One of the things I see time and again is people going heavy on the protein and fats while avoiding most vegetables and fruits when following a low-carb diet. While you're definitely minimizing carbs by only eating meat, fat, and leafy greens, you're also missing out on the benefit of including a large variety of plant foods in your diet.
Acid Base Balance
One concern regarding carbohydrate restriction is its adverse effect on the acid-base balance in the body. A high intake of acidic foods (primarily animal proteins) can result in loss of calcium in the urine and an increased risk of bone loss. This fact is often pointed out by advocates of vegan or plant-based diets. Protein plays an important role in appetite regulation and satiety, as well as performing many vital physiological functions. Compared to plant proteins, animal protein is both complete and of higher biological value. But at what level of protein intake does calciuria occur? Researchers found subjects to be in negative calcium balance on diets containing around 100 grams of protein per day, which isn't hard to exceed on a low-carbohydrate diet, or even the standard American diet. However, increasing intake of alkaline foods like vegetables and fruits has been shown to improve acid-base balance and significantly decrease urinary calcium losses. Adding plant sources of calcium (green beans, broccoli, greens, cauliflower, carrots, and oranges) would strengthen this effect.
Many low-carb advocates downplay the importance of fiber in the diet. While its cholesterol-lowering properties have proved to be less dramatic than once believed (who can forget the oat bran craze about 20 years ago?), there's new evidence that it may be quite beneficial for people with diabetes or prediabetes. Researchers looked at 15 studies conducted over a span of 30 years and concluded that increasing dietary fiber consumption by an average of 18 grams per day lowered fasting blood sugar by an average of 15 mg/dl, a significant amount. Of course, we are all unique in our response to nutrients, so not everyone who increases their fiber will experience the same results, but these results are encouraging. I find it particularly interesting that the fasting value rather than the postprandial value (which wasn't measured, as far as I can tell) decreased. The Hgb A1c also improved, but only fell by an average of 0.26%. However, since many of the studies were of relatively short duration and A1c is typically measured every 12 weeks, the actual improvement in long-term control may well have been greater.
Some of us tend to overestimate the amount of fiber we get. I recommend shooting for at least 30 grams a day, but more is even better. Here's a user-friendly online calculator to determine your total fiber intake for the day. I had 40 grams yesterday from vegetables, fruits, and nuts as part of a total carb intake of 106 grams (66 net carbs for the day).
Perhaps the most important reason to eat a wide variety of produce is the many types of phytonutrients (literally "plant nutrients") they contain. The color or pigment of a fruit or vegetable determines its type and potential benefits. The list of phytochemicals, their properties, and the plants that contain them is extensive, but here's a quick overview of a few of them:
Sulforaphane, isothocyanate, and indoles found in cruciferous vegetetables (broccoli, cabbage, kale, and Brussels sprouts) have been shown to protect against cancer.
Lycopene is found in tomatoes, grapefruit, and watermelon. Its consumption is linked to decrease incidence of certain types of cancer, including prostate cancer.
Anthocyanin-containing foods include berries, plums, red grapes, cranberies, and pomegranates. Protection from cancer and reduction in inflammation are attributed to this red/purple pigment.
Carotenes/carotenoids are found in carrots, pumpkin, apricots, cantaloupe, and yams. Properties including decreasing inflammation by quenching free radicals and assisting in the communication between cells.
For those interested in a more exhaustive list with references to phytochemical research, check out the Linus Pauling Institute web page.
While I'm definitely in favor of increasing fruit and vegetable intake for just about everyone, I'm also aware that many individuals have problems tolerating certain foods. Plants in the nightshade family (including tomatoes, eggplant, bell peppers, and potatoes) may be problematic for those with inflammatory conditions such as rheumatoid arthritis. People with irritable bowel syndrome or other fructose malabsoprtion may benefit from following the FODMAP approach, which limits foods containing Fermentable Oligosaccharides, Disaccharides, Monosaccharides,And Polyols (I plan to blog on this concept at a future date). Others may not feel great after eating certain plant foods or simply not find them appetizing. Obviously, if something doesn't agree with you, you shouldn't eat it no matter how healthy it's purported to be. Fortunately, there are a number of vegetables and fruits that each person can tolerate and enjoy; it may just take some time and experimentation to find them.
I see great benefit to eating animal protein and fats on a daily basis. But in order to make a low-carb diet a truly healthy way of eating, liberal intake of plant foods is important too.
1.Barzel US, Massey LK: Excess dietary protein can adversely affect bone. J Nutr Jun; 128: 1051-1053, 1988
2. Post, RE, et al. Dietary Fiber for the Treatment of Type 2 Diabetes Mellitus. J Am Board Fam Med Jan-Feb; 25:16-23, 2012
3.Galland L. Diet and Inflammation. Nutr Clin Pract Dec; 25(6):634-40, 2010
Franziska Spritzler, RD, CDE