Imagine you are a participant in a study on willpower. You arrive to the laboratory having fasted for the last 12 hours, so your body is in dire need of some nourishment.
Before you are two different milkshakes:
The first milkshake is a chocolaty, creamy, old-fashioned milkshake that people have been enjoying for decades.
The second milkshake has the exact same amount of calories and nutrients as a regular milkshake, but is just a gloppy milk product that is as tasteless as a glass of water.
Because both milkshakes contain the same nutrients, you will be adequately nourished regardless of the one that you choose.
We know from the cookies and radishes experiment that resisting temptations can deplete your willpower and ability to persevere through challenging tasks. So it would make sense that if we are given the chocolate milkshake, rather than the tasteless one, we will be much better at taking on a challenging task immediately after drinking it. Right?
WHAT FUELS WILLPOWER?
Researchers conducted this same experiment with participants to see what it was that really fueled our willpower muscle . They believed that those who indulged in the real milkshake would be able to persevere far longer than those who drank the tasteless glop of milk product.
However, when they conducted tests of perseverance after each group drank the milkshakes, there was no difference in the participants’ results! Both the milkshake drinkers and the glop drinkers spent about twice as much time on challenging tasks than those in a control group which didn’t have either shake.
What researchers found was that it was not the indulgence that fueled our willpower, it was the calories! 
GLUCOSE – THE WILLPOWER FUEL
When the body takes in food, it creates a chemical known as glucose that travels through the blood stream. The brain uses glucose as its source of fuel to think, create, and exert willpower. Because both of the milkshakes in the experiment had the exact same nutrients, they produced the same amount of glucose in the body. Thus, the brains of the participants in each group had the same amount of “willpower fuel” to work with. 
GOOD GLUCOSE AND BAD GLUCOSE
Any food that contains calories will give your brain glucose to work with. But not all glucose is created equally. Sugary foods will cause a quick spike of glucose, giving you willpower fuel for the short-term, but will cause a subsequent crash that depletes your willpower just as fast.
The best thing you can do is keep the glucose level in your bloodstream steady. This will give your brain a consistent reserve of fuel to exert willpower for the long-term. To accomplish this, researchers suggest a low-glycemic diet. 
Here are some low-glycemic foods that will give you long-term willpower fuel:
1. Lean proteins
Nothing fancy is required – just lean cuts of beef, poultry, pork and fish.
Specifically those nuts that are high in omega-3 fatty acids like walnuts, pecans and cashews. (Note: this does not include legumes like peanuts).
3. Fresh fruit
Fresh fruit is preferred over dried fruit because dried fruits have a high concentration of sugar in them. This will result in the glucose spike for the short term and lead to a subsequent crash. Some good choices are bananas, blueberries, apples and cherries.
All vegetables will help build your long-term willpower, but specific veggies that pack a willpower punch are root-based. These include sweet potatoes, potatoes and carrots which will all give you some serious willpower fuel!
Our willpower is like a muscle. And like all muscles in the human body, it requires fuel to be at its best. The fuel that it uses is a chemical found in our blood stream known as glucose. If your goal isn’t about dieting, you can eat or drink something sugary to get a quick willpower boost (this is especially good if you’re craving a cigarette, for example). However, for a long-term, sustainable, reservoir of willpower fuel, eat plenty of lean protein, nuts, fresh fruit and vegetables. If you can develop the habit of eating these foods, there are not limits to what you can do with that extra willpower fuel!
- Gailliot, M., Baumeister, R., DeWall, C., Maner, J., Plant, E., Tice, D., ... Schmeichel, B. (2007). Self-control Relies On Glucose As A Limited Energy Source: Willpower Is More Than A Metaphor. Journal of Personality and Social Psychology, 325-336.
- Baumeister, R., & Tierney, J. (2011) Willpower: Rediscovering the Greatest Human Strength. New York: Penguin.
- Pollan, M. (2009) Food Rules: An Eater's Manual. New York: Penguin.