Many locavores (and I could probably include myself among these) have claimed that local food is more nutritious. It seems like a simple fact: food that is grown closer to home, often with greater care as to the inputs and picked closer to peak harvest should have a higher nutritional content. But is it true? Naturally, the answer is not simple. There are an abundance of factors that have to be taken into consideration, and it is not a simple matter of correlating the more local the food is, the more nutritious it is. Primary among the complications is that both “local” and “nutritious” can be defined in a variety of ways. And if there are multiple definitions, then there are of course multiple ways to answer the questions, with data to support each statement. But my statistics professor always said data can be manipulated to prove anything you want.
So here’s what we’re looking at: by the time you sit down and take a bite of food that you have purchased at the supermarket, farmers market, roadside stand, bulk food store, etc., several factors have already happened that determine the food’s nutritional value. And by nutritional value, people mainly are talking about vitamin and mineral content, although some do take into consideration phytochemicals and other antioxidants. A tomato is not a tomato. There are several different varieties of tomatoes that have different nutritional values affixed to them. Additionally, methods used during the growing of the plant, when the plant was harvested in its cycle, how it was handled after harvest, how it was stored and transported for sale, the type of processing undergone, and the distance/time from harvest to you, as well as how you have treated and stored the food after you’ve bought it – these all can alter the vitamin and mineral content of the food.
Let’s break it down, starting with the basic element of your produce: the seed. The variety of plant does matter. Plants bred for industrial farms are bred primarily for short growing periods, high yields, and durability post-harvest and during transport. Nowhere in there is the vitamin level prioritized. To be fair, I cannot say that your local farmer is looking for varieties with the highest vitamin content either. However, she usually is looking for the most flavorful product possible. Additionally, local produce is not treated with ethylene gas to speed up the ripening process. There is little evidence that the synthetic ethylene used to ripen foods harms us consumers, and in fact there is some evidence that ethylene treatment actually increases antioxidant activity in kiwis. But the ethylene is produced in the petrochemical industry, in an extremely energy intensive process and then the foods are kept in a highly controlled, computerized room during the 24-48 hours of treatment – more energy consumption. There have been several studies on the loss of vitamins A, E and B of various products (tomatoes, citrus) after ethylene treatment.
Next factor is production method. This refers to the use of fertilizers or cover crops; it’s about the health of the soil the plants are grown in. Composted manures or other organic fertilizers release the nutrients slower than chemically-based fertilizers, which allows plants to establish deeper roots and enhance overall nutrient uptake by the plants. This creates healthier, stronger plants and – one would assume – healthier, better tasting fruit/vegetables. Another part of production method is the use of pesticides during plant growth. There is no evidence that I have found that pesticide use on plants changed the vitamin, mineral, or antioxidant content of the foods, but there is certainly residue that is left behind on many plants that could be adverse for your health. Click here for a list of conventional foods that tend to have very high levels of pesticides in them.
Post-harvest handling can also impact the nutritional value of foods. Foods need to be treated carefully to avoid bruising and extreme temperature changes. Bulk handling – as with mechanical harvesters and truck-loads of produce – increase these types of damages and therefore decrease the nutritional value. Local foods tend to be handled by people along all stages of harvest and packaging, which can minimize damages if done carefully. Post-harvest processing – even as something as simple as washing or cutting stems – can start an enzymatic reaction that will begin nutrient degradation. Those bags of washed, cut, ready-to-eat “baby” carrots may be handy for a quick snack, but have long ago reached their nutritional peak. (Not to say they are still not healthy to eat, mind you!!)
Storage and transportation impact the nutritional content of food as well. Texture, appearance and flavor can all be altered during the journey unless care is taken to keep the produce at an optimum temperature (which of course varies depending on the type of product being shipped). Fruits and vegetables shipped across the country (or from other countries) have to be kept in refrigerated containers, often with climate-controlled atmosphere to change humidity. The fruits and vegetables continue to respire and their enzymatic activity can alter the overall humidity of their environment, so it must be constantly monitored. Naturally, the longer the journey, the more chance for this alteration to occur especially with minimally processed foods. If your food was transported from a mile or two away, there is less time for these changes to occur. However, I will concede that many small local farmers do not have access to expensive refrigerator trucks and that their produce’s atmosphere was probably not computer-controlled. But the produce was also most likely picked just that morning.
In summary, it is likely that food picked within 24 hours of purchase and handled by far fewer people and grown from seeds that a farmer has selected for flavor and diversity and sold in the open air will have a higher nutritional value than food grown thousands of miles away from seeds selected to be tough, handled by dozens of people and displayed under fluorescent lights. However, there are many factors at play – it’s not cut and dry – and there is not an easy way to determine if local foods are more nutritious for you. Organic food, on the other hand, is a bit clearer and quite a bit of local food is also organic. So best to go with that! Read through this report on how organic foods have higher levels of certain nutrients and lower levels of pesticides.