High Protein, Low Carbohydrate (HPLC) diets have been one of the most popular nutritional trends over the past decade. Encompassing weight loss programs like Atkins and the Dukan Diet, ketogenic diets adopted by athletes to improve energy usage and endurance, and lifestyle movements like Paleo, carbohydrates have fallen out of favour with a lot of people.
HPLC diets alter the body's metabolism - once the glycogen stores are depleted, which usually occurs within the first week, the tissues no longer have easy access to glucose. While glucose can still be produced from some amino acids, much of the body's energy now comes from ketone bodies, which are molecules formed from the body's store of fatty acids. These can be used directly by many tissues and organs in the body, including the brain.
A baffling paradox of these type of diets is that they allow the dieter to consume almost unlimited quantities of rich food, yet they are very effective for weight loss. Scientists of course were intrigued, investigated, and came to the conclusion that people following HPLC diets quite simply use more energy than they take in.
It has since been established that a high protein intake suppresses the appetite, but we do not know why. Many theories have been thrown about to explain this, ranging from long digestion time and lack of insulin stimulation to the unpalatability and monotony of the food.
A Scottish team decided to take a deeper look into one of the major theories behind appetite suppression - that ketone bodies are responsible for the change in appetite. Ketosis can occur in the body during times of fasting or starvation and the scientists postulated that the reduced appetite may be a result of the areas of the brain involved in appetite responding to ketone bodies as fuel and moderating hunger accordingly.
Two groups of overweight men were placed on either an HPLC diet, or a diet providing the same amount of energy with a normal intake of carbohydrates for one month. The groups were swapped over and placed on the other diet for a month. At the end of each month, scientists looked at the way the brain took up and used glucose and ketone bodies, examining 54 seperate regions. The amount of ketones in the blood were measured and the men also completed a questionnaire that measured their hunger levels.
As expected, men on the HPLC diets had much higher ketones and lower hunger levels. Although overall glucose usage by the brain was reduced by an average of 5% in the HPLC dieters, the authors were surprised to see no differences in the amount of glucose taken up by various parts of the brain when the two diets were compared.
The group concluded that the lower hunger levels in the HPLC diets were not caused by differences in the use of glucose by the brain. While the reasons for reduced appetite in HPLC diets remain a mystery, and are likely a combination of many factors, these diets are still, for many people, an unconventional yet effective way to lose weight, reach their fitness goals, or improve their overall wellbeing.
Lobley GE, Johnstone AM, Fyfe C, Horgan GW, Holtrop G, BremnerDM, Broom I, Schweiger L, Welch A. Glucose uptake by the brain on chronic high-protein weight-loss diets with either moderate or low amounts of carbohydrate. Br J Nutr. 2014 Feb;111(4):586-97.