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Intermittent fasting itself will not make your extra kilos disappear

Intermittent fasting is popular. However, you should not expect to lose weight unless you also restrict your caloric intake. But there are still many important health benefits to intermittent fasting.

By Birgitte Svennevig, , 1/2/2024

Not eating anything for 16 or 18 hours every day. Or not eating anything for two days a week. These are typical examples of popular intermittent fasting protocols, often followed by people who want to lose weight. The idea is that the body begins to tap into its fat stores when it doesn't receive food during the fasting period, resulting in weight loss over time.

“There are indeed many health benefits to intermittent fasting, but fasting itself does not lead to significant weight loss”, says Philip Ruppert, a postdoc at the Department of Biochemistry and Molecular Biology, where he studies the body's energy metabolism and has a particular interest in ketogenesis; the state where the body burns fat instead of carbohydrates.

Together with Sander Kersten, a professor at Wageningen University and Research in the Netherlands, he has authored a review article summarizing and discussing existing research on metabolic processes such as ketogenesis and fatty acid oxidation that come into play during fasting. The article is published in Trends in Endocrinology and Metabolism and can be seen here.

More research in obesity than fasting

“When you do intermittent fasting, the fundamental rule still applies that we should consume fewer calories than we burn, if we want to lose weight. This means that intermittent fasting does not give you a “free pass for eating unlimited quantities of food. It's basic physiology, and fasting can't change that”, says Philip Ruppert.

Although intermittent fasting itself does not have a slimming effect, Philip Ruppert advocates for the new fasting trend.

The body's ability to deposit fat and thereby store calories for periods with limited food availability developed through years of evolution and ensured our survival before we developed all the food storage and preservation techniques we’re used to today.

“Today, there is a lot of research into what goes awry in our metabolism when we become obese. But there is much less attention to what happens to metabolism when we fast. I believe that more research in this area can lead to more effective treatment strategies for obesity and its consequences”, says Philip Ruppert.

The phases of fasting 

Phase 1: The nutrients from food are digested, absorbed, and stored in the body.

Phase 2: Once, the intestines have finished absorbing nutrients, the liver taps into its glycogen stores to maintain blood sugar. At the same time, body fat begins releasing fatty acids to fuel the body’s energy need.

Phase 3: The liver's glycogen stores are depleted. The liver starts producing ketones from bodyfat to fuel the body.

Phase 4: Ketone production is in full swing in the liver, providing the body and the brain with energy. At this point, the body gets almost all its energy from body fat. This phase can last for several weeks.

Final phase: There is no more body fat left. Proteins in the muscle mass are the last resort for obtaining energy to prevent death.

Depending on the body's fat percentage, a person can tolerate fasting for a few weeks up to several months.

Health benefits

Fasting, even in the absence of weight loss, has a range of health benefits, he believes. Benefits of intermittent fasting (16-18 hours a day) include:

  • Learning how the body reacts to eating and fasting, gaining insight into what the body needs.
  • Feeling more energized.
  • Getting in sync with your natural circadian rhythm and sleeping better. This aligns metabolic processes and it is currently believed that it makes them more efficient.
  •  Other benefits, suggested by scientific studies are improved blood pressure, lower resting heart rate, increased insulin sensitivity, steady blood sugar levels and improved skin.

Euphoric long term fasters

Some people fast for longer periods; several days or even weeks. Research has shown that such prolonged fasters may experience a significant change after 2-3 days of fasting:

“Initially, they are hungry, but that will go away after 1-2 days, and then they may become almost euphoric. It's a bit like runner's high, and it may last until they start eating again”, says Philip Ruppert, referring to the unique feeling of happiness and absence of pain and strain that many athletes experience during prolonged exercise.

He himself fasts for seven days once a year and has also experienced the disappearance of hunger and the onset of well-being.

“You experience a clarity in the brain that is hard to describe”, he says.

We all react differently to fasting

Science has yet to provide a definitive explanation for why fasting can lead to a feeling of clarity in the brain and perhaps even euphoria, but one possibility could be that ketones are formed during fasting. These are special energy molecules that function as fuel for the brain.

“The brain is fed with ketones during fasting. Maybe that's why you may experience this clarity”, says Philip Ruppert, emphasizing that there is no definitive research on this and that it is for now only a hypothesis.

Everyone is different, and therefore, one should expect that everyone reacts differently to fasting regimens, depending on factors such as age, gender, genetics, and lifestyle.

“Factors like seasons could also have an impact. For example, in winter, we get less vitamin D than in summer, and we know that vitamin D is important for many metabolic processes. Hormones could also play a role, for example, whether a woman is in menopause or has her period. All sorts of factors can have an influence, and that's something I want to continue researching so that each individual person can get solid, research-based recommendations that suit that person specifically, says Philip Ruppert.

Meet the researcher

Philip Ruppert is a postdoc at the Center for Functional Genomic Studies and Metabolism, Department of Biochemistry and Molecular Biology.

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Editing was completed: 02.01.2024