Dear Zain
I am sorry to hear that are frail and weak. Basically individuals who struggle to gain weight do not ingest sufficient energy for the high demands of their Basic metabolic rate and/or physical activity. I have written some articles on this subject. For basic information click on 'Diet' at the top of this page and then on 'Diet Basics' and 'Weight gaining' and read the articles. Companies like Fresenius Kabi and Nestle, among others, make meal replacement supplements Phone Fresenius on (011) 545-0000, or the Nestle Consumer Services at 0860 09 6789 and ask about Nutritional Supplements to boost your energy intake. You can purchase liquid meal supplements at most chemist shops. Talk to the pharmacist about your need to gain weight and see which products he or she will recommend. Basically you need to eat more food either by gradually increasing how much you eat at each meal, or by taking supplements IN ADDITION to your regular meals (not to replace meals), or by having more snacks between meals. If you use a meal supplement such as Fresubin, with or without fibre, at the rate of 200 ml 3x a day, then you should gain 1 kg per week.
Best regards
DietDoc