"Never be limited by other people's limited imaginations. If you adopt their attitudes, then the possibility won't exist because you'll have already shut it out." -Mae Jemison, astronaut

Sunday, June 16, 2013

Just How Phat is a High Fat Diet?

Carbs are out and protein is in.
Fat is out and grapefruit is in.
Eat for your blood type.
Carbs are out and fat is in.
Grain is out and liver is in.
Tapeworms, anyone?

Another case of our human desire for the "quick fix." Because our diet affects weight management, athletic performance, health, and mood, most of us are all too readily excited by new "fads" that come on the market. This applies whether it's a diet to lose weight or a diet as in our "lifestyle" day-to-day diet.

The one that seems to be making its way to my desk/android phone/computer screen/ears the most these days is the high fat diet. It may or may not be "paleo" - the focus is not so much about eating as a caveman, but merely on eating a diet high in fat.

So what's the deal with a high fat diet, from a running athlete's perspective?

Well, the theory is good. We have a limited store of carbohydrate to use as an energy source for exercise. A wee bit, like 5 grams, circulates in the blood. A slightly larger amount, in the range of 60-120 grams (body size dependent), is stored in our liver and provides fuel for the brain. Then, there's the muscle glycogen. The muscles store carbohydrate for use when we exercise. Although it can be increased to some extent in trained athletes and those who are careful to get enough carbs in their diet, particularly replenishing after long runs, there are limits to how much can be stored. Again, women/smaller individuals, with smaller muscles, can store less. The range is something like 200-500 grams. Since 300g is an oft-touted number for a 70kg man, I'm going to guess I'm only able to store closer to 200g.

Look mum, no glycogen stores!
That equates to 800-900 calories for me. Again, most general advice out there is that glycogen stores are depleted after about 1.5 hours of running (less in those less trained). And the "wall" that so many talk about in the marathon...it's just a term for the glycogen "fuel tank" being emptied. At that point, the body is left with two choices: (1) slow down in order to run on fat. And whilst my own body probably has about 9kg of 'excess' fat, which, at 9 cal/g is 81,000 calories, the body converts fat to fatty acid/ATP more slowly than it does carb to glycogen/ATP, so I would have to run slower ... or (2) get some exogenous carbs. That is, eat carbs whilst running. And again, we're limited by how much we can stuff down our throats whilst running hard, without feeling sick (typically less than 240 cals/hr).

Therefore, any endurance athlete would surely want to spare their carbs in storage during a long event and run longer on their fat stores. This concept is known as "carbohydrate sparing" or "glycogen sparing" and a phrase I'm seeing more often lately is "Become a better fat burner!" by going on a high fat diet.

Right. I checked into the research and sure enough, it's true. A high fat diet (70-85% calories/day from fat) induces metabolic changes that do indeed cause us to become better fat burners. When exercising at less than about 60-65% of VO2max, runners burn off less of their stored carbs compared to runners who are on the more typical 65%+ carb diet. They are burning more fat compared to carbs. Their carbs are being "spared." But 65% of VO2max is our LSD pace: 20% slower than marathon or no more than 75% of max heart rate. That's slow.

Anecdotally, I know this. I can go on a long 4 or 5 hour run alone or with mates, at a leisurely pace, even on trails, and eat nothing. Somewhere between 1.5 to 2 hours into the run, I will get a weird hunger pang that I used to think was nausea coming on. I describe it as a feeling kind of like the stomach is trying to eat itself. I used to need to fuel immediately when this came on (if not before, in order to avoid it entirely), as I was less fit and conditioned. So I was working harder at any given pace. In fact, I used to have that feeling come on closer to 1 hour into the run.
Long run Sunday: fueling optional if the pace is easy!

Now, I have more fitness in the legs, lungs, and heart. More base. So I can run an easy pace (less than 75% of max HR) probably all day without eating. However, I just need one mate to push the pace suddenly on a hill and my body screams, "Stop!" Of course, I don't. But I know what it means. It means I'm trying to go from my "neutral" speed where I can effectively fat burn, into gear with lots of low-end torque, where I need faster delivery of fuel. I need carbs or I need to slow down. And my body will slow me down if I don't give it carbs!

But an argument has been made that at least for ultra runners, we are often running at LSD pace. True? Well, that depends on the race distance, training background, and race goals of the runner. And no matter what pace (i.e., heart rate) we start at, after a while, even at the same pace, it won't feel "LSD" anymore! The HR will climb as we heat up and become fatigued. So, either we slow right down to compensate, or we accept that the race gets harder over time. Most of us won't stay below 65% of max through an entire race.

