Grazing or Fasting – and Why It Matters for Weight Loss

cows grazing in a fresh green field

Grazing throughout the day could be bad for your weight. In this post you’ll learn why and what to do instead.

In order to understand how the body gains and loses weight, you must understand how it uses energy. The body really only exists in one of two states – the fed and the fasted state. When we eat, the hormone insulin goes up and insulin is released. Now all foods stimulate different amounts of insulin release but few foods except for pure fat cause no insulin release at all. Insulin is really a type of nutrient sensor. It senses the ingestion of both carbohydrate and protein containing foods. Refined foods, particularly carbohydrates cause the highest release of insulin.

TwoTrack1 copyOur bodies need a continual source of energy for basic metabolic housekeeping – keeping the heart pumping blood, the liver and kidney detoxifying, the lungs sucking air, brain function etc. Obviously we need a source of energy for all that work and it must be continuously available. Since we do not eat food all the time, we have a system of storing food energy (in the liver and as body fat) for times where we are not eating.

The main mistake people make is believing that weight loss is a simple one compartment problem. That is, people think that all calories go into a single compartment and taken out of that same one.

Consider the energy balance equation: Fat = (Calories In) – (Calories Out). This is always true. Suppose that your weight is stable and you eat 2000 calories and burn 2000. What if you want to lose weight? You hope that you reduce dietary calories to 1500, and body fat will provide the other 500. Over time you lose body fat. That’s exactly what does not happen.

There are really two different places where our body can obtain energy:

  1. Food
  2. Stored food energy (glycogen in liver, or body fat)

But here’s the CRITICAL point. You can only get energy from one or the other, but not both at the same time.

Like a railroad track

Imagine a railroad track. Suppose you need 2000 calories to keep basic metabolic function normal. There are two different tracks where energy can come from – either food or stored food. You may only obtain energy from one source at a time. If you take energy from the first track, you cannot get any from the second and vice versa.

TwoTrack3 copyIn the fed state, when you are eating, insulin levels are high. During that time, it makes sense to derive your energy from the food that you are eating. So what happens is that we shut down burning of stored food energy in the form of fat and glycogen. For all you technically inclined people, we say that insulin inhibits lipolysis and gluconeogenesis. This is a well known physiologic fact.

Throughout most of the day, assuming you eat 3 meals a day, this is the normal state of affairs. But what happens when you go to sleep? Because you are not eating, you are fasting. Insulin levels fall. You now need to pull some of the food energy you’ve stored away to keep your vital organs running. This is the reason you do not die in your sleep every single night.

TwoTrack2 copyAs you fast, insulin levels fall. This is the signal to switch energy sources from food to stored food. You pull stored energy out from the liver (glycogen) and if that is not enough, body fat. Technically speaking, we say that we start glycogenolysis, gluconeogenesis and lipolysis when insulin levels fall.

If you fast for 24 hours, for example, what happens is that your body wants 2000 calories for that day’s energy bill. Since you are carrying around plenty of body fat, it’s no problem to supply that 2000 calories. Approximately 1/2 pound of fat will supply that easily and the body says “Whoa, I have tons of fat, take all you want’. It is important to realize that this is a completely natural process. Humans have evolved this mechanism of food storage, and there is nothing inherently unhealthy about fasting. It’s all part of a natural balance of being in the fed state and the fasted state.

How this applies to weight loss

Another way to put it is that this. You either burn fat or store it. You can’t do both at the same time. The body is just not that stupid. If food is plentiful, you store food energy. If food is scarce, you burn food energy (body fat). The key hormonal regulator here is insulin. The change in insulin levels is what signals our body to go into fat storing mode or fat burning mode.

So what happens now if you are trying to lose weight by adopting conventional advice to reduce the dietary fat and calories, and eat 6 times a day. By doing so, you keep insulin levels high because you are eating lots of low-fat bread, pasta and rice and eating all the time. This also happens in type 2 diabetes, where insulin resistance causes insulin levels to stay elevated.

Since insulin is high, you must get your energy from food, and cannot get any from your body fat stores. You reduce your calorie intake from 2000 calories to 1500 and hope against hope that you will lose weight. You do, at first, but then your body must adjust. Since you cannot get at your fat stores, if you are only getting 1500 calories in, you must reduce you calorie expenditure to 1500 as well.

TwoTrack4 copySo, you feel tired, hungry, cold because your body’s metabolism is starting to shut down. But the worst part? You don’t lose any more weight! Your weight loss starts to plateau, but you feel like crap. Over time, you start to regain some of that weight. So you decide that you’ve had enough and increase your caloric intake to 1700 – still lower than when you started. But, because you are taking in 1700 calories but only burning 1500 calories, your weight quickly goes back to what it was before you started the diet. Sound familiar to anybody?

The key to successful long-term weight loss is not reducing calories. It’s reducing insulin. Because insulin is the switch that decides whether your body is burning food energy or stored food energy (body fat). If you are burning food, then you are not burning fat. It’s as simple as that. The key to accessing your body fat stores is to reduce insulin. You must let your body go into the ‘fasted’ state.


Jason Fung

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3 Comments

  1. Cleo
    I read that fasting is encouraged on the keto diet but I’ve always been told that fasting will lower metabolism because the body thinks it is experiencing famine. I already have very low metabolism. Could someone clarify this?
  2. 1 comment removed
  3. EB
    Cleo, you can try Keto without fasting first and see if it helps. Once you are doing Keto for a while, your body should have learned to burn fat.

    So when you start fasting, your body knows to get the energy from fat, which means it's still metabolising. So you are not reducing your metabolic rate.

    At least that's how the theory goes.

  4. Chris
    Longest record fast is 382 days! He went from 456 to 180 pounds. His metabolism went down slowly only due to the fact that he was losing fat and the body naturally needs less calories to maintain lower body weight.

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