|Dr Clyde Reading the post (Long Version)
To start the conversation, consider that fasting can reduce your muscle mass more than your fat:
Intermittent fasting burns you up & out: you can lose twice the lean tissue as fat skipping breakfast
Fasting’s metabolic benefits come at a high cost: your body consuming itself. No matter how much fat you burn during fasting, lean tissue loss is inevitable. This is because your body needs more than just fat. Intermittent fasting by skipping breakfast therefore burns up to twice as much lean tissue as body fat: Research Link
Skipping breakfast might sound like an easy way to cut Calories and rejuvenate the body into youth and health, but at the cost of the very tissue you are trying to help. This raises the question of why we eat. If fasting is so amazing, why ever stop? Why ever eat anything at all? While this question might sound absurd, the popularity of fasting begs it to be asked, and it leads to four deeper questions whose answers are not so obvious: at what point does fasting begin to hurt you (immediately), are the benefits of fasting better than caloric restriction (no), is caloric restriction always better than fasting (not if you eat processed food), and how can you include some processed food in your diet (by combining it with vegetables). Now in a bit more detail:
When are fasting’s benefits outweighed by its detriments? The short answer: immediately. Caloric and carbohydrate restriction (instead of fasting) support the cells of our vital organs to maintain their continuous needs. Cells cannot turn off their needs just because we are not feeding them, so they eat each other by the brain stimulating the release of stress hormones to break down muscle mass to benefit vital organs.
Do human studies show caloric restriction provides the benefits we are trying to achieve with fasting? Yes. Every study that bothers looking shows fasting provides no benefits beyond its caloric restriction. There is therefore no additional unique benefit of fasting beyond its underlying caloric restriction, and fasting accelerates lean tissue loss, whereas caloric restriction does not (if eating healthy foods).
Is caloric restriction enough to get the same benefits as fasting? No, it’s not. Processed food, in particular processed carbohydrates, trigger an insulin response in the body sending out the signal that you are over-fed even if you are under-fed. This is like filling your car’s gas tank so fast that gas overflows out onto your feet even though the tank is still empty: the delivery of gas to the tank has to be slow enough to go through the delivery line or it comes shooting out as if the tank were full. Likewise, the delivery of carbohydrates through your bloodstream must be slow enough to be taken up by your cells or else it will skyrocket your blood sugar. Flooding carbs into your bloodstream therefore forces your body to go into an over-flooded response even if you have barely eaten anything; think candy, soda, chips, juice, cookies, muffins, and anything else made of sugar and/or flour. Fasting followed by processed carbs is therefore the worst combination, whittling away at lean tissue both when you are eating as well as when you are not.
How can I continue to occasionally eat the processed foods I love without them hurting me so much that I keep having to struggle with diets to make up for it? Eat processed foods right after exercise when your lean tissue is absorbing Calories faster, and at all other times eat processed foods together with unprocessed foods within the same meal to slow the rate of nutrient digestion and delivery. For example, having vegetables or salad in the same meal as processed carbohydrate provides a profound clinical benefit to diabetics: Research Link
The detriments of processed carbohydrate and unhealthy fats are the main driving forces behind the obesity and diabetes epidemics as well as our search for the best diet to counter how we wish we could eat that lead to those epidemics in the first place (inactivity and stress both worsen the effects of a poor diet and increase our tendency to have a poor diet). The belief that it just comes down to Calories (lending itself to cutting fats out of the diet) was a century ago replaced with a focus on what (not simply how much) we should. Diets therefore emerged with opposing viewpoints as their creators over-emphasized one food group over another, some touting vegetables, others protein, and more recently keto diets espousing mainly fats (the extreme opposite of what dieters used to focus on). All of these work in that they steer adherents away from processed carbohydrate, but none of them eliminate our psychological need for those carbs, nor do any ensure that someone’s full balance of nutrient needs are being met. There is no diet that can keep you healthy if it does not meet your needs since health and meeting your needs are the same thing. Assuming that your needs are met both with respect to specific nutrients and enough Calories to avoid losing lean tissue, dieting can then finally deliver the healthy weight loss we are after.
What to eat is covered in a separate “research update” tab on the homepage. The best and most current Caloric need equations from which you could estimate your maximum 25% Caloric restriction (beyond which you health and lean tissue are at greater risk) was published by the National Academy of Medicine in 2023 in this free online text with the equations on page 80:
As you look at these equations you can see how our Caloric needs scale with age, height, weight, and activity levels, with our height and weight having ever greater impact on needs as our activity levels increase. This gives you a sense of how much energy it takes to accelerate (based on mass) and create torque (based on height) on your limbs when active.
We know from published research that non-exercise movement throughout the day can double our Caloric needs (Levine, 2015; Research Link) and that inactivity and dieting can cut your needs in half because of how the body adapts (my research at UCSF; Research Link). This is why calculating your Caloric needs only gives you an average need, but you might need double that or only half that even without considering exercise, which increases the range even more. I recommend shifting how you eat based on what you are already doing and your goals without counting Calories at all. You already know what foods and how much of them you eat. And you know your goals. Shift what it looks like on your plate instead of crunching numbers. Because our body can rev up or shut down so much with respect to how many Calories it burns, without a focus on providing your body enough Calories and nutrients to keep it going, it can easily backfire and work against you as you struggle to achieve your goals. The worst-case scenario is the combination of hard exercise and barely eating, where health and fitness goals can be elusive because of the body being denied an ability to heal and function, making the strict workouts and diets worse than ineffective.