A study of more than 20,000 U.S. adults, average age 48 years, found that those who limited their eating to a period of less than eight hours a day had almost double the risk of dying from heart disease, compared to those who ate meals over 12-16 hours per day (Abstract P192 presented March 18, 2024 at the American Heart Association Epidemiology and Prevention | Lifestyle and Cardiometabolic Health Scientific Sessions). An increased death rate was also seen in people who used this pattern of intermittent fasting and had cancer.
This study found an association between intermittent fasting and death rate. It did not show that intermittent fasting causes heart disease or cancer deaths. The authors speculated that intermittent fasting might be harmful specifically for people who suffer from heart disease or cancer.
Limits of the Study
My main concern with this study is that the participants were given no guidance whatsoever on the foods they should eat; they were basically told to eat anything they wanted as long as they only ate during a period of eight hours or less. None of the intermittent fasting programs I have used and recommended have failed to give guidance on healthful eating. Other possible reasons why this association was found between fasting more than eight hours per day and death from heart disease or cancer may include:
• People who restricted eating to less than eight hours per day had less lean muscle than those who ate in 12-16 hours per day. Heart-attack risk is strongly associated with lack of active movement and having smaller and weaker muscles (J of the Amer Heart Assoc, Feb 2024;13(4)).
• People who suffered from heart attacks and cancers in this study may have eaten a pro-inflammatory diet that has been proved to be associated with increased risk for heart attacks and cancers (J Multidiscip Healthc, Aug 30, 2023;16:2543-2556). Plant-based diets such as the DASH diet or Mediterranean diets are strongly associated with reduced risk for heart attacks and cancers.
• People who suffer from heart attacks and cancers are far more likely to have developed their diseases by following other heart attack-and-cancer-risk factors such as eating a diet loaded with sugar and meat, not exercising, being overweight, smoking, drinking alcohol, being exposed to pollutants and so forth (JACC Cardio Oncol, Aug 2023;5(4):431-440). This study showed that the group that fasted for 16 hours per day had more smokers and more men. Both factors could account for the increased death risk from heart attacks (J Thorac Dis, July 2020;12(7):3866-3876).
• Many previous studies show that some forms of intermittent fasting improve several measures related to heart health, including lowering high blood pressure, high blood sugar and high cholesterol levels (BioMed Central Trials, Jan 9, 2024;25:32).
• Intermittent fasting does not help many people to lose weight and keep it off, but it may help some people to lose weight (Obesity, Feb 2018;26(2):254-268) and slow down their increased risk for diabetes, heart attacks, strokes and many cancers (BioMed Central Trials, Jan 9, 2024; 25:32). Most overweight people will not stay on any type of constant calorie-restriction program, will almost always gain back the lost weight and be left with a slower metabolism that increases their risk for regaining their lost weight (Am J Clin Nutr, 2005;82(1):222S-225S).
• After you have fasted for about 12 hours, you start to lose body fat because your body is forced to change temporarily from its main energy source, glucose (sugar) to fat from the fat stored in your body, and using these fatty acids that are converted to energy to produce ketones that are also used for energy (Nutrients, Feb 2022;14(3):631). This is called a “metabolic switch”, which is defined as the shift from using glucose to using fatty acids and fatty-acid-derived ketones for energy, and then shifting back to glucose when eating is resumed (Cell Research, October 17, 2017; 27:1309-1326). Because of this, some forms of intermittent fasting have been shown to help people reduce their risk for becoming diabetic (Obesity, Feb 2018;26(2):260).
Cautions on Intermittent Fasting
Certain conditions can be worsened by fasting. Check with your doctor, particularly if you are diabetic, have low blood pressure, take medications, are underweight, have eating disorders, or are pregnant, trying to become pregnant or breast-feeding.
A review of 92 articles on intermittent fasting in athletes found that generally, intermittent fasting did no harm (Nutrients, Jan 2024;16(1):168). For example, the fasting during Ramadan helped athletes lose 6 to 12 percent of body fat in men and between 12 and 18 percent of body fat in women, without losing muscle mass (Br J Sports Med, 2022;56:1001-1002). However, I do not recommend intermittent fasting for endurance athletes because it may reduce the amount of exercise that they can perform (Nutrients, May 2020;12(5):1390). I personally found that when I fasted from 4PM to 8AM the next morning, I could not maintain my 150-mile bicycling weeks and had to take many days off from riding because of muscle pain and repeated muscle tears, particularly in my lower hamstrings and groin. Eating more food during the day did not help me at all.
Try Not to Eat at Night
Perhaps the easiest way to use intermittent fasting for weight loss or weight control is just to eat during the daytime and avoid eating at night. The most fattening time to eat is in the evening or just before you go to bed because you don’t contract your muscles very much when you sit on the couch or lie down. After you eat, your blood sugar rises and there are only three places that store sugar: your muscles, liver and fat cells. Resting muscles remove almost no sugar from your bloodstream and what little they can remove requires insulin to do so. On the other hand, contracting muscles remove tremendous amounts of sugar from your bloodstream and don’t even need insulin. When you eat and do not move much afterwards, blood sugar levels can rise very high. To keep blood sugar levels from rising too high, your pancreas releases insulin which drives sugar into muscles and liver, and then converts what extra sugar remains to a type of fat called triglycerides. The extra triglycerides are stored as fat in your liver, other organs and fat cells.
My Recommendations
This recent study was done by respected researchers, and it reinforces my advice that you should always check with your doctor before you start a program of intermittent fasting. People who are obese are at increased risk for many diseases that increase risk for premature death. If your doctor clears you for intermittent fasting, realize that what you eat is likely to be far more important than when you eat. You need to follow all the rules for a high-plant diet and other anti-inflammatory lifestyle factors:
• Keep foods you want to avoid out of your house (sugared drinks, sugar-added foods, fried foods, foods made from flour, red meat, processed meats and so forth)
• Stock lots of fruits, vegetables, whole grains, beans, nuts and other seeds
• Eat a healthful breakfast such as oatmeal with raisins
• Avoid or severely limit alcohol
• Avoid smoking and second hand smoke
• Keep your vitamin D levels above 30 ng/mL
• Try to exercise every day, and keep moving throughout the day
