Concerning butter/peanuts and saturated fats, there is a bit of a story. Let me keep it short.
When I was just beginning to study sport science, I was very fortunate to spend over a year in the company of some sport scientists from Eastern Europe. This was in the days just post-Perestroika. We had talks on many subjects (within language limitations), and one that came up was how they scoffed at the West's advice to almost completely reduce saturated fats in the diet. I often went to their lodgings and the table was rich with salad creams, creamy butter (on bread, on potatoes, on anything) and full fat cream desserts. I enjoyed these immensely and listened a lot.
Although doing high mileage at the time, I occasionally suffered on extra (2hr +) long runs at the end of a tough week. I would "bonk", hitting a mini wall. One particular long run was on the early Sunday morning after a Saturday night at my science friends' for dinner. It was effortless, and my wife and I went over it later with the bike. We knew we had run 3hrs, and calculated we had passed the full marathon somewhere around 2.50. We had talked the whole way.
I began experimenting and found that 50gms of peanuts (chewed until "milk" before swallowing) had a similar effect on me. As I moved into coaching I discovered that many men had a similar problem, although women less so. I soon got them onto the Saturday peanut routine (everybody eats them, some people are funny about eating butter). Soon, as well as having more endurance (being able to go further) a few of them began to notice a drop in aches and pains...
Now, I always like to know the WHY of a thing. Why does this work? Because I live in the home of olive oil. It is on just about every meal. And this particular Mediterranean diet is praised worldwide as one of the most healthy on the planet. Why did the peanuts work when the olive oil did not? As an aside, science always works like this for me: Have a problem. Find a solution. Figure out why the solution worked.
So, here are just a couple of pertinent papers from the research I have read over the intervening years.
Researchers at Univ of Buffalo, New York put runners on a low fat (16%) and medium fat (31%) diet. They also put other runners on a high fat (44% diet). All for 4 weeks.
Results: runners on the low fat diet ate 19% fewer calories than those on the other diets (did not wish to eat more). Body weight, body fat percent, VO2max and anaerobic power were not affected by level of dietary fat. (nobody got fatter or less fit).
Now the good stuff: endurance time increased from the low fat to medium fat diet by 14%. Those on the high fat diet even had reduced lactate levels (39%) after an endurance run (suggesting they had been relying more on burning fat in their aerobic system). Conclusion: runners on a low-fat diet consume fewer calories and have reduced endurance performance than on a medium or high fat diet.
(Full story here for those interested:
J Am Coll Nutr 2000 Feb;19(1):52-60
The effects of varying dietary fat on performance and metabolism in trained male and female runners.
Horvath PJ, Eagen CK, Fisher NM, Leddy JJ, Pendergast DR.
Department of Physical Therapy, University at Buffalo, New York 14214, USA.)
Now we have proved this over a number of years to our own satisfaction with my wife (2.36 marathoner). She runs best on a 30%+ fat diet. And loves almonds and peanuts.
But what about butter? Many of you will grab a handful of waistline fat and believe that you have enough already thanks, and don't need to eat more to run long. Wrong. For best energy, you want that fat stored in the muscle cells or available in the bloodstream. Think of it like a millionaire walking into a shop to buy something and finding no money in his pocket. No good telling the shopowner how much he has in the bank, he is unable to use it at this exact moment. The shopowner wants to see the money RIGHT NOW in his hand. Adipose fat (the stuff round your waistline) is not easy to access and use for running.
Another study in Louisiana State University found that after a 2-hr treadmill run (where DID they find rthe runners who volunteered for that?) the intramyocellular fat stores in women were replaced within 22hrs by a 35% fat diet. But were NOT replaced, even after 70hrs by a 10% fat diet. (so a longish run even 3 days later is gonna be tough). The fat stores had fallen by 25% during the 2-hr run.
But why butter and not vegetable oil?
Scientists in Paris, France fed Zucker rats a 30% fat diet over 11 weeks, with the fats coming from either soybean oil or butter (2 groups). Each group was further subdivided into obese and lean rats. So there were now 2 groups (1 fat, 1 skinny) being fed 30% butter, and another similar 2 groups being fed 30% soybean oil.
Body weight increased in fat (obese) rats and decreased in lean rats after butter feeding. BUT, the body weight gain in obese rats was due mainly to an increase of lean tissue. Body fat did not change. Resting metabolic (energy use) rate and the amount of fat burned after eating rose in obese rats. So if they ATE more, they BURNED more. The researchers concluded that this stopped the rats increasing fat accumulation in the long term. In the lean rats, butter feeding favoured (increased) fat burning by working muscles. They conclude by stating that this is an observation that deserves further investigation in terms of endurance and performance.
(Full story here for those interested:
Am J Clin Nutr 2002 Jan;75(1):21-30
Body weight, body composition, and energy metabolism in lean and obese Zucker rats fed soybean oil or butter.
Rolland V, Roseau S, Fromentin G, Nicolaidis S, Tome D, Even PC.
Laboratoire de Neurobiologie des Regulations, CNRS, Paris, France.)
Another recent thread on here was about one guy who "bonked" on a 17-miler. I told him to get on the peanuts, while others chimed in with suggestions of ice-cream which they had found important during their long mileage days.
A long post, but the short answer is don't go nuts (little pun, there. Lighten things up a bit), but do add in some butter, some almonds or peanuts, to your diet, even the odd ice-cream, especially before the long runs. Ahh, the simple pleasures of being a distance runner.
Last point for middle-distance runner: anaerobic power was not compromised by a medium fat diet. Meaning it won't stop you running or training fast.