There is a lot of confusion and outright falsehoods about fasting. You can search the internet for clarity and end up more confused than when you started.
The fact is, fasting can be a very effective tool to improve your metabolic health, increase insulin sensitivity and help you lose weight. But, who should and shouldn’t use fasting for weight loss and how long to fast for effective weight loss are questions that need to be answered before you rush into fasting.
The main thing to consider is to what end? What’s your why?
If you’re considering using fasting as a tool to grind out a weight loss goal more quickly than without it, stop right there.
You’re stuck in diet thinking, and that will backfire.
If you’re considering using fasting as a tool to improve your metabolic health and it’s also a tool you are considering using long-term, then you’re on the right track.
As a physician, certified in obesity medicine, and someone who has gone through my own weight loss journey, I support fasting as one of the tools to help patients and clients achieve their metabolic health and weight goals.
Important disclaimer: I do not recommend fasting for teens.
As physicians, we know that we aren’t getting anywhere when it comes to helping patients address insulin resistance, metabolic syndrome, and obesity as a chronic disease. The old calories in calories out diet mentality, along with the Food Addiction Business Model’s highly successful conditioning of our brains to over desire ultra-processed foods, have caused harm and perpetuated the epidemic of insulin resistance, overweight, and obesity in the U.S.
Diet culture has created and perpetuated the myth that our weight is simply calories in minus calories out. We’ve also been taught to eat small meals frequently throughout the day, both of which have perpetuated the epidemic of insulin resistance and comorbidities.
If insulin resistance and its metabolic consequences are the problems, then the solution is not to restrict overall calories but rather to use tools that will increase insulin sensitivity.
Dr. Jason Fung’s book, The Obesity Code, has upended the status quo that has kept Americans stuck in a never-ending cycle of weight gain and disease. Obesity is hormonal and not simply caloric. It’s the quality and composition of calories and the timing of eating that counts. Not all calories are created equal.
What to eat and when to eat is what’s important.
The Complete Guide to Fasting by Dr. Jason Fung is also an excellent resource. The benefits of fasting are hormonal, specifically, fasting consistently lowers insulin levels, which leads to increased insulin sensitivity, where your body becomes more responsive to insulin.
Based on my personal experience with fasting and coaching many adult clients on weight loss using fasting, it’s best to start dialing in on what you are eating first, specifically reducing processed carbohydrates and refined sugars and eating a balance of quality carbohydrates and more protein, and healthy fats.
Starting with when you eat, or fast, can be miserable if your body is carb-adapted and not fat-adapted. In other words, when your body is a “sugar burner,” it’s not used to mobilize your fat stores for energy. Becoming fat-adapted, where your hunger comes in waves instead of tsunamis, allows you to fast without discomfort and really benefit metabolically from fasting. Becoming fat-adapted typically takes several weeks.
To start fasting, go slowly by narrowing your eating window and then gradually decrease your eating window over time. You may start by not eating after dinner, so fasting for 12 hours overnight.
Weight loss doesn’t always happen right away, so don’t make it mean there is something going wrong if you’re not losing weight when you start fasting.
Stay hydrated. This is critical.
Make sure you have your own back when you’re fasting. Tune in to your body and don’t be in a rush to get to your goals.
Whether you start with alternate-day fasting, 5:2 or intermittent fasting is up to you, your physician, and dependent on your metabolic health goals.
Make sure you stay out of all or nothing thinking with fasting. Make sure you eat the same amount of quality food you would in a day, just in a narrower eating window. Your body needs to know it will get fed or your body will start to decrease energy expenditure and activate your hunger hormones to tell you to eat, eat, eat!
Fasting is intentional and should never be punishing or used to restrict yourself. If you end up needing to break a fast, make sure you plan ahead to eat quality food, so you don’t end up binging on processed carbs. If you end up breaking a fast or overeating, don’t punish or restrict yourself by fasting the next day if you hadn’t planned on it. Don’t use fasting to restrict calories. If you do, then it’s just a diet and will keep you stuck in the extremes of indulgence and restriction and away from your metabolic health goals.
Focus on the metabolic health benefits of fasting, so your non-scale victories. It really helped me to focus on increasing insulin sensitivity as the main benefit of fasting.
Who should consider fasting? If you are an adult patient with overweight or obesity and have insulin resistance, you will most likely benefit from fasting. You should always seek consultation with your physician before fasting, especially if you are taking medications for comorbidities like type 2 diabetes. You can also go to the American Board of Obesity Medicine to find a physician near you who will help you manage weight and improve your metabolic health.
Who should not consider fasting? Teens should never intentionally fast. If you are an adult with binge eating, a history of binge eating or an eating disorder, fasting could potentially create more harm.
If you are on medications, please consult your physician before fasting. Your mindset starting out with fasting is critical. If you are high achieving, motivate yourself with negative self-talk, or feel in a hurry to get to your end goal (all physicians), stop right there. I recommend taking a pause and working on mindfulness, self-care, and self-compassion before considering fasting.
As physicians, we have minimal training in nutrition and weight management.
Make sure you lean into becoming educated and gathering resources before prescribing fasting to your patients.
Close follow-up and support are needed to help your patients use fasting as an effective tool to improve their metabolic health.
Karla Lester is a pediatrician, certified life and weight coach, and diplomate, American Board of Obesity Medicine. She is founder, IME Community, and can be reached on Twitter @DrKarlaA, TikTok, Instagram @ime_community, Facebook, and YouTube.
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