Tag: Intermittent fasting

Does Fasting Slow Aging? What the Science Says

On By Dr. Scott RennieIn Longevity & Healthy AgingLeave a comment

Aging cannot be stopped. The pace of it may be influenced by what and when we eat, and that idea is old rather than new. Since the 1930s, animal studies have shown that cutting calories without causing malnutrition extends lifespan. Rodents held at a 30 percent reduction lived longer with less cancer and less heart disease.

The proposed mechanisms are lower oxidative stress, better mitochondrial function, and less inflammation. Less fuel in, less metabolic wear.

Then you move from mice to people and it gets complicated.

The CALERIE trials brought calorie restriction into human research. CALERIE II found that cutting roughly 15 percent of calories for two years improved blood pressure, cholesterol, and markers of oxidative stress, and those benefits appeared without major weight loss (Kraus et al., 2019). The obstacle is adherence. Sustained restriction is hard on mood, on social life, and on nutritional balance, and most people will not hold it.

Which is how fasting patterns entered the conversation. Intermittent fasting covers several approaches. Some people fast on alternate days. Others follow 5:2, eating normally five days a week and restricting on two. Time-restricted feeding, which confines eating to an 8 to 10 hour window, has drawn the most recent attention.

Alternate-day fasting does produce fat and weight loss, but hunger is a serious barrier and dropout rates run high. Modified versions have not clearly outperformed standard calorie restriction (Trepanowski et al., 2017).

Time-restricted feeding looks more workable. Sutton and colleagues tested an early schedule with all meals between 7 a.m. and 3 p.m. Participants did not lose weight, but insulin sensitivity, blood pressure, and oxidative stress markers all improved, and they reported less hunger through the evening (Sutton et al., 2018). Other short-term work points the same direction, particularly on appetite control and metabolic flexibility (Hatori et al., 2012; Varady et al., 2022).

What does that mean in practice? Calorie restriction has the strongest animal data and some genuinely promising human results, and almost nobody sustains it. Alternate-day fasting rarely survives contact with real life. Time-restricted feeding is the one most patients can actually keep, and even moving to a 10 hour window and dropping late-night meals is a defensible starting point.

When I talk this through with patients, I usually suggest starting at a 12 hour window and tightening it only if they tolerate the change well. What goes in the window still matters more than the window does, and nutrient density should not get lost in the enthusiasm about timing.

Fasting is one tool among several, and which one fits depends on the person, their health, and what they can hold onto for longer than a month.

Scott Rennie, D.O.

References

Ravussin E, Redman LM et al. 2015.

Kraus WE et al. Lancet Diabetes Endocrinol. 2019.

Sutton EF et al. Cell Metab. 2018.

Trepanowski JF et al. JAMA Intern Med. 2017.

Heilbronn LK et al. 2005.

Spadaro PA et al. 2022.

Hatori M et al. Cell Metab. 2012.

Varady KA et al. 2022.

Board Certified in Obesity Medicine and Family Medicine

This blog is for educational purposes only and does not constitute individual medical advice. Always consult your own physician before making changes to your health, medications, or treatment plan.

How Does Aging Change Your Metabolism? What Research Shows

On By Dr. Scott RennieIn Weight Loss & Metabolic HealthLeave a comment

Obesity, diabetes, cardiovascular disease, and frailty get treated as though they were the price of getting older. Work from researchers like Luigi Fontana points somewhere else. Decades of metabolic strain do most of this damage, and diet, activity, and the choices stacked on top of them shape how much strain accumulates.

The scale is hard to ignore. Chronic disease accounts for close to 90% of U.S. healthcare spending, most of it aimed at managing complications rather than preventing them (Martin et al., Health Affairs, 2021). Life expectancy in this country fell by more than two years between 2019 and 2021. Obesity now affects 40.3% of adults aged 20 and older, with 9.7% in the severe range (1). Excess adiposity drives hypertension, diabetes, fatty liver disease, and several cancers.

