Overfeeding studies are really interesting tests of boundaries. I’ve looked at studies which overfeed protein and showed that overfeeding protein doesn’t lead to weight gain (Too Much Protein?). I’ve looked at studies which show overfeeding carbs leads to weight gain (Carbs Make Me Fat? I Thought Fat Made Me Fat!).
But what about overeating fat? There are some in the keto community who imply that you can pretty much eat all the fat you want and still lose weight but it’s clearly not working for them (Fat Jimmy).
In a previous post (Does Fat Make You Fat?) I took a look at an n=1 case of Keto Savage who ate 4,000 calories of keto a day and gained a significant amount of body fat (from 4.3% body fat to 10.1% body fat in two months).
But, anecdotes aside, what does the actual science say? This study took a look at the question (T J Horton H Drougas A Brachey G W Reed J C Peters J O Hill. Fat and carbohydrate overfeeding in humans: different effects on energy storage. The American Journal of Clinical Nutrition, Volume 62, Issue 1, 1 July 1995, Pages 19–29). Here’s the abstract:
We overfed isoenergetic amounts (50% above energy requirements) of fat and carbohydrate (for 14 d each) to nine lean and seven obese men. A whole-room calorimeter was used to measure energy expenditure and nutrient oxidation on days 0, 1, 7, and 14 of each overfeeding period. From energy and nutrient balances (intake-expenditure) we estimated the amount and composition of energy stored.
Carbohydrate overfeeding produced progressive increases in carbohydrate oxidation and total energy expenditure resulting in 75-85% of excess energy being stored.
Alternatively, fat overfeeding had minimal effects on fat oxidation and total energy expenditure, leading to storage of 90-95% of excess energy.
Excess dietary fat leads to greater fat accumulation than does excess dietary carbohydrate, and the difference was greatest early in the overfeeding period.
So the science seems pretty clear. If you eat extra fat in your diet it’s not coming off your stomach. At least in this study, overfeeding fat didn’t lead to a kicked up metabolism.
The reason low carb diets work is that they break the sugar addiction and hunger that goes along with the blood sugar roller coaster. The amount of fat you choose to eat in a day is your choice once the carb addiction is under control.
A weakness of this study (from the keto perspective) is that the group that was overfed fat was not on a ketogenic diet. The argument could then be made that overfeeding fat and being very low carbohydrate at the same time is the magic formula but where is the science showing that? The Keto Savage (n=1) showed that high fat is not some magic formula for weight loss.