Fed vs. Fasted State: How You Metabolize Your Food for Energy

Our bodies are truly amazing. We know when to utilize certain macronutrients depending on our current status. What does it mean to be in a fed or fasted state? There is so much happening in your body from the time you finish eating to the hours between your meals and even during exercise. Your body is in a fed state immediately after consuming a meal when you start undergoing anabolic processes, which are those that build compounds in our body. Protein synthesis, cholesterol synthesis, triglyceride synthesis, glycogenesis – these are some of the active pathways in a fed state. There are several hormones that stimulate fed state metabolism, but I won’t get into all that detail here. Similarly, in a fasted state you have several hormones that regulate the breakdown of the foods you consume for energy. Your body is in a fasted state in between your meals and during exercise once your food has been digested, absorbed, and taken up by tissues. Protein degradation, glycogenolysis, gluconeogenesis, lipolysis, and ketogenesis can all occur at different points in a fasted state.

While the science is quite complex, I just want to focus on what types of substrates, or molecules, are primarily being used during each state. In a fed state, glucose is the main substrate being metabolized to make ATP (energy) and in a fasted state fatty acids are being metabolized. When we eat a balanced diet of carbohydrates, protein, and fat, all these macronutrients are broken down and synthesized in the body through different pathways in order for us to use glucose as our energy source. When we haven’t eaten for a while and need a source of energy, our body takes these stored sources and breaks them down.

In a fed state, our blood glucose levels are high, so our tissues want to take up glucose and use it as energy. Insulin is released and signals glucose uptake by tissues. Glucose is then used in glycolysis to make ATP and glycogenesis to make glycogen, or a stored form of glucose. Did you know that glucose is the required energy source for our brain? Yep, our brain loves carbs and needs them to properly function! Lipids, or fats, are broken down and used to make triglycerides (TAGs) which can enter various pathways to make ATP too, or are used to make cholesterol (Note: We need some cholesterol! Don’t think it is all “bad” for you). Proteins are constantly synthesized and degraded in the body, so during a fed state insulin and amino acids stimulate protein synthesis (and several mechanisms block protein degradation).

In a fasted state, fatty acids (which are stored as TAGs) are metabolized and used as a primary fuel source. Fatty acids go through lipolysis and then beta oxidation, producing acetyl CoA which can be used in the TCA cycle, producing ATP. Long story short, fatty acids are a source of energy! They are the preferred substrate in a fasted state because we want to preserve our glycogen stores as long as possible, such as during times of stress or increased energy demand. Glycogenolysis only takes place in the liver and muscle and is the breakdown of glycogen into glucose for ATP production. At 18-20 hours of intense fasting, gluconeogenesis takes place, which is the formation of glucose from non-carbohydrate sources. All of our glycogen stores have been used up and we need a source of energy.

So, can you guess these non-carbohydrate sources for glucose production? That's right. Our body will break down fatty acids and protein to gives us that energy we need. Sounds interesting? You want fat breakdown right? Not exactly. See, fatty acids are broken down and from that ketone bodies are produced in instances of extreme starvation or very low carbohydrate diets. Remember when I said the brain needs glucose as an energy source? Well, ketones can also be used by the brain for energy. Obligatory glucose users like the brain will use ketones as its source of energy when glucose is scarce and other tissues will utilize protein breakdown for energy, not fatty acids. You do not want protein breakdown in your tissues!

This is one reason athletes are not advised to eat low-carb diets. When we exercise we fluctuate between glucose and fatty acid oxidation. During low intensity activities like walking, we utilize fat oxidation and high intensity exercise like HIIT utilize glucose/carb oxidation. It takes quite a while for beta oxidation to get going, which oxidizes (breaks down) fatty acids, so only after we have been exercising for 20 minutes does fatty acid oxidation occur. As we increase our intensity, we crossover to carb oxidation because fatty acids cannot keep up with the increased energy demand.

Interestingly, highly trained athletes can utilize fats as an energy source longer which is advantageous because they want to spare their glycogen stores for later in their competition. This all goes to show that how we fuel for exercise is a critical aspect of health. Have you heard of carbo-loading before exercise? One study (Spencer et al. 1992) found that athletes with high glycogen stores prior to exercise had a lower heart rate, increased glycogen leftover, and a higher RER (meaning they oxidized more carbs). Athletes need carbs to perform efficiently and effectively! Even if you may not be a professional athlete, you NEED carbohydrates for your body to properly function. Here’s your reminder to ditch diet culture shaming you for eating carbs – it’s all about filling your diet with more complex carbs.

All of this information may not be crucial for your understanding, but I want this to be your biggest takeaway – carbohydrates and healthy fats are needed in your diet!! There is a ton of craze over low-carb, low-fat diets which, while providing possible benefits for some, are not the only way you need to eat to live healthy. Our metabolism thrives off of both carbohydrates and fats, so by severely cutting one of them out does more harm than good to your body. For example, those with Type 2 Diabetes or obesity are metabolically inflexible, meaning they are unable to utilize the correct substrate during the correlated fed or fasted state. This can lead to insulin resistance and cardiovascular problems. This is why I preach eating a balanced diet is the best way you take control of your nutrition. Don’t restrict yourself because that could result in dysfunctional metabolism, leaving you farther from your nutrition goals. Clearly, this is just my personal opinion and if a certain diet works for you and you have discussed this with a registered dietitian or physician and have done your research, by all means do what is right for you! But for those of you hesitant or concerned that you have to cut out all these food groups to "get healthier," I’m here to tell you that science proves that theory wrong.

I challenge you to try a new carbohydrate and healthy fat source this week! Know that all three macronutrients are very much needed for our bodies to produce energy. Here are some great examples to get you started:

Complex carbs:

- sweet potato

- brown rice

- quinoa

- black beans

- fruit

- peas

- broccoli

Healthy fats:

- extra virgin olive oil

- nut butter

- avocado

- salmon

- nuts & seeds

In your workouts, let’s push a little harder this week. Convince yourself to run that extra half mile, increase your weighted resistance by 5 pounds, or get in over 10,000 steps each day. Training positively affects our metabolism by increasing the rate of breakdown of fatty acids after exercise. This means our body will continually burn fat even after we finish our workout! Higher trained individuals also have an increased capacity for their muscles to take up blood glucose and utilize it as energy, rather than having more glucose circulating in the blood. Most importantly, if we do not have energy we cannot survive – eat up and exercise to make sure you are giving your body the energy it needs!

1. Spencer MK, Yan Z, Katz A. Effect of low glycogen on carbohydrate and energy metabolism in human muscle during exercise. Am J Physiol. 1992;262(4 Pt 1):C975-979.