Intermittent Fasting Benefits May Differ Based On Your Age
Intermittent fasting has become one of the most popular ways to manage weight and support metabolic health, and for good reason. The research behind it is solid (even though this eating pattern definitely isn't for everyone). But most of that research has treated all adults the same, as if a 26-year-old and a 58-year-old would respond to fasting in exactly the same way.
A new systematic review and meta-analysis1 challenges that assumption directly. Researchers found that age may be one of the most important factors in determining how (and how well) intermittent fasting actually works for you.
About the study
The parse out the age-related differences, researchers reviewed and pooled data from 28 randomized controlled trials involving 1,833 generally healthy adults. Participants were divided into three age groups: under 30, 30 to 44, and 45 and older. Researchers tracked body weight, fat mass, lean muscle mass, cholesterol levels, blood sugar markers, and blood pressure.
Weight loss was universal, but everything else diverged
Intermittent fasting reduced body weight and BMI across all three age groups, but the similarities largely ended there.
- Under 30: Fat mass dropped and fasting insulin improved, but LDL cholesterol rose by about 6.75 mg/dL and lean muscle loss averaged just under 2.2 pounds
- Ages 30 to 44: Results were the most variable. Fat loss didn't reach statistical significance, and there were no meaningful improvements in cholesterol, blood pressure, or blood sugar. Higher stress, inconsistent sleep, and elevated cortisol in this life stage may blunt fasting's metabolic benefits; relatively stable sex hormones may also reduce the body's drive to adapt.
- Ages 45 and older: This age group had the most pronounced metabolic benefits. Triglycerides dropped by about 7.83 mg/dL and systolic blood pressure fell by nearly 5 mmHg. However, nearly 1 kg of lean muscle was lost on average, with greater consequences given age-related muscle loss. Additional analysis also revealed a small but significant LDL increase not obvious in the standard analysis.
Why your age shapes your fasting results
In young adults, the liver is highly metabolically flexible. After breaking the fast (especially if the eating window includes high-glycemic carbs or saturated fats), the liver may overcompensate by producing more LDL cholesterol. Both time-restricted eating (like fasting for 16 hours and having an 8 hour eating window) and alternate-day fasting were associated with LDL increases, regardless of age.
In the 30-to-44 group, the blunted response likely comes down to a mix of lifestyle and hormonal factors. Chronic stress raises cortisol, which promotes insulin resistance and can trigger overeating during eating windows. Meanwhile, relatively stable sex hormones in this age bracket provide a kind of metabolic buffer, reducing the body's drive to adapt.
For adults 45 and older may have a more dramatically positive change in metabolic markers, as fasting may counter some of the metabolic changes that declining sex hormone levels brings. But aging also makes it harder for the body to build and maintain muscle, which is why lean mass loss becomes a more serious concern here.
Should you fast?
The decision to practice intermittent fasting is entirely personal. Some people love the structure this eating pattern provides and the guardrails on when to eat. Others, don't mesh well with it.
If you are a highly active person and strength training and participate in rigorous cardiovascular exercise a week, prolonged fasting may get in the way of your muscle recovery efforts. Intermittent fasting is also generally not recommend for pregnant or breastfeeding women.
The takeaway
Intermittent fasting works for weight management across all adult age groups, but the metabolic effects and the risk of losing muscle should be noted. The goal of weight loss is to not lose lean mass, so eating enough protein and strength training as especially important if you follow any time of fasting regimen (no matter your age).