Article written by Ben Greenfield, writer for Training Peaks

Question: As a triathlete, should I be changing my diet through the year depending on my training? Like eating more protein in the winter, more carbohydrate in the summer, etc? In other words, do I change my diet throughout the training year?

Answer: This is actually a more complex question than you may realize! In training, we often hear the term "periodization" thrown around, which refers to splitting a training year into specific blocks that focus on specific aspects of training, like endurance, power or speed.

When addressing nutrition, periodization refers to dividing your yearly nutritional plan into several distinct segments that apply to daily, weekly, or monthly cycles of training. While there are many different ways to put together your annual training plan, the basic idea behind nutrition periodization is that your dietary habits should match your training habits.

Unfortunately, fueling your body for multi-sport training is not as simple as fueling a car. With a car, you simply put fuel in the gas tank when you're running low or when you're prepping for high mileage days. With triathlon, however, you not only have several different physiological systems, or "engines", that you're fueling, but you also have three different types of fuel: fat, protein, and carbs.

The key to nutrition periodization is to match the amount and timing of these three fuel types with the volume and intensity of your training (which affect which "engine" you're using).

So you are essentially optimizing carbohydrate intake for glucose and glycogen derived energy, optimizing protein intake for lean muscle mass and immune system recovery, and optimizing fat intake for long fuel sources, as well as physiological balance of hormones and other fat dependent functions.

The final key is to manage your body weight (which ultimately affects your power:weight ratio) by adjusting your nutrition on days or periods of physical inactivity.

Sound complicated? A little bit, but not as much as it sounds. Let's look at a few examples, using general terms to describe each period of the multi-sport season.

Base/Foundation training
This is the time of the season when you are laying down the groundwork of aerobic conditioning. Workouts are typically long and intensity fairly low, meaning that you're using quite a bit of fat for fuel, and a good portion of carbohydrates as well. Remember, the lower the intensity, the more fat is used as a fuel for your "low-intensity engine", and the harder you work, the more you turn to carbs as a fuel for your "high-intensity engine". Not a huge amount of speed and force work is taking place, and with this decreased muscle repair demand, protein needs are lower (protein will be used to fuel your "muscle-building engine"). A sample carbohydrate/protein/fat percentage ratio might be about 60% carbs, 13% protein, and 27% fat.

Build/Intensity/Preparation
Your longer, harder efforts are increasing, and you are spending a greater period of time at lactate threshold. Many of your workouts include intervals and high intensity workouts, so dependence on carbohydrate as an energy source is increasing, and use of fat will decrease. Carbs now increase to 65%, protein increases to 15%, and fat drops a bit, down to 20% of total dietary intake.

Racing/Specialization/Peak
Efforts during this time of season are hardest, and involve difficult, high-intensity lactate threshold, above lactate threshold, and VO2 max efforts. A good deal of time is spent developing power, speed, and strength, which places a high demand on the body for carbohydrate (for high intensity energy) and protein (for muscle recovery). To enhance power:weight ratio, unnecessary amounts of subcutaneous fat should also be kept at a minimum during this phase, meaning your body fat percentage should be the lowest of the year. Ratios now increase to about 68% carbs, 17% protein, and decrease to 15% fat.

Recuperation/Strength/Recovery
As you transition into the off-season, and focus on goal setting, strength training, and cross-training, carbohydrate and total caloric needs will decrease, while protein will be more necessary for muscle building and recovery. A sample ratio would be back down to 60% carbs, with 18% protein, 22% fat.

Although it requires getting into more detail, these nutrient values can also be defined in terms of grams of fuel per kilogram of body weight. You have probably seen very high recommendations for protein intake in athletes, but the importance of carbohydrates and fat as fuel takes precedence for endurance athletes. At the most, protein only supplies about 15% of the total energy for exercise, and more than 1.4 grams of protein per pound of body weight has not been shown to give much benefit.

To help you have a more dynamic diet, I have created a periodized nutrition plan for TrainingPeaks called "Complete Nutrition Plan for Ironman Triathlon". It can be used for Half-IM or IM triathletes, and it actually takes all these concepts and puts them together for you with zero guesswork. The plan is downloadable to your TrainingPeaks calendar. You can also check out Ben Greenfield's Books on Amazon. Enjoy!