Intermittent fasting – can it be harmful?

31st March 2024, A/Prof Chee L Khoo

Time restricted eating

In case you have not noticed, there was some disquiet in the cardiovascular community this week on the potential harm intermittent fasting can cause. The headline presentation says “8-hour time-restricted eating linked to a 91% higher risk of cardiovascular death”. Well, any study regarding “diets” are always going to be sus. This was presented at the American Heart Association (AHA) Epidemiology and Prevention/Lifestyle conference recently. The study results should be quite robust, right? We better look at what is being studied and how the conclusion came about. It’s going to be out in the lay press soon. And you know what that means.

What is intermittent fasting?

Intermittent fasting may be practised in different ways. Some of the common ones include alternate day fasting that involves fasting for 24 h every other day, and the 5:2 method that involves a fast of 24 h twice a week and a very low-calorie diet consumed 2 other days of the week. Fasting could be on consecutive or non-consecutive days in this method. Time restricted feed is another popular approach in which fasting occurs every day with variable hours, but usually the feeding occurs during a 6-h period with breakfast in the morning and dinner prior to 3 p.m., allowing for the duration of the fast to be 14–18 h.

The “benefits” of intermittent fasting

The theory behind intermittent fasting is that after 12 or more hours of fasting, the body switch to starvation mode where metabolism switch from using glucose to using ketone bodies for fuel. This switch from lipid/cholesterol synthesis and fat storage to the mobilisation of fat through fatty acid oxidation and fatty-acid derived ketones, preserving both muscle mass and function. Thus, it has been hypothesised that intermittent fasting regimens that induce this metabolic switch have potential to improve body composition in overweight individuals.

In clinical studies, intermittent fasting regimens have been mainly targeted counteracting obesity and maximizing effects on healthy living by reducing progression to cardiovascular disease, metabolic syndrome, hypertension, and T2DM. Importantly, there have not been a sufficient number of long-term comparative studies to indicate whether one of these dietary protocols results in long-term efficacy.

So, the results from the recent study is a bit of a surprise.

The study presented was a poster presentation at the AHA. Chen M et al looked at 20,078 adults from the US National Health and Nutrition Examination (NHANE) 2003 – 2018 survey database. These are participants who had data available for two days of dietary recall. They looked at mortality outcomes (all cause, cardiovascular and cancer deaths) and eating duration.

They reported:

Overall, using participants who ate 12-16 hours per day as the reference group, participants who ate <8 hours per day had a hazard ratio for CV mortality of 1.91 (p = 0.006). When they looked at participants who had CVD at baseline, eating <8 hours per day had a hazard ratio for CV mortality of 2.07 (p = 0.02). With cancer mortality, participants who ate <8 hours had a hazard ratio of 3.04 (p = 0.004).

Eating duration of >16 hours were associated with lower risk of cancer mortality in participants who already had cancer.

All very unexpected results. Obviously, the authors did not offer an explanation for the unexpected results. There have been comments online about the way the results were publicised. First, this was a poster study and have not been peer reviewed yet. It’s acceptable to have poster presentations representing preliminary research but this should be made clear to the public when news of the study was released. Further criticisms were levelled at the lack of information on the diet quality of the time restricted diet which can make a difference in the results. Furthermore, we don’t know whether some of participants may be shift worker who is using time restricted eating during their overnight shifts which are independent CV risk factors.

Some patients who has established CVD may be using TRE as a means to lose weight.

It seems very implausible that patients who already have a cancer diagnosis is likely to have lower cancer mortality if they ate more 16 hours a day.

This study has raised more questions that it seeks to answer.

References: Accessed 31/3/2024