On the other hand, underweight people can suffer from medical problems, ranging from chronic fatigue and anemia to lowered resistance to infection and clinical depression. Inflammation, certain medications, and depression associated with a chronic illness may lessen your appetite or upset your stomach, making it difficult for some people with spondylitis to maintain a healthy weight. This is especially true for those who have spondylitis with inflammatory bowel disease or Crohn's disease who experience gastrointestinal problems on top of arthritis symptoms. Any severe weight loss to should be reported to your doctor. 
Focus on exercise: The most compelling studies favor physical activity for mental acuity, says Gary W. Small, M.D., the director of the UCLA Longevity Center and a coauthor of The Alzheimer’s Prevention Program ($14, amazon.com). A study from the Annals of Internal Medicine found that people who were fitter at midlife had a 36 percent lower risk of developing dementia later in life than did their less-fit peers. “When people exercise, the areas that control memory, thinking, and attention increase in the brain,” says Small. “Regular exercisers also have less of the abnormal protein deposits in the brain that have been linked to Alzheimer’s.” That’s not to say that diet has no impact, says Small: “It’s just that the effects of exercise are more pronounced based on the evidence we have now.” Foods rich in omega-3 fatty acids (such as fish, nuts, and flaxseed) and antioxidant-rich fruits and vegetables (like strawberries and spinach) have been shown to improve brain health, while refined sugars and processed foods can have the opposite effect.
Sodium: This is critical for nerve impulse transmission and helps to maintain cells' normal fluid balance. The guidelines for sodium consumption are to consume less than 2,300 mg (approximately 1 tsp of salt) of sodium per day and to choose and prepare foods with little salt. At the same time, consume potassium-rich foods, such as fruits and vegetables.
Fiber aids digestion, prevents constipation, and lowers cholesterol -- and can help with weight loss. Most Americans get only half the fiber they need. To reap fiber's benefits, most women should get about 25 grams daily, while men need about 38 grams -- or 14 grams per 1,000 calories. Good fiber sources include oatmeal, beans, whole grain foods, nuts, and most fruits and vegetables.
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