Back to the research. The research then says that although we can actually burn more fat and less of our carb stores up if we go with the high fat diet, it does not result in a performance gain. Yup, that's right. No performance advantage. I found only one study that showed high-fat-diet cyclists were able to cycle longer to exhaustion than high-carb-diet cyclists, but only when the intensity was kept to 60% of VO2max. At a high intensity, there was no performance advantage to being a "better fat burner." Indeed, more studies have reported that those on a high-fat diet report a higher heart rate and greater perceived exertion.

Another other downside to not fueling (with carbs, naturally) during training sessions is that because after 1.5 hours or so when you run out of glycogen, you will not be able to run as fast. How much benefit is there to running every run at less than 65% of VO2max? Gosh, I hope I don't need to answer that one :)

A version of a common graph, showing glycogen depletion within 3 days
Also, if choosing a high fat diet, one will never be able to fill the glycogen stores (after a long run or otherwise). Though research studies have tried to address this by carb-loading the high-fat-diet athletes for 24 hours before their "race" (whatever test they have lined up in the research study), there has still not been a performance advantage for them.

Does this mean fat is bad? No, of course not. Or that carb is king? No, not that, either. But runners do need to keep topping up their glycogen stores if they want to keep making gains in performance, running longer and harder (and enjoying their runs more, too!). It seems to me that the practical approach to take is to figure out our own personal glycogen needs (the size of our own fuel tanks) and then check out our diets to ensure we are getting that much carb in the daily diet. Then, calculate protein needs, and the leftover is fat! (Or more carb and protein if you prefer, but who doesn't want an avocado or handful of walnuts or piece of dark chocolate??)

Although the traditional adage is that 60-70% of our diets (as athletes) should come from carb, this does not take into account individual variations in body size (heavier people have more muscles, so more storage) and training volume (more volume means you're depleting stores more). So, here's how to calculate your own carb needs, summarised and paraphrased by me, from Anita Bean's "The Complete Guide to Sports Nutrition: 6th Edition" - all assuming a 70kg person (so girls like me, reduce!)

If you train "moderately" 3-5 hours per week, you need 4-5g carbs/kg of body weight (per day)
5-7 hours per week...5-6g/kg
1-2 hours/day...6-7g/kg
2-4 hours/day...7-8g/kg
More than 4 hours/day...8-10g/kg

I decided to go with 7 grams for my estimate of carb intake/needs. This week, for example, I exercised for a total of 14 hours, which is an average of 2 hours/day. I am only 52kg, so perhaps I only need 6 grams. But I'll err on the safe side for glycogen storage. So that means I need 364 grams of carbs per day on average (7g x 52kg). At 4 calories per gram in a carb, that's 1456 calories per day from carbs.

How many total daily calories do I need?

Again, referencing Anita's tables, resting metabolic rate in female athletes 31-60 years is (MYWEIGHT x 8.7) + 829. That's 1281 calories (kcals, of course, but I'm just using the generic 'calories' we're all used to).

Sleeping Beauty needed about 1300 cal/day
So no one is left out, the numbers for you guys to do your calculations are...
females 18-30 years = (WEIGHT x 14.7) + 496
males 18-30 years = (WEIGHT x 15.3) + 679
males 31-60 years = (WEIGHT x 11.6) + 879

Once you have your calories necessary to sustain life (mine being 1281), you need to add any for how active you are during the day (before exercise). I sit all day, so I multiply base by 1.4. If you deliver flyers door-to-door all day, you can multiply base by 1.7. If you lay bricks or pound nails all day, multiply by 2.0.

Now I'm up to 1793 cals (1281 x 1.4). And it's time to add the calories I burn during exercise. A sad by-product of getting fit is that I only burn a measly 450 calories per hour on most runs. Add that in to my daily average (450 cal x 2 hr/day average) = 2693 cals/day. There are my needs for weight maintenance, on average. So, back to the carbs - 1456 cal/day - that is 54% of 2693.

Protein. This is simple. 1.6g/kg. So I need 83g/day, which at 4 cal/gram, is 332 cal - 12%. I could even up that a little to be safe, considering I do some strength training. Using 1.7g/kg, I need 88g/day or 352 cal, which is still only 13%.

Fat. The leftover! Knowing I need about 2700 cal/day, 1450 cal should come from carbs to fill up my glycogen stores and 350 cal should come from protein, I am left with 900 cal to come from anywhere I like. So that's up to 33% fat for me personally. At a whopping 9 calories per gram, of course, I am limited to "only" 100 grams of fat. Interestingly, the scientists in endurance athletics recommend 20-33% fat in daily diet. Coincidence that my fat number worked out as it did? I think not. But remember to eat good anti-inflammatory, unsaturated, omega 3 types like those in avocado, salmon, nuts and seeds.

After all this, what are we left with? Gee, the science still supports a balanced diet. Oh, how boring!

P.S. Alcohol isn't in my calculations, as I have no taste for it anymore - just remember it has carbs, plus the alcohol itself is 7 calories per gram.

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