Calorie restriction and fasting

What it is

• In a fasting diet, a person does not eat, or eats very little, during certain times of the day, week, or month.
• Calorie restriction is a steady pattern of eating fewer calories each day, while fasting plans focus on how often a person eats.
• Time-restricted feeding means eating all meals within a limited number of hours each day.
• On alternate-day fasting, people eat freely every other day and eat little or nothing on the days between.
• The 5:2 pattern means eating freely for 5 days a week and limiting calories on 2 days.

Source: National Institute on Aging: Calorie restriction and fasting diets: What do we know? ↗

How it's thought to help

• Scientists still don't know why calorie restriction extends life and delays age-related diseases in lab animals.
• During fasting, the body uses up glucose (sugar) and glycogen, then turns to energy stored in fat, which is released as ketones.
• Some researchers think ketones may protect against aging-related decline in the central nervous system.
• Ketones are thought to lower insulin levels in the blood, which could protect against type 2 diabetes.
• An NIA review said the key mechanism is metabolic switching, where the body changes its energy source from glucose to ketones.
• The review authors said this ketone activity helps the body defend against stress and remove or repair damaged molecules.

Sources: National Institute on Aging: Calorie restriction and fasting diets: What do we know? ↗ · National Institute on Aging: Research on intermittent fasting shows health benefits ↗

What people use it for

These eating patterns are being studied as possible ways to stay healthy and live longer.

Source: National Institute on Aging: Calorie restriction and fasting diets: What do we know? ↗

What the research has found so far

• In many animal experiments, calorie-restricted feeding delayed age-related disorders and, in some studies, extended lifespan.
• The NIA says there are no human data on calorie restriction and longevity.
• An NIA-conducted review said studies show intermittent fasting can improve conditions such as obesity, diabetes, and heart disease.
• The NIA says there is not enough evidence to recommend any calorie-restriction or fasting diet.

Sources: National Institute on Aging: Calorie restriction and fasting diets: What do we know? ↗ · National Institute on Aging: Research on intermittent fasting shows health benefits ↗

On calorie restriction and fasting and aging, PubMed lists 129 trials in people, 73 studies that watched people over time, 2,885 animal and lab studies. See the counts and searches

Side effects and risks

• In the CALERIE trial, calorie restriction caused slight declines in bone density, lean body mass, and aerobic capacity.
• Some CALERIE participants had brief episodes of anemia, meaning fewer red blood cells that carry oxygen.
• In some mouse strains, calorie restriction shortened lifespan instead of extending it.

Source: National Institute on Aging: Calorie restriction and fasting diets: What do we know? ↗

Who should be careful

• MedlinePlus says intermittent fasting is not safe for everyone, especially people with diabetes, kidney disease, or heart-related issues.
• Researchers don't know if these eating patterns are safe or doable over the long run.
• Trials mostly enrolled adults age 60 or younger, so results may not apply to children or people older than 60.
• The review authors said it is not yet known if intermittent fasting is safe for people at a healthy weight, or who are younger or older.
• The NIA says clinical monitoring is recommended even for the level of calorie restriction used in CALERIE.

Sources: MedlinePlus (NIH): 5 questions about intermittent fasting ↗ · National Institute on Aging: Calorie restriction and fasting diets: What do we know? ↗ · National Institute on Aging: Research on intermittent fasting shows health benefits ↗

Questions to ask your doctor

• Is calorie restriction and fasting a good fit for me, given my health and the medicines I take?
• Is there anything I should avoid or watch for, given my health conditions?
• Where should I start, and how much is right for me?
• How would we tell whether it is helping, and when should we review it?