Throughout our evolutionary history, humans have had to contend with periodic food shortages. We have evolved, both metabolically and behaviourally, to adapt to these shortages. When food is abundant, we tend to consume more of it than we strictly need, storing the excess energy as fat in preparation for the next famine.
But what happens when food is always instantly available, as has become the case in much of the world? Animals, when provided with an unlimited source of food, will often continue to eat until they become obese. Humans are not an exception, and the high availability of inexpensive, addictive and calorie-rich foods has led to an epidemic of obesity and associated diseases throughout the developed world.
As humans, we have the ability to recognise this conflict between our present environment and that in which we have evolved, and to make efforts to address it. The extreme of this is the practice of fasting – the complete abstinence from food, usually for a period as short as one day or as long as several weeks. Intermittent fasting is an eating pattern that cycles between periods of fasting and eating. When this article mentions fasting, you can assume this includes intermittent fasting, with the caveat that there is some dispute over whether this practice is truly more effective than calorie restriction when it comes to weight loss.
Fasting has been practised throughout history, both for its health benefits and as part of spiritual traditions. It is only within the last 100 years that we have begun to study and truly understand the physiological benefits that fasting brings.
It turns out that the benefits of fasting are far more profound than simply preventing obesity. In this series of articles, we will explore these benefits, from boosting brain function to the potential for preventing cancer and even slowing the ageing process. But let’s start with perhaps the most obvious reason to fast: weight loss. Why is fasting (both intermittently and long-term) so much more effective than other types of dieting at getting rid of excess fat?
Weight loss is often simplified as a matter of ‘energy in, energy out’. Consume fewer calories each day than you burn, and your body will access your fat stores to make up the difference – the ‘eat less, move more’ approach. While this description is technically correct, it doesn’t paint a complete picture. A simple reduction in calorie intake can work, but can also have seemingly no effect on your weight – especially if you are already overweight to begin with.
The factor missing from the equation is that your basal metabolic rate (and therefore, daily energy expenditure) can vary in response to what you eat. In the introduction, we mentioned how humans have evolved to build up energy stores in preparation for the next food shortage. Where possible, therefore, the body tends to lower its basal energy consumption to avoid having to access its ’emergency stores’. You can read a slightly more detailed explanation of this concept in this article.
The ‘switch’ that controls access to stored fat is the hormone insulin. Insulin is released by pancreatic beta cells in response to blood sugar levels. It’s the signal by which cells ‘know’ that nutrients are available, and there is no need to access fat stores. Overweight people tend to have higher insulin levels than normal. This means that even when calorie intake is reduced, the presence of insulin locks fat stores away.
There are two ways to address this problem: either increase ‘energy out’ another way (in other words, exercise), or reduce insulin levels. Fasting is effective because it reduces insulin levels significantly (more so than even a sharp reduction in calories), and does so over an extended period. Notice from the graph below how it takes nearly 12 hours of ‘starvation’ for the rate of lipolysis (the breakdown of fat) to get close to its maximum level.
Another advantage of fasting is that it doesn’t just cause weight loss, but also reduces your propensity to gain weight in the future. This is because after a period of fasting, cells become more sensitive to the effects of insulin, meaning that less is required to achieve the same reduction in blood sugar. As discussed earlier, lower insulin reduces fat storage. These effects also reduce the risk of developing type II diabetes mellitus. There are other important benefits linked to reduced insulin levels as well, but we’ll cover those in a later article.
Reducing how much you eat is not always effective for weight loss, especially if you are overweight, because chronically high insulin levels prevent the burning of fat and force your metabolism to slow down instead. Fasting, whether intermittently or long-term, addresses this problem by lowering insulin levels. Fasting also has a lasting impact on insulin sensitivity, which protects against further weight gain and the development of diabetes.
Differential Effects of Alternate‐Day Fasting Versus Daily Calorie Restriction on Insulin Resistance: https://doi.org/10.1002/oby.22564
The Effectiveness of Intermittent Fasting to Reduce Body Mass Index and Glucose Metabolism: A Systematic Review and Meta-Analysis: doi: 10.3390/jcm8101645