences support specific timing for meals/fasting
• You would like to see how your personal biometrics respond to periods of fasting