Fat Loading Revisited
Ellen Coleman, RD, MA, MPH © 2004

Adaptation to a high fat diet or "fat loading" is one strategy utilized by endurance athletes to promote fat oxidation, slow the rate of carbohydrate utilization, and enhance performance. Compared to a high carbohydrate diet (60-70% energy from carbohydrate), "fat loading" (60-70% energy from fat) increases the contribution of fat oxidation to total energy expenditure and spares muscle glycogen during submaximal exercise (<70% of VO2max). While previous studies have used two to seven week periods of fat adaptation, it is not practical for an athlete to maintain such a radical diet. A high fat diet may also impair high intensity training and have adverse health consequences over the long term.

Burke and colleagues evaluated the effects of a five-day fat adaptation period followed by one day of carbohydrate restoration on fuel usage and performance during two hours of submaximal cycling followed by a 30 minute time trial (see Burke et al, 2000). A five-day timeframe represented a more manageable period for extreme dietary change and would minimize the potential health and training disadvantages caused by longer periods of fat adaptation.

The fat loading diet provided 4 g of fat/kg (65% of energy) and 2.4 g of carbohydrate/ kg (<20% of energy). The isoenergetic control diet supplied 0.7 g of fat/kg (<15% of energy), 9.6 g of carbohydrate/kg (70-75% of energy), and 3,852 calories (16.18 MJ). The supervised training program was individualized for each athlete based on fitness level and training load and designed to simulate road cycling.

All athletes consumed a high carbohydrate diet (10 g of carbohydrate/kg) for one day following both diets and rested to normalize muscle glycogen stores independent of previous dietary treatment. This brief carbohydrate recovery period restored muscle glycogen to similar levels in both groups without negating the metabolic adaptations favoring increased fat metabolism in the fat-loading group. After an overnight fast on day seven, the athletes underwent a performance ride that consisted of two hours of cycling at 70% of VO2max followed by a time trial in which the athletes completed a set amount of work (7 kJ/ kg) as quickly as possible.

Next: "Part 2"