Jason Fung’s The Hunger Code presents a provocative argument about food, hunger and body weight. Rather than viewing weight primarily as a simple matter of calories consumed and calories burned, Fung places considerable emphasis on insulin, hormones, hunger and the timing of eating.
Fasting is central to this argument. Fung suggests that periods without food can change the hormonal environment in the body and make it easier for the body to use stored energy. He also connects fasting with broader ideas about metabolic health and, ultimately, longevity.
But how much of this is supported by modern research?
The answer is more complicated than either the enthusiastic claims made about fasting or the dismissive idea that fasting is simply another diet.
The basic idea behind Fung’s approach
Traditional dieting often concentrates on reducing calorie intake: eat less, move more and the resulting energy deficit should produce weight loss.
Fung argues that this is incomplete.
His model gives much greater importance to insulin, a hormone involved in regulating blood glucose and energy storage. In his view, frequent eating—particularly in the context of a diet high in refined carbohydrates—can maintain insulin levels in a way that makes access to stored body energy more difficult.
Fasting, by contrast, creates periods during which no dietary energy is coming in. The body consequently changes its fuel use, moving progressively toward stored energy and fatty acids.
This basic physiological switch is real. During fasting, the body does alter its use of glucose, glycogen, fatty acids and, with longer periods without food, ketones.
The more controversial question is how much this translates into additional long-term health benefits compared with other ways of achieving an appropriate energy intake.
Fasting and weight loss
There is now a substantial body of research on intermittent fasting.
A 2025 BMJ systematic review and network meta-analysis examined 99 randomised clinical trials involving 6,582 adults. It found that intermittent-fasting strategies generally reduced body weight compared with unrestricted eating. However, when fasting was compared with conventional continuous energy restriction, the differences were generally small.
That is an important finding.
It suggests that fasting can be an effective dietary pattern for weight reduction in adults, but it does not establish that fasting has a uniquely powerful effect independent of overall energy intake.
A Cochrane review reached a similar broad conclusion: compared with conventional dietary advice, intermittent fasting may make little or no difference in weight loss.
In other words, Fung’s central idea has some physiological plausibility, but the human evidence does not support the strongest version of the claim—that fasting is categorically superior to conventional approaches.
Where insulin fits in
Insulin is undoubtedly important.
It helps regulate blood glucose and influences whether the body is storing or mobilising energy. When someone has not eaten for a period of time, insulin generally falls and the body increasingly relies on stored fuels.
This is one reason Fung’s explanation can be intuitively attractive.
However, metabolism is not controlled by insulin alone. Energy intake, protein, carbohydrate and fat intake, activity, sleep, genetics, appetite regulation and many other physiological processes interact.
The current evidence therefore doesn’t justify reducing the entire question of body weight to one hormone.
What about longevity?
This is where the distinction between interesting biology and proven human outcomes becomes particularly important.
Research in animals has produced fascinating findings. Intermittent fasting and related forms of energy restriction have been associated with changes in metabolic pathways, cellular stress responses and lifespan in several animal models. A 2024 review notes that fasting has extended lifespan in rodent studies.
That does not, however, demonstrate that fasting makes humans live longer.
Human studies have generally been much shorter and have concentrated on outcomes such as body weight, blood glucose, blood pressure and cholesterol rather than lifespan.
A 2024 review of randomised controlled trials examining ageing-related outcomes in adults found that intermittent fasting and conventional calorie restriction were broadly similar across the outcomes examined. The authors also emphasised the need for more comparative research.
So the statement that fasting may promote longevity is biologically interesting, but it remains considerably less certain than the claim that fasting can influence body weight and some metabolic measures.
The concept of metabolic health
One of the most useful aspects of Fung’s work is that it encourages people to think beyond the number on the scales.
Metabolic health includes factors such as:
- blood glucose regulation
- insulin sensitivity
- blood pressure
- blood lipids
- waist circumference
- cardiovascular risk
Some studies of time-restricted eating and intermittent fasting have reported improvements in several of these measures. A 2024 systematic review found modest improvements in weight and several metabolic measures among people with overweight or obesity. However, the analysis also suggested that the energy deficit itself explained much of the benefit, rather than eating time alone.
That is an important qualification.
Is fasting a special metabolic switch?
There certainly are metabolic changes during fasting.
The body does not behave identically immediately after a meal and many hours later without food. Fuel use changes, insulin generally falls, fatty-acid utilisation increases and ketone production can increase as fasting continues.
But this doesn’t mean there is a magical point at which the body suddenly changes from “bad” metabolism to “good” metabolism.
Human physiology is continuous and adaptable.
This is one area where popular discussions of fasting can become considerably more dramatic than the evidence warrants.
The most interesting part of Fung’s argument
Perhaps the most interesting idea in The Hunger Code is not simply “fasting causes weight loss.”
It is the suggestion that hunger, eating frequency and the hormonal environment surrounding food deserve more attention.
For decades, dieting discussions often concentrated almost entirely on calories. Fung helped bring greater public attention to the possibility that when and how often we eat can influence appetite and metabolic behaviour as well.
That contribution is worth considering even if one does not accept every conclusion in the book.
What the evidence doesn’t yet establish
Several claims should be treated cautiously.
There isn’t currently strong human evidence demonstrating that intermittent fasting:
- uniquely extends human lifespan;
- is inherently superior to conventional calorie restriction for long-term weight loss;
- produces a special longevity effect independent of other aspects of diet and health;
- or is automatically healthier simply because periods of fasting are involved.
The latest large evidence synthesis instead suggests that intermittent fasting and conventional energy restriction produce broadly similar effects, with some modest differences between particular fasting strategies. Longer and higher-quality studies are still needed.
A balanced reading of The Hunger Code
Jason Fung’s work is valuable because it challenges a simplistic view of metabolism.
His emphasis on insulin, appetite, eating patterns and fasting raises legitimate questions about whether our understanding of nutrition should be broader than simply counting calories.
At the same time, the modern evidence suggests a more moderate conclusion.
Fasting can be a meaningful metabolic state, and intermittent fasting can produce weight loss in adults. But the evidence does not establish fasting as a uniquely powerful route to weight loss or longevity.
The longevity question remains particularly fascinating. Animal research provides intriguing clues, while human research is still trying to determine whether those biological effects translate into longer or healthier lives.
For that reason, perhaps the most useful way to read The Hunger Code is not as a set of guaranteed answers, but as a challenge to examine the relationship between food, hormones, hunger, metabolism and ageing—and then compare those ideas with what rigorous human research actually shows.
Note: Fasting and restrictive eating can carry particular risks for adolescents and are not appropriate as a do-it-yourself weight-loss strategy. Nutritional needs during adolescence are different from those of adults.