Reclaiming the health of western society may require matching our diet and lifestyle to those that our bodies and minds evolved for, notes Julie Devinsky, RD, an inpatient dietitian of the Department of Clinical Nutrition at The Mount Sinai Hospital.
Julie Devinsky, RD
Before the modern age, we ate to survive. Our bodies and metabolism were more opportunistic and adaptive to what was intermittently available. There was no such thing as breakfast, lunch, or dinner. For most of human history, it was a mix of scavenging, gathering plants, and hunting. Diets varied by season, differing enormously across climates. At times, it was feast or famine.
Intermittent fasting—a term for prolonged cycles between eating and fasting—aims to mimic what early humans experienced by interrupting the regular flow of calories into our bodies.
Because intermittent fasts can be considered more of an eating pattern, rather than a diet, they may be more beneficial and can be used to complement a low-fat, low-carbohydrate, plant-based, or high-fat animal-based diet. However, this pattern works best for a very disciplined person and may not be realistic for many American lifestyles.
There are three types of intermittent fasting: the 5:2 method, alternate-day fasting, and time-restricted fasting.
- The 5:2 method involves eating normally for five days and fasting for two. On fasting days, roughly 500 calories are consumed. Based on an archetypal “hunter-gatherer” diet, the proposed benefits of the 5:2 diet are based on the premise that fasting increases insulin sensitivity and decreases leptin levels. A hallmark of the fasting phase of this diet is mobilizing fatty acids and ketosis, a process in which the body uses fat rather than glucose for energy. Glucose is stored in the liver as glycogen, which runs out after about 10-12 hours (without exercise), forcing the body to burn fat.
- Alternate-day fasting (ADF) requires eating no more than 500 calories every other day. In an October 2010 study published in the International Journal of Obesity, approximately 100 obese women on a ADF diet lost an average of 13 pounds over six months with reductions in cholesterol, blood pressure, triglycerides, and insulin. The mechanism of high-fat alternate-day fasting is inducing ketosis similar to the 5:2 regimen. Even lower-fat ADF diets were shown to be effective for weight loss. Further, the dropout rate in the ADF study was only 10-20 percent, which is considerably lower than other diet trials. Interestingly, researchers found that on the non-fasting days, participants ate less than expected. Instead of “feasting” to make up for the fasting days, participants only ate 10-15 percent more than baseline.
- The time-restricted feeding diet requires that the entire day’s worth of calories be consumed over six to eight hours and the remaining 16-18 hours are to be spent fasting. Research is limited, but preliminary studies suggest this diet may lower cancer risk and help with weight maintenance.
The jury is still out on fasting diets. Initially, many experience a two- to three-week adjustment period where symptoms often include fatigue and nausea. The long-term effects remain to be seen. Fasting may slow metabolism and cause persistent fatigue while some studies show marked changes in weight, blood glucose, insulin sensitivity, and lipid profiles.
These regimens are not for everyone, especially those with diabetes, pregnant and breastfeeding women, people taking certain medications, or those with serious medical conditions. However, in this age of escalating obesity and when the failure of “dieting” is commonplace, intermittent fasting may be useful as part of a weight loss regimen.