The biology is familiar. Oxidative stress, inflammation, and insulin resistance sit underneath most of these conditions (Fontana & Partridge, Cell, 2015). Abdominal fat is active tissue rather than storage. It fuels insulin resistance and chronic inflammation, and it also activates bone marrow. In the PESA cohort of 745 apparently healthy adults imaged with FDG-PET, bone marrow activation tracked with every component of metabolic syndrome, and the activated group carried metabolic syndrome at 22.2% versus 6.7% (Devesa et al., Eur Heart J, 2022). Obesity changes the terrain of the body long before symptoms show up.

Calorie restriction without malnutrition is the clearest intervention we have. The animal data are strong. In rhesus monkeys, long-term restriction slowed age-related brain atrophy (Colman et al., Science, 2009), preserved muscle and function (Colman et al., J Gerontol, 2008), and lowered frailty and chronic disease burden (Yamada et al., J Gerontol, 2018).

This literature gets oversold, and the primate data are where it happens. The 2017 joint reanalysis of the two big studies found a survival benefit in the Wisconsin animals and no significant survival effect in the National Institute on Aging cohort (Mattison et al., Nat Commun, 2017). Health measures improved in both. Lifespan did not reliably follow. That is a more honest summary than the one usually quoted.

Human data are thinner but point the same direction. CALERIE tested roughly a 25% calorie reduction over two years and found improvements in blood pressure, cholesterol, glucose, inflammatory markers, heart rate variability, and insulin sensitivity (Meydani et al., Aging, 2016; Stein et al., Aging Cell, 2012; Weiss et al., Am J Clin Nutr, 2006). Even the more realistic reduction people actually achieved, closer to 12%, produced meaningful metabolic change (Kraus et al., Lancet Diabetes Endocrinol, 2019).

Calories are one variable. Nutrient quality and timing matter too. Studies in mice suggest high-protein diets may shorten lifespan, while restricting methionine or branched-chain amino acids improves metabolic health and longevity markers (Solon-Biet et al., Cell Metab, 2014). A trial in men with prostate cancer found that a single month of protein restriction lowered fat mass, cholesterol, and insulin (Fontana et al., Cell Rep, 2016). Protein source appears to matter as well, with plant-based sources holding an advantage over animal-based ones.

Timing has drawn its own attention. Intermittent fasting, whether through time-restricted eating or alternate-day fasting, extends lifespan in animal studies and protects against age-related disease including diabetes and cancer (Mattson et al., PNAS, 2014). Human studies suggest benefits on body fat, insulin sensitivity, and metabolic markers (Tosti et al., Aging Biology, 2022).

The gut microbiome adds a layer underneath all of it. Diet shapes microbial diversity and function, which in turn shapes inflammation, immunity, and metabolism (Thorburn et al., Immunity, 2014; Griffin et al., Cell Host Microbe, 2017). Fiber, protein type, and eating pattern all shift that balance. Nutrition never acts alone. It works through microbial partners.

What Fontana and others argue is that the real challenge lies less in treating diseases once they appear and more in holding metabolic integrity across a lifespan. That means healthcare built around prevention rather than reaction. Whole-food, plant-predominant eating. Fewer excess calories. Less reliance on protein-heavy patterns. Fasting strategies where they fit the patient. Attention to gut and immune health.

Aging isn’t a disease. Metabolic dysfunction is. Treat it early and seriously, through diet, lifestyle, and the interventions that have evidence behind them, and the curve bends toward years worth having.

Scott Rennie, D.O.

