In response to a recent forum post on a bodybuilding forum I frequent, I proposed the following response to the question "What’s the purpose of carbing up on the weekends on a low-carb bodybuilding diet? Also, how are proteins metabolized into glucose?"
For more details about how protein is metabolized into glucose, please Google "gluconeogenesis". It’s a well-documented process, and Wikipedia has a great article introducing the concept. For its application to low-carb eating, Google "gluconeogenesis low-carb". You’ll find a lot of useful links.
For a great article on what carb-loading does in a cyclical ketogenic diet (CKD) like MANS, check out this article: http://www.bodybuilding.com/fun/keto2.htm . CKDs been very popular with bodybuilders for many years because they give you results similar to old-school anabolic steroids without the steroids part: the ability to diet away body fat while maintaining or gaining skeletal muscle.
Basically, we have two biologically opposed processes that we’re trying to maximize with a CKD like MANS:
Lipolysis, or your body catabolizing (that is, reducing structure and stored energy from) fatty tissue.
Hypertrophy, or your body anabolizing (that is, adding structure and stored energy to) muscle tissue.
Your body is catabolizing or anabolizing on a sliding scale dictated by:
The amount of insulin secreted by your pancreas in response to blood glucose levels. Key to remember is that when insulin is high, it’s a purely anabolic process: all your tissues are storing raw materials from your bloodstream. Fat cells are storing glucose and glycerol as triglycerides, while muscles are storing the same stuff as glycogen, which we’ll get to in a minute.
The need of your tissues. If your muscles need protein, amino & fatty acids, they’ll grab it from your blood. Muscles can’t grab glucose in large quantity without the presence of insulin, though… and your fat cells are just as eager for the stuff.
OK, so 5.5 days out of seven, you’re low-carbing. Insulin is very low and only present in more than tiny quantities after a really big helping (around 40g+, varies by individual) of protein. The anabolic process is very slow for muscles and fat. Your metabolism, however, is running rock-steady, and your liver is converting lots of fat into ketones and lots of protein into glucose to power those tissues which require those food sources. Usually, your fat cells are giving up a substantial amount of triglycerides to the liver, which converts them into glucose and their fatty acid components. This is the heart of the low-carb fat-loss process, and we spend most of the week in this state because to spend too much of the week in an anabolic state results in gaining large amounts of fat more than large amounts of muscle.
1.5 days a week, you’re high-carbing, and on MANS you can really eat whatever you want. Seriously, just eat whatever you’ve felt deprived of for the week, and it will still work for you.
However, if you’re interested in maximizing fat loss, you want to keep the carb-ups "clean" by eating slow-digesting carbs. Keep away from white rice, white flour, refined sugars, high-fructose corn syrup, and soda pop. Get your carbs instead from fruits and starchy vegetables. Also, if you’re trying to maximize fat-loss while still carbing up, glycogen storage will account for roughly 2% of your lean body weight when you’re "full" of glycogen. My lean body weight is 171 pounds, so I’ll gain around 3-4 pounds on a carb-up. This is very typical, and is around 1/2 water, 1/2 sugar in the polysaccharide called "glycogen". This glycogen storage is the majority of the so-called "water weight" people refer to: an easy 3-5 pounds that is lost as easily as gained in any given week, and illusory weight loss.
Note that means you’re only eating around a pound to two pounds of carbohydrates while carbing up for the most efficient, non-fat-gaining carb-up. That’s around 800-1000g of carbs for an adult male of my size; your amount will vary based on your lean body weight.
The purpose of the carb-up is:
To provide glycogen for your real heavy lifting. You’ll notice your ability to push heavier weights for more sets goes up with glycogen storage. Glycogen is purely short-term muscular energy storage, and won’t help beyond the first mile or two of a marathon, for instance.
To provide an anabolic environment for muscles to grow with insulin as a transport for several essential nutrients.
To increase the glycogen capacity of muscles by depleting and refilling, which also equates to muscle growth.
Hope I’ve been helpful!
Regards,
Matt B.