Skipping food until noon is a form of fasting, and as of 2018 there was not enough evidence to recommend any type of calorie-restriction or fasting diet. A lot more needs to be learned about how effective and safe these diets are, especially in older adults. If you already eat much less than you once did, any pattern you try needs to give you a safe level of nutrition, so talk with your own doctor about the benefits and risks before you make any significant change to your eating pattern.
Which eating pattern is being recommended to you
Skipping food until noon limits the hours you eat, which is the idea behind the pattern named time-restricted feeding, in which meals are eaten within a limited number of hours each day, such as 6 to 8, with nothing eaten during the other hours. Eating from noon until bedtime can take up more hours than that example. Time-restricted feeding is one of several fasting diets, sometimes grouped as intermittent fasting, and others you may hear about limit eating by whole days rather than by hours:
- alternate-day fasting, with unrestricted eating every other day and no or minimal calories on the days in between
- the 5:2 pattern, with unrestricted eating for 5 straight days each week, followed by 2 days of restricted calories
- periodic fasting, with calories restricted for several days in a row, such as 5 days once a month, and unrestricted on all other days
If someone advising you says fasting without saying which kind, ask them which pattern they mean and, for an eating window, how many hours it lasts, because each one sets different hours or days.
Fasting and calorie restriction are defined separately. Calorie restriction means eating fewer calories on average each day than is typical or habitual, without malnutrition or going short of essential nutrients, while fasting diets are mainly about how often you eat. Both are being studied as possible ways to maintain good health and live longer, rather than as temporary weight-loss plans, even though most trials in people up to 2018 focused on weight loss.
A fasting diet may or may not involve eating fewer calories in the hours when you do eat, but a practical effect of one may be fewer calories, because there is less time for regular eating. If you already eat very little, a shorter eating window may lower a daily total that is already low, and nobody can say by how much without knowing what you eat now. If you are unsure whether what you eat now is enough, sort that out with your doctor before you change anything (see No appetite at all? Start by writing down one day of eating).
Who the research behind fasting was done in
Laboratory and animal models, from yeast cells to primates, are where most of the work on fasting and calorie restriction has been done, and those findings do not necessarily apply to people. In humans, most clinical trials have been short, lasting a few weeks or months, have focused on weight loss, and were conducted in participants who were overweight. The trials have typically recruited adults aged 60 or younger, so the results do not necessarily apply to children or to people older than 60, though researchers are beginning to study older adults.
Only a few small clinical trials of fasting itself have been done, mostly in people with obesity, so more work is needed to find out which, if any, types of fasting diet have long-term benefits. There is still much to learn about how fasting and calorie restriction affect people who are not overweight, and whether these eating patterns are safe or even doable in the long run is not yet known.
An eating pattern that helped the person recommending it might not have the same effect on you, because people differ from each other in gender, size, age, genetics, environment, lifestyle and other factors.
What the longest human trial recorded
People told to eat fewer calories for 2 years, in a trial called CALERIE, ended up with lower blood pressure and lower cholesterol than people who kept to their usual diet, and also with slight declines in bone density, lean body mass and aerobic capacity. As of 2018 it was the longest trial in people, and 2 years is not long enough to learn about long-term health effects. It tested calorie restriction, not a fasting diet, in young and middle-aged adults who were of normal weight or moderately overweight, and the restriction was not a starvation diet: the body weights reached on it stayed within the normal or overweight range.
High blood pressure and high cholesterol are both risk factors for age-related diseases such as diabetes, heart disease and stroke, so the lower readings meant reduced risk factors, and no adverse effects were found on quality of life, mood, sexual function or sleep.
The declines in bone density, in lean body mass, which is the weight of everything in the body except fat, and in aerobic capacity, the body's ability to use oxygen during exercise, were generally no more than expected from the weight the participants lost. Some participants also had brief episodes of anemia, a reduced number of the red blood cells that carry oxygen through the body (see Ask for a blood count if eating far less has worn you out).
Other short-term studies have found that combining physical activity with calorie restriction protects against losses of bone, muscle mass and aerobic capacity, so if you are eating less, ask your doctor which activity is safe for you alongside it (see Strength work twice a week, and why it matters during weight loss).
Overall, the findings indicate that the degree of calorie restriction used in the trial is safe for people of normal weight or with moderate obesity, and that clinical monitoring is recommended. That degree was set against what each person had regularly eaten before the trial, and your starting point may be much lower, so if you plan to eat less on purpose, by fasting or any other way, ask your doctor about the benefits and risks first, and whether you would need any monitoring.
How to check a practice someone swears by
The research questions written for weight-loss program staff to answer can be asked of anyone who recommends fasting, too: has it been formally studied to find out if it is safe and effective, have the study results been published in a scientific journal, and can you get a copy or a link to the study findings? If someone sends you a study to back the advice, discuss the findings with your doctor, and say what you eat now.
If the advice arrives with a plan, an app or a product to buy, stay away from programs that make promises such as weight loss without diet or exercise, or a large number of pounds in a short time, and look out for very small print, asterisks and footnotes, which may make it easy to miss important information. Another warning sign is a personal endorsement that seems too good to be true, and claims like these are worth putting to your doctor too.
If you are thinking of fasting to lose more weight, treat it like any weight-loss diet and check with your doctor first, since your doctor can tell you what a healthy weight is for you, help you set goals and give you tips on losing weight. Before the first skipped breakfast, these are worth asking your doctor:
- Does what I eat now give me a safe level of nutrition?
- If I ate only within a shorter window each day, how much lower might my total go, and how low is too low for me?
- If I tried a fasting pattern, would I need any monitoring, and what would it involve?
- Which physical activity is safe for me while I eat less?
A NicoCares treatment plan, if you have one, was built around your health profile and your goals, so send the same questions to your care team as well, in Spruce, the app their replies come through, and say what you eat on an ordinary day now.
While the research on fasting is still unsettled, there is plenty of evidence for other actions you can take to stay healthy as you age:
- a balanced diet with nutritious food in moderate amounts
- regular physical exercise
- alcohol in moderation or not at all
- not smoking
- an active social life
- a good night's sleep