References:

1. National Center for Health Statistics. Prevalence of Overweight, Obesity, and Severe Obesity Among Adults Age 20 and Older: United States, 1960–1962 Through August 2021–August 2023. https://www.cdc.gov/nchs/data/hestat/obesity-adult-17-18/obesity-adult.htm

2. Martin AB, et al. Health Affairs. 2021.

3. Fontana L, Partridge L. Promoting health and longevity through diet: from model organisms to humans. Cell. 2015;161(1):106-118. https://pubmed.ncbi.nlm.nih.gov/25815989/

4. Fontana L, Kennedy BK, Longo VD, Seals D, Melov S. Medical research: treat ageing. Nature. 2014;511(7510):405-407. https://pubmed.ncbi.nlm.nih.gov/25056047/

5. Devesa A, et al. Bone marrow activation in response to metabolic syndrome and early atherosclerosis. Eur Heart J. 2022;43(19):1809-1828. https://pubmed.ncbi.nlm.nih.gov/35567559/

6. Colman RJ, et al. Science. 2009;325(5937):201-204.

7. Colman RJ, et al. J Gerontol A Biol Sci Med Sci. 2008;63(6):556-559.

8. Yamada Y, et al. J Gerontol A Biol Sci Med Sci. 2018;73(3):273-278.

9. Mattison JA, et al. Caloric restriction improves health and survival of rhesus monkeys. Nat Commun. 2017;8:14063. https://pubmed.ncbi.nlm.nih.gov/28094793/

10. Weiss EP, et al. Am J Clin Nutr. 2006;84(5):1033-1042.

11. Meydani SN, et al. Aging (Albany NY). 2016;8(7):1416-1431.

12. Stein PK, et al. Aging Cell. 2012;11(4):644-650.

13. Kraus WE, et al. Lancet Diabetes Endocrinol. 2019;7(9):673-683.

14. Solon-Biet SM, et al. Cell Metab. 2014;19(3):418-430.

15. Fontana L, et al. Cell Rep. 2016;16(2):520-530.

16. Mattson MP, et al. Proc Natl Acad Sci USA. 2014;111(47):16647-16653.

17. Tosti V, et al. Aging Biology. 2022.

18. Thorburn AN, et al. Immunity. 2014;40(6):833-842.

19. Griffin NW, et al. Cell Host Microbe. 2017;21(1):84-96.

Board Certified in Obesity Medicine and Family Medicine

This blog is for educational purposes only and does not constitute individual medical advice. Always consult your own physician before making changes to your health, medications, or treatment plan.

Does Meal Timing Affect Your Metabolism and Health?

On By Dr. Scott RennieIn Weight Loss & Metabolic HealthLeave a comment

When you eat can matter as much as what you eat. Not a fad, a measurable physiological pattern, and one most people never think to adjust. Timing affects weight management, metabolic health, and even the risk of chronic disease, and the field studying the connection between circadian rhythms and eating patterns has a name: chrononutrition.

Typical eating habits in the U.S. don’t line up well with what the research suggests is healthiest. Most people eat less than a quarter of their daily calories before noon, and more than a third after 6 p.m. The average eating window stretches nearly fifteen hours a day (Gill and Panda, 2015). That long, backloaded pattern is linked to higher body fat, worse glucose tolerance, and poorer metabolic outcomes.

Evidence favors eating earlier. In one trial, people who made breakfast their largest meal had better glucose and insulin control, felt more satisfied during the day, and lost more weight than those who made dinner the main meal (Jakubowicz et al., 2013). That tracks with circadian biology: the body handles nutrients more efficiently in the morning than late at night.

Breakfast does more than supply calories. Regular breakfast eaters tend to take in more vitamins and minerals overall (St-Onge et al., 2017), and they show lower rates of obesity and more stable weight over time. Skipping breakfast, by contrast, tracks with higher BMI, larger blood sugar swings, and greater cardiovascular risk.

Time-restricted eating fits the same pattern. Narrow the eating window to eight or twelve hours and total calorie intake often falls on its own, insulin sensitivity improves, and fat oxidation increases (Gill and Panda, 2015). Early time-restricted eating, where meals cluster in the morning and early afternoon, improves appetite control and fat metabolism specifically (Ravussin et al., 2019).

