WOD: REST DAY!Question from E:I heard that the Paleo diet is too low sugar to be sustainable over a long period of time. Especially in the case of attempting to uphold a sustained level of higher level cognition and learning in the brain. what is an 'appropriate' amount of sugar for humans to consume?I was also told that HFCS is GOOD because it's actually partially broken down and can save your body energy (not to say that I'm actually believing what my source told me)————————————————————————————–You are correct in that our brain does prefer to run on glucose; however the misconception lies in the assumption that because our brain needs glucose, then we must get it from our diet. This is false; we are perfectly capable of producing all the glucose our brain needs to 'uphold a sustained level of higher level cognition and learning' from amino acids and glycerol (and, our brain can use ketones when glucose becomes scarce) without eating any carbohydrate at all.Therefore, I disagree with the blanket statement that 'the paleo diet is too low in sugar to be sustained over a long period of time', however–and here's the important distinction–our bodies are not capable of synthesizing enough glucose to support brain function AND that which is necessary to fuel intense, chronic, anaerobic exercise–the work demand of, say, crossfit.If you are sedentary, or only participate in low-intensity, aerobic exercise (i.e. walking), then you are capable of burning fat to fuel you during periods of low-intensity movement thereby sparing precious glucose that you've manufactured from protein to keep your brain functioning. If you participate in metcon workouts, sprints, any fast-twitch muscle movements, or endurance events that depend on muscle glycogen (cycling, marathon) then your demand for glucose will be higher and must be supplied by glucose from the diet.How much do you need? That depends on your sport and goals, but for the average 3x/wk crossfit athlete who would like to perform well and remain lean, somewhere around 80-100 grams/day of starchy carbs (with the majority consumed following exercise) is a good rule of thumb. Basically, eat as many carbs as you need to do the exercise you want to do.Now, the second part of the question: High-fructose corn syrup. In the previous paragraph, I was talking about glucose—an essential monosaccharide that is the primary sugar found in starchy carbohydrate. Fructose—also a monosaccharide—is a completely non essential sugar; there is not a single reaction or process in our bodies that requires fructose. HFCS is composed of glucose and fructose (just like sucrose, or table sugar) somewhere in the range of 55 to 90 % fructose. It is 'partially broken down' in that it occurs in a slurry form with weak, easily broken bonds which allows the fructose and glucose to enter the portal vein very rapidly.The sugars are carried immediately to the liver where 80 % of the fructose (and only 20% of the glucose) is extracted before the blood continues on to systemic circulation. This is our first clue that fructose is potentially a bad molecule: the liver extracts almost all of the fructose—just like it extracts almost all the ethanol when we drink alcohol. The liver is in charge of dealing with toxins before they can reach the rest of the body and the liver must deal with fructose just as it deals with ethanol.We have a huge capacity to store glucose in the form of liver and muscle glycogen, however we have no way to store fructose, so we try to burn it off quickly yet 30 % of the fructose we ingest ends up as fat. This is because the fructose molecule enters glycolysis (energy-producing reaction) at a later point, following a key rate-limiting enzyme step. This means that there is no regulation as to how much energy is being produced because there is no enzyme keeping tabs on the fructose. It's like a nozzle on a hose: With glucose, the stream is controlled by a dial on the hose matching the amount of glucose entering the energy-producing pathway to the amount of energy demand at the other end. With fructose, there is no dial on the hose; it is open full-bore all the time regardless of the body's energy demand on the other end.What happens when you have excess energy constantly entering a system that is not utilizing said energy? The liver has to protect itself by exporting all the by-products in the form of fat and when the the liver cannot continue to match the rate of export with the generation of new fat, fat droplets begin to accumulate in the liver. This is exactly what happens in the liver of an alcoholic, but with fructose we call it Non-Alcoholic Fatty Liver Disease.So, E, your source stating that 'HFCS is good because it saves your body energy' is an understatement: HFCS is bad because it creates a huge energy surplus that, when consumed chronically, causes damage by overwhelming the liver's capacity to deal with the fructose toxin. Now, if you are someone who can utilize the extra energy generated by fructose (by participating in regular exercise), consuming up to 25 grams of fructose/day is probably no problem for the normal, healthy person. Fructose can also be used to generate extra energy in the liver for a boost immediately before exercise (such as a small piece of fruit right before activity). But for the most part, fructose should be treated as alcohol; a toxin that is best minimized or consumed in small doses.For an excellent video explaining this entire process, check out Dr. Robert Lustig: 'Sugar, the Bitter Truth'Jessica Kuzma MS, RD