Bioavailability is the fraction of an administered substance that the body can use. The difference between the dose and that fraction is waste. As my doctor once described unnecessary supplements, the excess is "just expensive pee."

The bioavailability of nutrients

How we absorb iron is a good example of bioavailability. For endurance athletes, iron is essential to the delivery of oxygen and energy production. But how much iron we need is not necessarily how much we can absorb. The bioavailability of iron depends on the quality of the source and the food consumed with it.

If you prioritize heme sources of iron, batch-consume sources together, and do so along with vitamin C, absorption is increased. Conversely, if you consume iron along with caffeine or calcium, absorption is decreased.*

The bioavailability of training

Training is no different. The training we think we need—or wish we could do—may be more than we can tolerate. And secondary components will enhance or inhibit the type and volume of training we can absorb.

To maximize the bioavailability of our training, we need to increase the enhancers and decrease the inhibitors. And we need to couple that strategy with the humility of knowing when enough is enough. Hubris in racing can be an enhancer; in training, it's an inhibitor.

Enhancers & inhibitors

Our habits and life constraints determine how much enhancers help and how much inhibitors interfere. Here are a few to consider:

Enhancers (increase these)

  • Sleep
  • Food
  • Appropriate training before & after
  • Supportive network

Inhibitors (decrease these)

  • Professional work load
  • Outside obligations
  • Stress
  • Discouraging network

If you can increase your volume of sleep, improve the quality and timing of nutrition, make wise training decisions, and surround yourself with people that believe in you and your goal, then you'll be able to tolerate more training.

In contrast, if you have a heavy professional life, a lot of outside obligations, a lot of stress, and many critics, your training will be greatly reduced. (You might even fall victim to the HIITster attitude and think there's a shortcut to performance.)

Think of bioavailability as a barrel

The barrel of bioavailability is limited by its construction.

Similar to Leibig's Law, as the staves of the barrel (the enhancers) get longer, you can tolerate more valuable training. With wise selection, you'll perform better. In contrast, if there are leaks in the barrel (the inhibitors) your available training types and volume will fall until they're below the leak.

"[Liebig's Law] states that a plant’s performance is affected not by the most abundant resource, but by the most deficient one."

When it's full, it's full.

When my great uncle was on leave from World War II, he followed the directions and put a tablespoon of bleach into his bath to get rid of the lice. Then he paused and thought, "If a tablespoon is good, then half the bottle must be better."

When he retold the story decades later, he said in his strong Scottish brogue, "Some o' the wee bits was stingin'!"

Like bleach in your bath, "more is better" in training is naive. Or worse, stupid. The extra effort is usually wasted or counter-productive. The goal of training is to stimulate a response. Overwhelming our capacity to respond will reduce the response, not magnify it.

Want a bigger barrel?

To increase the size of the barrel, you need to lengthen the staves and plug the leaks. Ask yourself:

Which enhancers are missing?

  • Are you getting at least eight hours of sleep per night?
  • Do your habits and lifestyle promote high-quality nutrition? Is the selection of fuel sources high-quality and well-timed?
  • Are your key training sessions supported by the proper speed, strength, and duration in the rest of your training?
  • Do your friends and family support your goal event?

Which limiters are present?

  • Do you have an overwhelming job?
  • Do you have too many extracurricular obligations?
  • Is your life high-stress?
  • Do your friends and family criticize your goals?

Leave some room.

Even when you have everything aligned, it pays to be conservative. Leave a margin of safety between what you think you can do in a training session and what you execute. That leaves some extra bandwidth for misjudgments and extra energy for the recovery process.

As Warren Buffett says, ""Be fearful when others are greedy, and greedy when others are fearful." " Be fearful in training so that you can be greedy in your goal event.

"Going to the well" is useful, but rarely. Leaving some room will speed and enhance recovery and, in the long-term, speed our rate of adaptation.