Intermittent fasting runs on the same logic. The most common version is 16:8: fast sixteen hours, eat during an eight-hour window. Alternate-day fasting has also been studied, and it produces weight loss similar to plain calorie restriction, though dropout rates run higher (Trepanowski et al., 2017).

What the science of chrononutrition shows is that timing matters. Shifting more calories to the first part of the day improves metabolism and weight control, breakfast and early meals set the body up for better glucose regulation and satiety, and time-restricted eating and intermittent fasting add tools on top of that, especially when they lean toward earlier eating windows rather than later ones. For patients, these strategies are often more about changing the clock than drastically changing the plate.

Scott Rennie, D.O.
Board Certified in Obesity Medicine and Family Medicine

This blog is for educational purposes only and does not constitute individual medical advice. Always consult your own physician before making changes to your health, medications, or treatment plan.

References:

  • Gill S, Panda S. A Smartphone App Reveals Erratic Diurnal Eating Patterns in Humans that Can Be Modulated for Health Benefits. Cell Metabolism. 2015;22(5):789-798. PMID 26411343. https://pubmed.ncbi.nlm.nih.gov/26411343/
  • Jakubowicz D, et al. High Caloric Intake at Breakfast vs. Dinner Differentially Influences Weight Loss of Overweight and Obese Women. Obesity (Silver Spring). 2013;21(12):2504-2512. PMID 23512957. https://pubmed.ncbi.nlm.nih.gov/23512957/
  • St-Onge MP, et al. Meal Timing and Frequency: Implications for Cardiovascular Disease Prevention. Circulation. 2017;135:e96-e121. PMID 28137935. https://pubmed.ncbi.nlm.nih.gov/28137935/
  • Ravussin E, et al. Early Time-Restricted Feeding Reduces Appetite and Increases Fat Oxidation But Does Not Affect Energy Expenditure in Humans. Obesity (Silver Spring). 2019;27(8):1244-1254. PMID 31339000. https://pubmed.ncbi.nlm.nih.gov/31339000/
  • Trepanowski JF, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults. JAMA Intern Med. 2017;177(7):930-938. PMID 28459931. https://pubmed.ncbi.nlm.nih.gov/28459931/

Can You Do Intermittent Fasting on Ozempic or Wegovy?

On By Dr. Scott RennieIn Weight Loss & Metabolic HealthLeave a comment

Weight comes up in almost every visit I do. For some patients it is fifteen pounds standing between them and a better blood pressure number. For others it is obesity that has already done damage, and the conversation starts further back. The work can feel overwhelming from the inside. What’s changed is that the tools finally match the size of the problem. Medications like Ozempic, Wegovy, Mounjaro, and Zepbound, the GLP-1 receptor agonists, have shifted how this is approached, and paired with a structure like intermittent fasting they help people lose weight and keep it off.

What GLP-1 Agonists Do

These drugs mimic glucagon-like peptide-1, a hormone that regulates appetite and blood sugar. Given as medication, they slow gastric emptying and push stronger satiety signals to the brain, so fullness arrives earlier and stays longer. They also improve insulin sensitivity, which is why they earned their place in type 2 diabetes first.

The weight effect is substantial. Wegovy and Zepbound carry FDA approval specifically for weight loss. Ozempic and Mounjaro are approved for diabetes and produce strong weight results as well, which is the source of most of the confusion patients arrive with about which drug is which.

How They Work With Intermittent Fasting

Intermittent fasting improves insulin sensitivity, supports fat loss, and helps regulate hunger hormones. Staying with it is the hard part. Many patients tell me they can’t get past the hunger. GLP-1 medications change that equation by blunting appetite and cravings, which makes a fasting schedule something a person can actually hold.

A patient of mine started a 16:8 fasting plan (16 hours fasting, 8 hours eating) while on a GLP-1 medication. Before starting the medication, she felt shaky and irritable during fasting. After starting, she was surprised by how manageable it felt. She ate smaller meals, felt full, and didn’t struggle to maintain the fasting window.

