鈥淓at your vegetables!鈥� is a command that just about every child has heard. 鈥淭hey鈥檒l make you stronger鈥� is a common follow up. The 鈥渟tronger鈥� connection is often traced back to Popeye the Sailorman鈥檚 guzzling spinach for its vitamin A content. Yes, vitamin A not iron! Contrary to the myth, Popeye never talked about iron in spinach making him stronger. In a 1932 comic strip, Popeye explicitly states, "Spinach is full of vitamin A, an' tha's what makes hoomans strong an' helty".
In any case, neither vitamin A nor iron in spinach has anything to do with strength. Spinach doesn鈥檛 even contain vitamin A, it contains beta-carotene that the body can convert into vitamin A. But that is neither here nor there because while vitamin A is important for a properly functioning immune system, it does not build muscle. Iron can indeed improve strength in someone who is anemic, but spinach would be a poor choice because the iron in it is bound up by phytates and is not readily absorbed. However, while spinach may not add muscle, it may add years to life. As long as we are using 鈥渆at more spinach鈥� as a stand in for 鈥渆at more veggies.鈥�
When we look for insight into nutrition, we look to the scientific literature. It is filled with studies touting the benefits of fruits and vegetables! The cornerstone for their link to health was laid by James Lind in the 18th聽century with his classic experiment demonstrating that scurvy can be prevented by the consumption of citrus fruits. This was then built upon by the identification of vitamins in plant products, and more recently, by the discovery that various polyphenols in plants can neutralize those nasty free radicals generated by biochemical processes.
While there has been much talk of the importance of consuming plant products based on animal experiments, cell culture studies and human epidemiological data, there has been ambiguity about quantizing the effects in terms of extending life. However, Chinese researchers have now analyzed data from the UK Biobank study that has for a decade followed over 100,000 participants who had submitted information about their diets and whose deaths were eventually recorded. It is common for Chinese scientists to mine studies carried out in the West for useful information that they can then publish. It is a way of increasing publications without doing any experiments. The conclusion of their analysis of the Biobank data was that subjects who had adhered to any one of the common 鈥渉ealthy鈥� diets, be it the 鈥淒ietary Approaches to Stop Hypertension (DASH),鈥� the 鈥淎lternative Healthy Eating Index,鈥� the 鈥淒iabetes Risk Reduction鈥� or the 鈥淢editerranean鈥� diet, had their life extended by up to 3 years.
The common feature of these diets is the consumption of a large variety and amount of whole grains, fruits and vegetables. While these contain loads of vitamins, minerals and polyphenols, all of which have been linked with preventing disease, the strongest association with increased longevity in the Biobank study was with a high fiber intake. Conversely, the biggest longevity reducer was a high intake of sugar-sweetened beverages.
Fiber is the component of food that resists breakdown by enzymes in the stomach and small intestine and therefore travels through to the colon, where depending on its specific composition it can have different effects. Soluble fiber, mostly complex carbohydrates such as beta-glucan in oats, pectin in fruit, psyllium from the Plantago ovata plant, or inulin from chicory, are fermented by colonic bacteria to produce short chain fatty acids. These serve as nutrients for cells in the lining of the colon and keep them healthy so that they can prevent nasty compounds from being absorbed into the bloodstream. Short chain fatty acids themselves can be absorbed into the bloodstream where they produce an anti-inflammatory effect. Soluble fiber also forms a viscuous gel in the small intestine that slows carbohydrate absorption and blunts glucose spikes after eating. In the colon, the gel binds bile acids preventing them from being reabsorbed and recycled. This forces the liver to make a fresh batch, which it does by pulling LDL cholesterol from the blood and converting it to bile acids, thereby effectively reducing blood cholesterol.
Insoluble fiber such as cellulose, lignin and wheat bran is not fermented by colonic bacteria, but does serve to increase the mass of stool that then results in speeding up its passage through the colon. This prevents the formation of diverticula, small bulging pockets in the wall of the colon that can cause diverticulitis with its associated fever, nausea and pain. Increasing the bulk of stool also reduces constipation as well as contact time of the colon with potential carcinogens.
The major problem with fiber is that people are not consuming enough of it. A ballpark figure to aim for is 35 grams a day for men, 25 for women. To get an idea, a slice of whole grain bread delivers about 3 grams, a cup of raspberries 8, an apple 4, a cup of cooked broccoli 5, half an avocado 6, a cup of blueberries 4, a banana 3 and two tablespoons of hummus 2. If you really want to strike it rich with fiber, go for a large bowl of lentil soup that will deliver about 15 grams. And that spinach? A cup cooked spinach has 4 grams.
Given that a meal of hamburger and fries has 4-6 grams of fiber, two slices of pizza have about the same amount and a peanut butter and jelly sandwich on white bread has about 4 grams, it is easy to see why the average daily consumption of fiber in Canada and the U.S. is roughly 50%, of the ideal amount. If you want to know the chance of your diet extending your life, make a rough calculation of your fiber intake and see how close it comes to 30 grams. But keep in mind that the Biobank study showed only an average of one and a half year extension by people consuming the 鈥渉ealthiest鈥� diet. Nevertheless, 鈥渆at your veggies鈥� is still good advice for one and all.
