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.

During the two hours of cycling, the fat loading group burned significantly more fat than the control group (94 g versus 61 g) and less carbohydrate (271g versus 342 g). There were no differences in plasma glucose uptake between treatments, indicating that muscle glycogen sparing accounted for the reduced carbohydrate oxidation. The time trial performance was 8% faster for the fat loading group (30.73 minutes) than for the control group (34.17 minutes) but this difference was not statistically significant (P = 0.21). The mean power output during the time trial was higher for the fat loading group (281 watts) than for the control group (260 watts) but this difference was not statistically significant (P = 0.24). This study demonstrated a significant increase in fat metabolism during submaximal exercise following a brief period of fat loading – an adaptation that persisted even after one day on a high carbohydrate diet. However, there was no clear evidence that fat adaptation improved cycling time-trial performance.

It is possible that fat adaptation may benefit ultraendurance athletes by sparing muscle glycogen, since these individuals compete at an intensity (>65% of VO2max) and duration (>4 hours) that significantly reduce the body’s carbohydrate stores. Furthermore, fat oxidation has the potential to meet a large proportion of the fuel requirements for ultraendurance events.

Carey and colleagues evaluated the effects of a six-day fat adaptation period followed by one day of carbohydrate restoration on fuel usage and performance during four hours of submaximal cycling followed by a one hour time trial (see Carey et al, 2001). The researchers also provided carbohydrate feedings before and during exercise to reproduce nutritional strategies commonly used during ultraendurance events and evaluate their influence on metabolism and performance. All athletes consumed a control diet for one day to standardize muscle and liver glycogen stores prior to commencing a seven day supervised diet and training program.

The fat loading diet provided 4.6 g of fat/kg (69% of energy) and 2.5 g of carbohydrate/ kg (16% of energy). The isoenergetic control diet supplied 1 g of fat/kg (15% of energy), 11 g of carbohydrate/kg (70% of energy), and 5,952 calories (25 MJ).
On day eight, all athletes consumed the control diet and rested. On day nine, the athletes consumed a breakfast (similar in size and composition to what they might consume before an ultraendurance event) that provided 3 g of carbohydrate/kg. One hour later, the athletes began four hours of cycling of 65% of VO2max. The athletes consumed 7 ml/kg of a 10% glucose solution every 30 minutes for an average intake of 100 g of carbohydrate per hour. Following the four-hour ride, the athletes underwent a one-hour time trial in which they rode as fast as possible.

During the four-hour ride, the fat loading group oxidized significantly more fat than the control group (171 g versus 119 g) and less carbohydrate (597 g versus 719 g). Total glucose utilization (ingested glucose plus plasma glucose) was similar for both groups. The mean power output during the time trial was 11% higher for the fat loading group (312 watts) than for the control group (279 watts) but this difference was not statistically significant (P = 0.11). There was also no significant difference in the distance covered during the one hour time trial – 44.25 km for the fat loading group and 42.1 km for the control group (P = 0.11).

Despite marked differences in fuel utilization favoring fat oxidation during four hours of submaximal exercise and maximizing carbohydrate availability before and during exercise, fat adaptation failed to enhance subsequent time trial performance compared to a high carbohydrate control diet. The researchers note, however, that the athletes were able to ride the time trial at a power output that was 11% higher after fat adaptation. Although this performance enhancement failed to reach statistical significance, it represented a 4% performance improvement, which would certainly be worthwhile and meaningful for an ultraendurance athlete.

Burke LM, Angus DJ, Cox GR, Cummings NK et al. Effect of fat adaptation and carbohydrate restoration on metabolism and performance during prolonged cycling. Journal of Applied Physiology. 2000;89(6):2413-21.
Carey AL, Staudacher HM, Cummings NK et al. Effects of fat adaptation and carbohydrate restoration on prolonged endurance exercise. Journal of Applied Physiology. 2001;91(1):115-22.