Side Effects and Adjustments

Nausea leads the list, and it’s worst early. Diarrhea and reflux show up too. Most of it settles as the body adapts. Start low, titrate slowly, and resist the urge to chase the next dose because the scale stalled for two weeks. Patients who stay in contact through the titration get their dose adjusted before they quit over side effects, and the ones who go quiet are the ones who stop the drug entirely.

Barriers to Access

Getting these medications is its own project. Cost is the main wall. Insurance coverage for weight loss remains inconsistent in a way that’s hard to explain to a patient who has just been told the drug would help, and out-of-pocket pricing is punishing. Demand has outrun supply, so delays and shortages are part of the picture.

Then there are the compounded versions. Some pharmacies sell them well below brand pricing, and they’re not FDA-approved. Safety and potency can’t be guaranteed. I tell patients to stay away from them, and I don’t soften that advice when someone pushes back on price.

Putting It Into Context

These aren’t quick fixes. They are tools, and they work when they sit on top of durable changes: balanced eating, regular activity, attention to mental health. Intermittent fasting is one workable way to structure eating alongside them. The lifestyle piece doesn’t become optional because a medication is doing part of the lifting.

When patients pair the medication with habits they can sustain, results hold longer and vary less. The goal is a set of strategies that still works three years from now, not the fastest possible drop on the scale.

Scott Rennie, D.O.

Sources

U.S. Food and Drug Administration. FDA Approvals: Wegovy, Zepbound.

Wilding JPH, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. N Engl J Med. 2021;384:989-1002.

Jastreboff AM, et al. Tirzepatide Once Weekly for the Treatment of Obesity. N Engl J Med. 2022;387:205-216.

American Diabetes Association. Pharmacologic Approaches to Glycemic Treatment: Standards of Medical Care in Diabetes, 2024. Diabetes Care. 2024;47(Suppl 1):S181-S202.

Board Certified in Obesity Medicine and Family Medicine

This blog is for educational purposes only and does not constitute individual medical advice. Always consult your own physician before making changes to your health, medications, or treatment plan.

Wall clock with illustrations of various foods representing mealtime hours

Does Intermittent Fasting Work? A Doctor Explains

On By Dr. Scott RennieIn Weight Loss & Metabolic HealthLeave a comment
snake-diet-fasting

Intermittent fasting, sometimes called therapeutic fasting or intermittent energy restriction, has gained attention both in the media and in clinical practice. Patients ask about it regularly, often because they’ve heard it can help with weight loss or with lowering blood sugar in diabetes.

I have no ties to pharmaceutical companies or research groups, and I don’t profit from sharing this information. My purpose is simply to explain what I’ve seen and what the research shows. For patients with diabetes or heart disease, or those taking medications, I always recommend making changes under the guidance of a physician. Fasting can alter medication needs quickly, sometimes within days.

Why Weight Matters

About 75% of adults in the United States are overweight or obese, and the number continues to rise. Excess weight contributes to metabolic syndrome, diabetes, hypertension, and heart disease, and it increases the risk of stroke and some cancers. Many patients struggle with arthritis or back pain from the mechanical stress of extra weight. Others develop sleep apnea, fatigue, or sexual dysfunction. Beyond the health issues, there are real-world effects: trouble traveling, difficulty exercising, and limits on daily activities.

Why “Eat Less, Move More” Falls Short

The common advice is to cut calories and exercise more. Most patients I’ve seen have tried this approach at some point. The problem is that it rarely works long term; weight usually returns, often within months.

A striking example came from contestants on The Biggest Loser. Many lost large amounts of weight during the show, but most regained it later. As one participant admitted, “we’re all fat again.” Research supports this. A 2012 study in the New England Journal of Medicine followed patients on calorie restriction and found no sustained improvement in BMI after five years. The Women’s Health Initiative produced similar results over nine years.

