Fasting, explained.

A clear, realistic guide to 16:8, 5:2, ketosis and what really works in practice

Fasting has become one of the most talked-about tools in nutrition.

Depending on where you look, it can seem like the answer to everything from weight loss to better energy, improved metabolic health and even longevity.

As with most things in nutrition, the reality is more nuanced.

Fasting can be helpful. It can also make things worse.

The difference usually comes down to how it is used and whether it actually suits the person doing it.

What do we actually mean by “fasting”?

One of the reasons fasting feels confusing is that it covers a wide range of approaches.

At its simplest, intermittent fasting is just an umbrella term for eating patterns that include periods without food, usually for twelve hours or longer.

Within that, there are several ways people apply it.

Time restricted feeding is probably the most common. This involves eating within a defined window each day, often somewhere between eight and twelve hours. The popular 16:8 approach sits here, where food is eaten within an eight hour window with the remaining 16 hours spent fasting.

Then there are approaches that focus more on reducing intake across the week rather than each day.

The 5:2 diet is a form sometimes called periodic fasting. You eat normally for five days of the week and then significantly reduce calories on two days (typically around 500 to 600 calories).

Alternate day fasting takes this further, alternating between days of eating and days with no calorie intake. A slightly less extreme version, known as alternate day modified fasting, allows a small intake on fasting days, usually around a quarter of normal energy needs.

Alongside these sits calorie restriction, which is not technically fasting but is often discussed in the same space. This involves consistently eating fewer calories each day without periods of complete fasting.

Although these approaches look quite different, they are all doing a similar thing. They reduce the amount of time the body spends in a fed state and allow insulin levels to fall, giving the body the opportunity to gradually shift from storing energy to using it.

What is happening in your body when you fast?

After you eat, your body primarily uses glucose for energy. Blood sugar rises, insulin is released and energy is readily available.

As time passes without food, insulin levels begin to fall and the body starts to draw on stored energy, primarily glycogen.

After around twelve hours, the body begins to rely more on these internal stores, including fat. This is a gradual shift rather than a sudden switch, but it is an important one.

If the fast continues, the liver starts producing ketones from fatty acids. These can be used as an alternative fuel source, particularly by the brain. This state is known as ketosis.

Ketosis often gets a lot of attention, but it is not unique to fasting. It can also occur on a very low carbohydrate diet, where glucose availability is reduced.

More importantly, you do not need to be in ketosis to burn fat or improve metabolic health. It is simply one end of a spectrum of fuel use.

Where does autophagy fit in?

Autophagy is often described as one of the key reasons fasting might support long-term health.

It is the body’s internal recycling process, helping to clear out damaged cellular components and reuse them. This process is always happening at a low level, but it can increase when food availability is reduced.

Longer fasting periods are thought to stimulate autophagy more strongly, which is why it is often linked to fasting windows of around 24 hours or more.

However, most of the detailed research on this comes from animal studies, and measuring autophagy directly in humans is much more difficult. In reality, it is likely a gradual process that increases as time without food extends, rather than switching on at a specific point.

What matters is how your body responds, not how long you can go without food.

For many people, especially those already dealing with stress or poor sleep, more aggressive fasting can increase strain on the body. Rather than improving energy, it often leads to fatigue, cravings and less stable energy overall.

This is particularly relevant in midlife. With hormonal shifts and a higher baseline stress load, the body is often less resilient to additional restriction.

Instead of better energy and appetite control, I often see the opposite: increased fatigue, stronger cravings and a cycle of under-eating followed by overeating later in the day.

Is fasting better than dieting?

Fasting is not inherently better than dieting. It is simply another way of structuring food intake.

It can be helpful when it creates clarity and reduces mindless eating. For example, someone who tends to snack throughout the day or eat late into the evening may benefit from having more defined boundaries.

But when fasting leads to under eating or is followed by overeating later, the benefits are reduced.

In practice, the improvements people experience often come from eating more intentionally rather than from the fasting itself.

A more practical approach

One of the biggest misconceptions is that longer fasting is more effective. In reality, a simpler approach often works better.

In clinic, I consistently see better results with a gentle 12 to 14 hour overnight fast combined with balanced meals that keep blood sugar stable. This approach is easier to maintain and tends to lead to steadier energy, fewer cravings and better long-term consistency.

It also helps avoid the common cycle of restriction followed by rebound hunger.

Final thought

Fasting can be a useful tool, but it is not essential and it is not a shortcut.

For most people, the biggest improvements come from doing the basics well. Eating enough, eating regularly and building meals that support energy and satiety will take you much further than pushing fasting windows longer and longer.

The real question is not how long you can go without food.

It is whether the way you are eating helps you feel and function at your best.

Nutrition is rarely about one tool in isolation. It’s about finding an approach that works for you.

If you’re focusing on fasting but still struggling with energy, cravings or consistency, the issue is often not the fasting itself but how you’re eating around it.

That’s something I can help you untangle. Book a free call here.

References

• de Cabo, R. and Mattson, M.P. (2019). Effects of intermittent fasting on health, aging, and disease. New England Journal of Medicine, 381(26), 2541–2551

• Longo, V.D. and Mattson, M.P. (2014). Fasting: Molecular mechanisms and clinical applications. Cell Metabolism, 19(2), 181–192

• Anton, S.D. et al. (2018). Flipping the metabolic switch: Understanding and applying the health benefits of fasting. Obesity, 26(2), 254–268

• Patterson, R.E. et al. (2015). Intermittent fasting and human metabolic health. Journal of the Academy of Nutrition and Dietetics, 115(8), 1203–1212