Energy metabolism basics: 5 conditions that your body needs to burn fat

What if you could look at the ‘latest’ diet and immediately understand why and how it works, as well as some advantages and disadvantages? Wouldn’t it be great not to feel you have to stick to an artificially rigid plan, because you understand the principles that underpin it and you can make your own decisions?

I’m not professing to cover the entire subject in this post (I have plans for New Year’s Eve too!). As with anything, the more your learn, the more you realise there is to learn! But these basic principles will get you a long way.

If you want to start the New Year on the right foot, make a note of these 5 points – stick them on your fridge or noticeboard and empower yourself to make healthy decisions regardless of what your body composition goals are.

Energy metabolism basics

Blood sugar is the basic fuel the body needs for most functions and there are two hormones that work to remove sugar from the blood when there is too much, or add sugar to the blood when there is too little:

  • Insulin removes excess sugar from the bloodstream and converts it to fat.
  • Glucagon releases stored glycogen (carbohydrates) and fats from around the body to be converted to energy.

So how is fat used for energy?

The process of converting fat to energy requires glucose (normally as released from stored glycogen) and plenty of oxygen. It requires a sequence of reactions in the body and as such is comparatively slow. It also requires the body to be in a relaxed state – the body usually wants to conserve energy stores, particularly if there is a sense of impending danger! In a stressed state when cortisol levels (the body’s main stress hormone) are high, the body is more prone to storing fat and using other fuel sources wherever possible.

Taking this information into account, fat burning happens when:

  1. Blood sugar is low
  2. Stored glycogen is available
  3. Oxygen is available
  4. The intensity of activity is sufficiently low that converting fat to energy will provide enough energy in time
  5. The body and mind is generally in a balanced, relaxed state

How do I tap into my fat burning potential by bringing these conditions about?

  1. Maintain low blood sugar as much as possible by consuming a diet that is low-glycaemic, i.e. low in foods that are quickly converted to blood sugar (find out more about the Glycaemic Index from Diabetes UK here.) As we know from how insulin and glucagon work, high blood sugar not only prevents fat from being used as a fuel, but also triggers insulin to remove the excess sugar and store it as fat.
    1. Low-glycaemic foods are generally those that are either low in simple carbohydrates and/or contain a lot of fibre or fat which slows down the absorption of the carbohydrates into the bloodstream. It’s worth pointing out, therefore, that not all low-glycaemic is especially healthy as high-fat and high-sugar foods would get a lower-glycaemic index score than low-fat, high-sugar even though both contain a lot of sugar and calories.
    2. High glycaemic foods include refined foods such as sugar (listed in ingredients as sugar, sucrose, glucose, fructose, maltose or as various syrups to name a few) as well as low-fibre starchy foods such as white rice, white bread, white pasta, peeled potatoes. The glycaemic index of fruits vary depending on their sugar and fibre content but the highest glycaemic-index fruits are melons, very sweet tropical fruits, grapes and ripe bananas, although the water content of these means that the quantity of sugars per serving is not as high as similar GI foods might be.
    3. You can balance the effect of these foods on your bloodstream by having small amounts of higher glycaemic-index foods with more high-fibre, low-glycaemic foods e.g. vegetables.
    4. Aim to stick to the latest government guidelines on refined sugar intake. This was revised in 2015 following an overwhelming body of research showing the risks posed by a high-sugar diet. Free or added sugars shouldn’t make up more than 5% of the energy (calories) you get from food and drink each day.
      1. That’s a maximum of 30g of added sugar a day for adults, which is roughly seven sugar cubes.
        1. In UK food packaging, sugar content is not distinguished as ‘free sugars’ and those naturally occurring so it can be hard to gauge precisely how much is in pre-prepared foods, but consume products where sugar (or a form of sugar) is high up on the ingredients list or and the grams of sugar per 100g of product is over 5g per 100g in moderation.
        2. It may be wise to avoid high-sugar foods with 22.5g or more sugar per 100g.
  2. Conversely, ensure your diet includes sufficient low-glycaemic-index starchy foods to keep glycogen stores topped up. Don’t totally exclude all starchy foods in the quest to keep blood-sugar levels low – your body can cope with low-glycaemic starchy foods in moderation and you need some stored glycogen in order to metabolise fat. What’s more, a diet that is too low in carbohydrates triggers a warning signal and stimulates cortisol (particularly in women), the stress hormone that leads the body to store fat and burn other types of fuel.
  3. Make rest and relaxation a necessity and not a luxury. Think about what strategies you can use to make the body feel less stressed and plan how you will implement them. The bonus is this will help with fat loss but often have a positive effect on other aspects of your life too!
  4. Exercise and move frequently, but understand that this only uses fat if the intensity is low enough that you can absorb enough oxygen for it to be aerobic (less than 7/10 effort, no burning sensations in the muscles, and feeling like you could maintain the effort for longer than 20 minutes continuously).
    1. A brisk (but not your fastest) walk burns about 5 times more energy than watching television and about 3.3 times more energy than sitting or standing activities like writing or working at the desk and is at the intensity that the body can use fat as a fuel source.
    2. The average UK woman burns around 86 calories per hour while doing sitting or standing activities, but an hour’s brisk walk, active gardening, cycling or tennis would require 295 calories – a little movement goes a long way!
  5. Understand that when you’re working out at 8/10 intensity or higher, you are burning almost completely blood glucose or stored glycogen without fat. This still has advantages for fat loss, but it’s worth understanding how they work:
    1. This will improve your cardiovascular fitness by increasing your lung capacity and making your heart muscle stronger, leading to better aerobic capacity (and thus more potential for fat burning) in future, even though it doesn’t burn much fat at the time.
    2. It will have the effect of using up current blood sugar, so that when you cool-down and go back to normal activity, the body will turn to fat stores for fuel until more energy is absorbed from food. This is effective for shorter workouts if you don’t take on carbohydrates for a while afterwards as there is still some glycogen in reserve to burn in conjunction with fat.
    3. Be aware that if you worked out at this sort of intensity for the best part of an hour or more (e.g. a football match or 10K running race), you would use up almost all of the stored glycogen so you wouldn’t be able to burn fat as a fuel afterwards. Instead you’d burn protein (liver or muscle tissue) for energy (this is almost never desirable and can carry health risks) which is why it’s important to refuel after a hard workout of 1-hour or longer duration.

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