The reason is physiology. When calorie intake drops, the body adapts by slowing its metabolism. If someone weighs 230 pounds and cuts back to 1500 calories a day, the body may drop its resting burn rate to 1300 calories. That person feels cold, sluggish, and hungry. Hormones like ghrelin keep signaling hunger, even long after dieting begins, and eventually weight creeps back.

The Role of Insulin

Energy comes from two sources: food and stored fat. When we eat, insulin rises and directs the body to store extra energy as glycogen or fat. At the same time, fat breakdown is switched off. This makes weight loss difficult if insulin stays high, especially with frequent meals or a high-carbohydrate diet.

How Intermittent Fasting Works

Intermittent fasting lowers insulin levels by reducing the frequency of eating. With food energy unavailable, the body taps into fat stores. Fasting can be combined with any diet, but it works particularly well with lower carbohydrate intake, since that also keeps insulin levels down.

Patients use different fasting schedules. A common one is the 16:8 approach, where meals are limited to an 8-hour window. Some prefer one meal a day. Others follow alternate-day fasting or the 5:2 pattern, eating normally for five days and restricting calories for two.

Water intake is essential. During fasting, the body needs extra water to help mobilize fat for energy. Electrolytes, especially sodium, potassium, magnesium, and calcium, also need attention, since ketosis increases their loss through the kidneys. Many patients avoid “keto flu” symptoms like dizziness, fatigue, or cramps by adding broth or electrolyte-rich foods.

What the Evidence Shows

Fasting does not slow metabolism. In fact, studies have shown a slight increase in metabolic rate during fasting, partly due to rises in norepinephrine (Zauner C, Am J Clin Nutr 2000;71:1511-1515). Hunger doesn’t keep building either. Ghrelin levels rise and fall around usual mealtimes, then settle back if food is skipped.

Research also shows fasting preferentially burns fat rather than muscle (Drenick EJ, Am J Public Health Nations Health. 1968;58(3):477-484). Men and women both respond with fat loss, averaging about half a pound per fasting day in controlled studies.

Beyond weight, there are possible additional benefits. Animal studies suggest longer lifespan, reduced cancer risk, and protection against neurodegenerative conditions. Human research is still developing, but some studies show improved insulin sensitivity, better blood sugar control, and even improved mood and cognition.

Safety and Practical Considerations

Fasting is not new. Hippocrates wrote about it, Plato praised it, and major religions have incorporated fasting traditions for centuries, from Ramadan to Lent to the Jewish high holy days. In most overweight patients it is safe when supervised. It is not appropriate for those who are pregnant, underweight, or struggling with eating disorders.

Patients with diabetes, especially those on insulin, need careful monitoring. Many see rapid improvements in blood sugar and may require lower doses. Some patients with type 1 diabetes using low-carb diets report more stable glucose and lower A1c.

For patients who like numbers, tools exist to track progress. Ketone meters can show if the body is burning fat. Blood sugar meters can help patients see how specific foods affect them. Insulin testing is available in labs, but not over the counter.

The Bottom Line

Intermittent fasting is not a fad. It is a structured way to reduce insulin, tap into fat stores, and improve metabolic health. Like any medical intervention, it should be individualized. With guidance, many patients find fasting more sustainable than calorie restriction and more effective for long-term weight control.

Scott Rennie, D.O.

Sources

  • Zauner C, et al. Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. Am J Clin Nutr. 2000;71:1511-1515.
  • Drenick EJ, et al. Prolonged starvation as treatment for obesity. Am J Public Health Nations Health. 1968;58(3):477-484.
  • Sumithran P, et al. Long-term persistence of hormonal adaptations to weight loss. N Engl J Med. 2011;365:1597-1604.
  • Look AHEAD Research Group. Long-term effects of a lifestyle intervention on weight and cardiovascular risk factors. N Engl J Med. 2013;369:145-154.

Board Certified in Obesity Medicine and Family Medicine

This blog is for educational purposes only and does not constitute individual medical advice. Always consult your own physician before making changes to your health, medications, or treatment plan.